[PATCH, v4] Fix reverse stepping multiple contiguous PC ranges over the line table

[PATCH, v4] Fix reverse stepping multiple contiguous PC ranges over the line table

[Luis Machado]

v4:
- Updated testcase to make it a bit longer so it can exercise reverse-stepping
  multiple times.
- Cleaned up debugging prints.

v3:
- Updated testcase.  The format for writing the DWARF program body in the
  testcase expect file changed.
  See commit gdb/testsuite/dwarf: simplify line number program syntax
  (commit d4c4a2298cad06ca71cfef725f5248f68205f0be)

v2:
- Check if both the line and symtab match for a particular line table entry.

--

When running GDB's testsuite on aarch64-linux/Ubuntu 20.04 (also spotted on
the ppc backend), I noticed some failures in gdb.reverse/solib-precsave.exp
and gdb.reverse/solib-reverse.exp.

The failure happens around the following code:

38  b[1] = shr2(17);		/* middle part two */
40  b[0] = 6;   b[1] = 9;	/* generic statement, end part two */
42  shr1 ("message 1\n");	/* shr1 one */

Normal execution:

- step from line 38 will land on line 40.
- step from line 40 will land on line 42.

Reverse execution:

- step from line 42 will land on line 40.
- step from line 40 will land on line 40.
- step from line 40 will land on line 38.

The problem here is that line 40 contains two contiguous but distinct
PC ranges in the line table, like so:

Line 40 - [0x7ec ~ 0x7f4]
Line 40 - [0x7f4 ~ 0x7fc]

The two distinct ranges are generated because GCC started outputting source
column information, which GDB doesn't take into account at the moment.

When stepping forward from line 40, we skip both of these ranges and land on
line 42. When stepping backward from line 42, we stop at the start PC of the
second (or first, going backwards) range of line 40.

This happens because we have this check in infrun.c:process_event_stop_test:

      /* When stepping backward, stop at beginning of line range
	 (unless it's the function entry point, in which case
	 keep going back to the call point).  */
      CORE_ADDR stop_pc = ecs->event_thread->stop_pc ();
      if (stop_pc == ecs->event_thread->control.step_range_start
	  && stop_pc != ecs->stop_func_start
	  && execution_direction == EXEC_REVERSE)
	end_stepping_range (ecs);
      else
	keep_going (ecs);

Since we've reached ecs->event_thread->control.step_range_start, we stop
stepping backwards.

The right thing to do is to look for adjacent PC ranges for the same line,
until we notice a line change. Then we take that as the start PC of the
range.

Another solution I thought about is to merge the contiguous ranges when
we are reading the line tables. Though I'm not sure if we really want to process
that data as opposed to keeping it as the compiler created, and then working
around that.

In any case, the following patch addresses this problem.

Validated on aarch64-linux and x86_64/Ubuntu 20.04/18.04. Carl Love has
verified that it does fix a similar issue on ppc.

Ubuntu 18.04 doesn't actually run into these failures because the compiler
doesn't generate distinct PC ranges for the same line.

I see similar failures on x86_64 in the gdb.reverse tests
(gdb.reverse/step-reverse.exp and gdb.reverse/step-reverse.exp). Those are
also fixed by this patch.

The included testcase (based on a test Carl wrote) exercises this problem for
Arm, ppc and x86. It shows full passes with the patch applied.

Co-authored-by: Carl Love <cel@us.ibm.com>
---
 gdb/infrun.c                                  |  22 ++-
 gdb/symtab.c                                  |  49 +++++++
 gdb/symtab.h                                  |  16 +++
 gdb/testsuite/gdb.reverse/map-to-same-line.c  |  55 +++++++
 .../gdb.reverse/map-to-same-line.exp          | 136 ++++++++++++++++++
 5 files changed, 277 insertions(+), 1 deletion(-)
 create mode 100644 gdb/testsuite/gdb.reverse/map-to-same-line.c
 create mode 100644 gdb/testsuite/gdb.reverse/map-to-same-line.exp

diff --git a/gdb/infrun.c b/gdb/infrun.c
index 02c98b50c8c..e9e14e58745 100644
--- a/gdb/infrun.c
+++ b/gdb/infrun.c
@@ -6917,11 +6917,31 @@ process_event_stop_test (struct execution_control_state *ecs)
 	 have software watchpoints).  */
       ecs->event_thread->control.may_range_step = 1;
 
+      /* When we are stepping inside a particular line range, in reverse,
+	 and we are sitting at the first address of that range, we need to
+	 check if this address also shows up in another line range as the
+	 end address.
+
+	 If so, we need to check what line such a step range points to.
+	 If it points to the same line as the current step range, that
+	 means we need to keep going in order to reach the first address
+	 of the line range.  We repeat this until we eventually get to the
+	 first address of a particular line we're stepping through.  */
+      CORE_ADDR range_start = ecs->event_thread->control.step_range_start;
+      if (execution_direction == EXEC_REVERSE)
+	{
+	  gdb::optional<CORE_ADDR> real_range_start
+	    = find_line_range_start (ecs->event_thread->stop_pc ());
+
+	  if (real_range_start.has_value ())
+	    range_start = *real_range_start;
+	}
+
       /* When stepping backward, stop at beginning of line range
 	 (unless it's the function entry point, in which case
 	 keep going back to the call point).  */
       CORE_ADDR stop_pc = ecs->event_thread->stop_pc ();
-      if (stop_pc == ecs->event_thread->control.step_range_start
+      if (stop_pc == range_start
 	  && stop_pc != ecs->stop_func_start
 	  && execution_direction == EXEC_REVERSE)
 	end_stepping_range (ecs);
diff --git a/gdb/symtab.c b/gdb/symtab.c
index 4b33d6c91af..de4cb5dd0eb 100644
--- a/gdb/symtab.c
+++ b/gdb/symtab.c
@@ -3433,6 +3433,55 @@ find_pc_line (CORE_ADDR pc, int notcurrent)
   return sal;
 }
 
+/* Compare two symtab_and_line entries.  Return true if both have
+   the same line number and the same symtab pointer.  That means we
+   are dealing with two entries from the same line and from the same
+   source file.
+
+   Return false otherwise.  */
+
+static bool
+sal_line_symtab_matches_p (const symtab_and_line &sal1,
+			   const symtab_and_line &sal2)
+{
+  return (sal1.line == sal2.line && sal1.symtab == sal2.symtab);
+}
+
+/* See symtah.h.  */
+
+gdb::optional<CORE_ADDR>
+find_line_range_start (CORE_ADDR pc)
+{
+  struct symtab_and_line current_sal = find_pc_line (pc, 0);
+
+  if (current_sal.line == 0)
+    return {};
+
+  struct symtab_and_line prev_sal = find_pc_line (current_sal.pc - 1, 0);
+
+  /* If the previous entry is for a different line, that means we are already
+     at the entry with the start PC for this line.  */
+  if (!sal_line_symtab_matches_p (prev_sal, current_sal))
+    return current_sal.pc;
+
+  /* Otherwise, keep looking for entries for the same line but with
+     smaller PC's.  */
+  bool done = false;
+  CORE_ADDR prev_pc;
+  while (!done)
+    {
+      prev_pc = prev_sal.pc;
+
+      prev_sal = find_pc_line (prev_pc - 1, 0);
+
+      /* Did we notice a line change?  If so, we are done with the search.  */
+      if (!sal_line_symtab_matches_p (prev_sal, current_sal))
+	done = true;
+    }
+
+  return prev_pc;
+}
+
 /* See symtab.h.  */
 
 struct symtab *
diff --git a/gdb/symtab.h b/gdb/symtab.h
index b1cf84f756f..226fe8803db 100644
--- a/gdb/symtab.h
+++ b/gdb/symtab.h
@@ -2285,6 +2285,22 @@ extern struct symtab_and_line find_pc_line (CORE_ADDR, int);
 extern struct symtab_and_line find_pc_sect_line (CORE_ADDR,
 						 struct obj_section *, int);
 
+/* Given PC, and assuming it is part of a range of addresses that is part of a
+   line, go back through the linetable and find the starting PC of that
+   line.
+
+   For example, suppose we have 3 PC ranges for line X:
+
+   Line X - [0x0 - 0x8]
+   Line X - [0x8 - 0x10]
+   Line X - [0x10 - 0x18]
+
+   If we call the function with PC == 0x14, we want to return 0x0, as that is
+   the starting PC of line X, and the ranges are contiguous.
+*/
+
+extern gdb::optional<CORE_ADDR> find_line_range_start (CORE_ADDR pc);
+
 /* Wrapper around find_pc_line to just return the symtab.  */
 
 extern struct symtab *find_pc_line_symtab (CORE_ADDR);
diff --git a/gdb/testsuite/gdb.reverse/map-to-same-line.c b/gdb/testsuite/gdb.reverse/map-to-same-line.c
new file mode 100644
index 00000000000..fa751ffe842
--- /dev/null
+++ b/gdb/testsuite/gdb.reverse/map-to-same-line.c
@@ -0,0 +1,55 @@
+/* Copyright 2022 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+/* The purpose of this test is to create a DWARF line table that contains two
+   or more entries for the same line.  When stepping (forwards or backwards),
+   GDB should step over the entire line and not just a particular entry in the
+   line table.  */
+
+int
+main ()
+{     /* TAG: main prologue */
+  asm ("main_label: .globl main_label");
+  int i = 1, j = 2, k;
+  float f1 = 2.0, f2 = 4.1, f3;
+  const char *str_1 = "foo", *str_2 = "bar", *str_3;
+
+  asm ("line1: .globl line1");
+  k = i; f3 = f1; str_3 = str_1;    /* TAG: line 1 */
+
+  asm ("line2: .globl line2");
+  k = j; f3 = f2; str_3 = str_2;    /* TAG: line 2 */
+
+  asm ("line3: .globl line3");
+  k = i; f3 = f1; str_3 = str_1;    /* TAG: line 3 */
+
+  asm ("line4: .globl line4");
+  k = j; f3 = f2; str_3 = str_2;    /* TAG: line 4 */
+
+  asm ("line5: .globl line5");
+  k = i; f3 = f1; str_3 = str_1;    /* TAG: line 5 */
+
+  asm ("line6: .globl line6");
+  k = j; f3 = f2; str_3 = str_2;    /* TAG: line 6 */
+
+  asm ("line7: .globl line7");
+  k = i; f3 = f1; str_3 = str_1;    /* TAG: line 7 */
+
+  asm ("line8: .globl line8");
+  k = j; f3 = f2; str_3 = str_2;    /* TAG: line 8 */
+
+  asm ("main_return: .globl main_return");
+  return 0; /* TAG: main return */
+}
diff --git a/gdb/testsuite/gdb.reverse/map-to-same-line.exp b/gdb/testsuite/gdb.reverse/map-to-same-line.exp
new file mode 100644
index 00000000000..efaa60a957f
--- /dev/null
+++ b/gdb/testsuite/gdb.reverse/map-to-same-line.exp
@@ -0,0 +1,136 @@
+# Copyright 2022 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# The purpose of this test is to create a DWARF line table that contains two
+# or more entries for the same line.  When stepping (forwards or backwards),
+# GDB should step over the entire line and not just a particular entry in the
+# line table.
+
+load_lib dwarf.exp
+
+# This test can only be run on targets which support DWARF-2 and use gas.
+if {![dwarf2_support]} {
+    unsupported "dwarf2 support required for this test"
+    return 0
+}
+
+if [get_compiler_info] {
+    return -1
+}
+
+# The DWARF assembler requires the gcc compiler.
+if {!$gcc_compiled} {
+    unsupported "gcc is required for this test"
+    return 0
+}
+
+standard_testfile .c .S
+
+if { [prepare_for_testing "failed to prepare" ${testfile} ${srcfile}] } {
+    return -1
+}
+
+set asm_file [standard_output_file $srcfile2]
+Dwarf::assemble $asm_file {
+    global srcdir subdir srcfile
+    declare_labels integer_label L
+
+    # Find start address and length of program
+    lassign [function_range main [list ${srcdir}/${subdir}/$srcfile]] \
+	main_start main_len
+    set main_end "$main_start + $main_len"
+
+    cu {} {
+	compile_unit {
+	    {language @DW_LANG_C}
+	    {name map-to-same-line.c}
+	    {stmt_list $L DW_FORM_sec_offset}
+	    {low_pc 0 addr}
+	} {
+	    subprogram {
+		{external 1 flag}
+		{name main}
+		{low_pc $main_start addr}
+		{high_pc $main_len DW_FORM_data4}
+	    }
+	}
+    }
+
+    lines {version 2 default_is_stmt 1} L {
+	include_dir "${srcdir}/${subdir}"
+	file_name "$srcfile" 1
+
+	# Generate the line table program with distinct source lines being
+	# mapped to the same line entry. Line 1, 5 and 8 contain 1 statement
+	# each.  Line 2 contains 2 statements.  Line 3 contains 3 statements.
+	program {
+	    DW_LNE_set_address $main_start
+	    line [gdb_get_line_number "TAG: main prologue"]
+	    DW_LNS_copy
+	    DW_LNE_set_address line1
+	    line [gdb_get_line_number "TAG: line 1" ]
+	    DW_LNS_copy
+	    DW_LNE_set_address line2
+	    line [gdb_get_line_number "TAG: line 2" ]
+	    DW_LNS_copy
+	    DW_LNE_set_address line3
+	    line [gdb_get_line_number "TAG: line 2" ]
+	    DW_LNS_copy
+	    DW_LNE_set_address line4
+	    line [gdb_get_line_number "TAG: line 3" ]
+	    DW_LNS_copy
+	    DW_LNE_set_address line5
+	    line [gdb_get_line_number "TAG: line 3" ]
+	    DW_LNS_copy
+	    DW_LNE_set_address line6
+	    line [gdb_get_line_number "TAG: line 3" ]
+	    DW_LNS_copy
+	    DW_LNE_set_address line7
+	    line [gdb_get_line_number "TAG: line 5" ]
+	    DW_LNS_copy
+	    DW_LNE_set_address line8
+	    line [gdb_get_line_number "TAG: line 8" ]
+	    DW_LNS_copy
+	    DW_LNE_set_address main_return
+	    line [gdb_get_line_number "TAG: main return"]
+	    DW_LNS_copy
+	    DW_LNE_end_sequence
+	}
+    }
+}
+
+if { [prepare_for_testing "failed to prepare" ${testfile} \
+	[list $srcfile $asm_file] {nodebug} ] } {
+    return -1
+}
+
+if ![runto_main] {
+    return -1
+}
+
+if [supports_process_record] {
+    # Activate process record/replay
+    gdb_test_no_output "record" "turn on process record"
+}
+
+gdb_test "tbreak main_return" "Temporary breakpoint .*" "breakpoint at return"
+gdb_test "continue" "Temporary breakpoint .*" "run to end of main"
+
+# At this point, GDB has already recorded the execution up until the return
+# statement.  Reverse-step and test if GDB transitions between lines in the
+# expected order.  It should reverse-step across lines 8, 5, 3, 2 and 1.
+foreach line {8 5 3 2 1} {
+    gdb_test "reverse-step" ".*TAG: line $line.*" "reverse step to line $line"
+}
-- 
2.25.1

Re: [PATCH,v4] Fix reverse stepping multiple contiguous PC ranges over the line table

[Bruno Larsen]

Hi Luis and Carl!

Sorry about not answering about this earlier, I had a few rushed weeks, but I have some nits about the .exp file

On 5/6/22 05:55, Luis Machado wrote:
> v4:
> - Updated testcase to make it a bit longer so it can exercise reverse-stepping
>    multiple times.
> - Cleaned up debugging prints.
> 

<snip>

> diff --git a/gdb/testsuite/gdb.reverse/map-to-same-line.exp b/gdb/testsuite/gdb.reverse/map-to-same-line.exp
> new file mode 100644
> index 00000000000..efaa60a957f
> --- /dev/null
> +++ b/gdb/testsuite/gdb.reverse/map-to-same-line.exp
> @@ -0,0 +1,136 @@
> +# Copyright 2022 Free Software Foundation, Inc.
> +
> +# This program is free software; you can redistribute it and/or modify
> +# it under the terms of the GNU General Public License as published by
> +# the Free Software Foundation; either version 3 of the License, or
> +# (at your option) any later version.
> +#
> +# This program is distributed in the hope that it will be useful,
> +# but WITHOUT ANY WARRANTY; without even the implied warranty of
> +# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> +# GNU General Public License for more details.
> +#
> +# You should have received a copy of the GNU General Public License
> +# along with this program.  If not, see <http://www.gnu.org/licenses/>.
> +
> +# The purpose of this test is to create a DWARF line table that contains two
> +# or more entries for the same line.  When stepping (forwards or backwards),
> +# GDB should step over the entire line and not just a particular entry in the
> +# line table.
> +

Could you add a comment here after the copyright blurb explaining why this testcase exists? Something similar to the find_line_range_start comment should be enough, jsut so the next unrelated person who looks at this code in 2 years has some context as to what is going on.

> +load_lib dwarf.exp
> +
> +# This test can only be run on targets which support DWARF-2 and use gas.
> +if {![dwarf2_support]} {
> +    unsupported "dwarf2 support required for this test"
> +    return 0
> +}
> +
> +if [get_compiler_info] {
> +    return -1
> +}
> +
> +# The DWARF assembler requires the gcc compiler.
> +if {!$gcc_compiled} {
> +    unsupported "gcc is required for this test"
> +    return 0
> +}

There should probably be a test here for supports_reverse. Or supports_process_record. I'm not sure what the difference is between these 2 checks.

> +
> +standard_testfile .c .S
> +
> +if { [prepare_for_testing "failed to prepare" ${testfile} ${srcfile}] } {
> +    return -1
> +}
> +
> +set asm_file [standard_output_file $srcfile2]
> +Dwarf::assemble $asm_file {
> +    global srcdir subdir srcfile
> +    declare_labels integer_label L
> +
> +    # Find start address and length of program
> +    lassign [function_range main [list ${srcdir}/${subdir}/$srcfile]] \
> +	main_start main_len
> +    set main_end "$main_start + $main_len"
> +
> +    cu {} {
> +	compile_unit {
> +	    {language @DW_LANG_C}
> +	    {name map-to-same-line.c}
> +	    {stmt_list $L DW_FORM_sec_offset}
> +	    {low_pc 0 addr}
> +	} {
> +	    subprogram {
> +		{external 1 flag}
> +		{name main}
> +		{low_pc $main_start addr}
> +		{high_pc $main_len DW_FORM_data4}
> +	    }
> +	}
> +    }
> +
> +    lines {version 2 default_is_stmt 1} L {
> +	include_dir "${srcdir}/${subdir}"
> +	file_name "$srcfile" 1
> +
> +	# Generate the line table program with distinct source lines being
> +	# mapped to the same line entry. Line 1, 5 and 8 contain 1 statement
> +	# each.  Line 2 contains 2 statements.  Line 3 contains 3 statements.
> +	program {
> +	    DW_LNE_set_address $main_start
> +	    line [gdb_get_line_number "TAG: main prologue"]
> +	    DW_LNS_copy
> +	    DW_LNE_set_address line1
> +	    line [gdb_get_line_number "TAG: line 1" ]
> +	    DW_LNS_copy
> +	    DW_LNE_set_address line2
> +	    line [gdb_get_line_number "TAG: line 2" ]
> +	    DW_LNS_copy
> +	    DW_LNE_set_address line3
> +	    line [gdb_get_line_number "TAG: line 2" ]
> +	    DW_LNS_copy
> +	    DW_LNE_set_address line4
> +	    line [gdb_get_line_number "TAG: line 3" ]
> +	    DW_LNS_copy
> +	    DW_LNE_set_address line5
> +	    line [gdb_get_line_number "TAG: line 3" ]
> +	    DW_LNS_copy
> +	    DW_LNE_set_address line6
> +	    line [gdb_get_line_number "TAG: line 3" ]
> +	    DW_LNS_copy
> +	    DW_LNE_set_address line7
> +	    line [gdb_get_line_number "TAG: line 5" ]
> +	    DW_LNS_copy
> +	    DW_LNE_set_address line8
> +	    line [gdb_get_line_number "TAG: line 8" ]
> +	    DW_LNS_copy
> +	    DW_LNE_set_address main_return
> +	    line [gdb_get_line_number "TAG: main return"]
> +	    DW_LNS_copy
> +	    DW_LNE_end_sequence
> +	}
> +    }
> +}
> +
> +if { [prepare_for_testing "failed to prepare" ${testfile} \
> +	[list $srcfile $asm_file] {nodebug} ] } {
> +    return -1
> +}
> +
> +if ![runto_main] {
> +    return -1
> +}
> +
> +if [supports_process_record] {
> +    # Activate process record/replay
> +    gdb_test_no_output "record" "turn on process record"
> +}
since we would have tested above if the target supports recording/reversing, we should be good to just use record here, without the if clause. I suggest doing it this way because there is no point in this whole test if the target doesn't support reverse execution.

> +
> +gdb_test "tbreak main_return" "Temporary breakpoint .*" "breakpoint at return"
> +gdb_test "continue" "Temporary breakpoint .*" "run to end of main"
> +
> +# At this point, GDB has already recorded the execution up until the return
> +# statement.  Reverse-step and test if GDB transitions between lines in the
> +# expected order.  It should reverse-step across lines 8, 5, 3, 2 and 1.
> +foreach line {8 5 3 2 1} {
> +    gdb_test "reverse-step" ".*TAG: line $line.*" "reverse step to line $line"
> +}

Cheers!
Bruno Larsen

RE: [PATCH, v4] Fix reverse stepping multiple contiguous PC ranges over the line table

[Carl Love]

Bruno:

On Fri, 2022-05-06 at 12:04 -0300, Bruno Larsen wrote:

<snip>

> >  >.
> > +
> > +# The purpose of this test is to create a DWARF line table that
> > contains two
> > +# or more entries for the same line.  When stepping (forwards or
> > backwards),
> > +# GDB should step over the entire line and not just a particular
> > entry in the
> > +# line table.
> > +
> 
> Could you add a comment here after the copyright blurb explaining why
> this testcase exists? Something similar to the find_line_range_start
> comment should be enough, jsut so the next unrelated person who looks
> at this code in 2 years has some context as to what is going on.

OK, updated the comment to say the purpose is to test the
find_line_range_start functionality.  I did reorganize the comment a
bit to make things flow better.  Specifically, started with the
expected behaviour of GDB followed by testing find_line_range_start to
ensure GDB works as expected.

> 
> > +load_lib dwarf.exp
> > +
> > +# This test can only be run on targets which support DWARF-2 and
> > use gas.
> > +if {![dwarf2_support]} {
> > +    unsupported "dwarf2 support required for this test"
> > +    return 0
> > +}
> > +
> > +if [get_compiler_info] {
> > +    return -1
> > +}
> > +
> > +# The DWARF assembler requires the gcc compiler.
> > +if {!$gcc_compiled} {
> > +    unsupported "gcc is required for this test"
> > +    return 0
> > +}
> 
> There should probably be a test here for supports_reverse. Or
> supports_process_record. I'm not sure what the difference is between
> these 2 checks.

Yes, I added if ![supports_process_record] test here to make sure the
system supports the record and replay.  

<snip>
> 
> > +
> > +if { [prepare_for_testing "failed to prepare" ${testfile} \
> > +	[list $srcfile $asm_file] {nodebug} ] } {
> > +    return -1
> > +}
> > +
> > +if ![runto_main] {
> > +    return -1
> > +}
> > +
> > +if [supports_process_record] {
> > +    # Activate process record/replay
> > +    gdb_test_no_output "record" "turn on process record"
> > +}
> since we would have tested above if the target supports
> recording/reversing, we should be good to just use record here,
> without the if clause. I suggest doing it this way because there is
> no point in this whole test if the target doesn't support reverse
> execution.

Yes, removed the supports_process_record test.
> > 

Will post v5 of the patch for review.

Thanks for the input. 

                          Carl Love

Re: [PATCH,v5] Fix reverse stepping multiple contiguous PC ranges over the line table

[Carl Love]

Bruno, GDB maintainers:

The patch has been updated per the comments on the testcase from Bruno.

The patch was retested on Power 10 to ensure the test still works
correctly.

Please let us know if there are any additional comments or if the patch
is ready to commit.  We thank you for your help with this patch.

                   Carl Love
--------------------------------------------------------
Fix reverse stepping multiple contiguous PC ranges over the line table
v5:- Updated test case comments on the purpose of the test.- Add test
to check system supports record-replay.- Removed now unnecessary reord
test when activating record.
v4:- Updated testcase to make it a bit longer so it can exercise
reverse-stepping  multiple times.- Cleaned up debugging prints.
v3:- Updated testcase.  The format for writing the DWARF program body
in the  testcase expect file changed.  See commit gdb/testsuite/dwarf:
simplify line number program syntax  (commit
d4c4a2298cad06ca71cfef725f5248f68205f0be)
v2:- Check if both the line and symtab match for a particular line
table entry.
--
When running GDB's testsuite on aarch64-linux/Ubuntu 20.04 (also
spotted onthe ppc backend), I noticed some failures in
gdb.reverse/solib-precsave.expand gdb.reverse/solib-reverse.exp.
The failure happens around the following code:
38  b[1] = shr2(17);		/* middle part two */40  b[0] =
6;   b[1] = 9;	/* generic statement, end part two */42  shr1 ("message
1\n");	/* shr1 one */
Normal execution:
- step from line 38 will land on line 40.- step from line 40 will land
on line 42.
Reverse execution:
- step from line 42 will land on line 40.- step from line 40 will land
on line 40.- step from line 40 will land on line 38.
The problem here is that line 40 contains two contiguous but distinctPC
ranges in the line table, like so:
Line 40 - [0x7ec ~ 0x7f4]Line 40 - [0x7f4 ~ 0x7fc]
The two distinct ranges are generated because GCC started outputting
sourcecolumn information, which GDB doesn't take into account at the
moment.
When stepping forward from line 40, we skip both of these ranges and
land online 42. When stepping backward from line 42, we stop at the
start PC of thesecond (or first, going backwards) range of line 40.
This happens because we have this check in
infrun.c:process_event_stop_test:
      /* When stepping backward, stop at beginning of line range	
 (unless it's the function entry point, in which case	 keep going
back to the call point).  */      CORE_ADDR stop_pc = ecs-
>event_thread->stop_pc ();      if (stop_pc == ecs->event_thread-
>control.step_range_start	  && stop_pc != ecs->stop_func_start	
  && execution_direction == EXEC_REVERSE)	end_stepping_range
(ecs);      else	keep_going (ecs);
Since we've reached ecs->event_thread->control.step_range_start, we
stopstepping backwards.
The right thing to do is to look for adjacent PC ranges for the same
line,until we notice a line change. Then we take that as the start PC
of therange.
Another solution I thought about is to merge the contiguous ranges
whenwe are reading the line tables. Though I'm not sure if we really
want to processthat data as opposed to keeping it as the compiler
created, and then workingaround that.
In any case, the following patch addresses this problem.
Validated on aarch64-linux and x86_64/Ubuntu 20.04/18.04. Carl Love
hasverified that it does fix a similar issue on ppc.
Ubuntu 18.04 doesn't actually run into these failures because the
compilerdoesn't generate distinct PC ranges for the same line.
I see similar failures on x86_64 in the gdb.reverse
tests(gdb.reverse/step-reverse.exp and gdb.reverse/step-reverse.exp).
Those arealso fixed by this patch.
The included testcase (based on a test Carl wrote) exercises this
problem forArm, ppc and x86. It shows full passes with the patch
applied.
Co-authored-by: Carl Love <cel@us.ibm.com>---
gdb/infrun.c                                  |  22 ++-
gdb/symtab.c                                  |  49 ++++++
gdb/symtab.h                                  |  16 ++
gdb/testsuite/gdb.reverse/map-to-same-line.c  |  55 +++++++
.../gdb.reverse/map-to-same-line.exp          | 141 ++++++++++++++++++
5 files changed, 282 insertions(+), 1 deletion(-) create mode 100644
gdb/testsuite/gdb.reverse/map-to-same-line.c create mode 100644
gdb/testsuite/gdb.reverse/map-to-same-line.exp
diff --git a/gdb/infrun.c b/gdb/infrun.cindex 6e5853ef42a..82c28961aeb
100644--- a/gdb/infrun.c+++ b/gdb/infrun.c@@ -6955,11 +6955,31 @@ if
(ecs->event_thread->control.proceed_to_finish 	 have software
watchpoints).  */       ecs->event_thread->control.may_range_step = 1;
+      /* When we are stepping inside a particular line range, in
reverse,+	 and we are sitting at the first address of that range,
we need to+	 check if this address also shows up in another line
range as the+	 end address.++	 If so, we need to check what line such
a step range points to.+	 If it points to the same line as the
current step range, that+	 means we need to keep going in order
to reach the first address+	 of the line range.  We repeat this
until we eventually get to the+	 first address of a particular line
we're stepping through.  */+      CORE_ADDR range_start = ecs-
>event_thread->control.step_range_start;+      if (execution_direction
== EXEC_REVERSE)+	{+	  gdb::optional<CORE_ADDR>
real_range_start+	    = find_line_range_start (ecs->event_thread-
>stop_pc ());++	  if (real_range_start.has_value ())+	    range_start
= *real_range_start;+	}+       /* When stepping backward, stop at
beginning of line range 	 (unless it's the function entry point,
in which case 	 keep going back to the call
point).  */       CORE_ADDR stop_pc = ecs->event_thread->stop_pc
();-      if (stop_pc == ecs->event_thread-
>control.step_range_start+      if (stop_pc == range_start 	  &&
stop_pc != ecs->stop_func_start 	  && execution_direction ==
EXEC_REVERSE) 	end_stepping_range (ecs);diff --git a/gdb/symtab.c
b/gdb/symtab.cindex 4b33d6c91af..de4cb5dd0eb 100644---
a/gdb/symtab.c+++ b/gdb/symtab.c@@ -3433,6 +3433,55 @@ find_pc_line
(CORE_ADDR pc, int notcurrent)   return sal; } +/* Compare two
symtab_and_line entries.  Return true if both have+   the same line
number and the same symtab pointer.  That means we+   are dealing with
two entries from the same line and from the same+   source
file.++   Return false otherwise.  */++static
bool+sal_line_symtab_matches_p (const symtab_and_line &sal1,+		
	   const symtab_and_line &sal2)+{+  return (sal1.line ==
sal2.line && sal1.symtab == sal2.symtab);+}++/* See
symtah.h.  */++gdb::optional<CORE_ADDR>+find_line_range_start
(CORE_ADDR pc)+{+  struct symtab_and_line current_sal = find_pc_line
(pc, 0);++  if (current_sal.line == 0)+    return {};++  struct
symtab_and_line prev_sal = find_pc_line (current_sal.pc - 1, 0);++  /*
If the previous entry is for a different line, that means we are
already+     at the entry with the start PC for this line.  */+  if
(!sal_line_symtab_matches_p (prev_sal, current_sal))+    return
current_sal.pc;++  /* Otherwise, keep looking for entries for the same
line but with+     smaller PC's.  */+  bool done = false;+  CORE_ADDR
prev_pc;+  while (!done)+    {+      prev_pc =
prev_sal.pc;++      prev_sal = find_pc_line (prev_pc - 1, 0);++      /*
Did we notice a line change?  If so, we are done with the
search.  */+      if (!sal_line_symtab_matches_p (prev_sal,
current_sal))+	done = true;+    }++  return prev_pc;+}+ /* See
symtab.h.  */  struct symtab *diff --git a/gdb/symtab.h
b/gdb/symtab.hindex b1cf84f756f..226fe8803db 100644---
a/gdb/symtab.h+++ b/gdb/symtab.h@@ -2285,6 +2285,22 @@ extern struct
symtab_and_line find_pc_line (CORE_ADDR, int); extern struct
symtab_and_line find_pc_sect_line (CORE_ADDR, 				
		 struct obj_section *, int); +/* Given PC, and assuming
it is part of a range of addresses that is part of a+   line, go back
through the linetable and find the starting PC of that+   line.++   For
example, suppose we have 3 PC ranges for line X:++   Line X - [0x0 -
0x8]+   Line X - [0x8 - 0x10]+   Line X - [0x10 - 0x18]++   If we call
the function with PC == 0x14, we want to return 0x0, as that is+   the
starting PC of line X, and the ranges are contiguous.+*/++extern
gdb::optional<CORE_ADDR> find_line_range_start (CORE_ADDR pc);+ /*
Wrapper around find_pc_line to just return the symtab.  */  extern
struct symtab *find_pc_line_symtab (CORE_ADDR);diff --git
a/gdb/testsuite/gdb.reverse/map-to-same-line.c
b/gdb/testsuite/gdb.reverse/map-to-same-line.cnew file mode 100644index
00000000000..dd9f9f8a400--- /dev/null+++
b/gdb/testsuite/gdb.reverse/map-to-same-line.c@@ -0,0 +1,55 @@+/*
Copyright 2022 Free Software Foundation, Inc.++   This program is free
software; you can redistribute it and/or modify+   it under the terms
of the GNU General Public License as published by+   the Free Software
Foundation; either version 3 of the License, or+   (at your option) any
later version.++   This program is distributed in the hope that it will
be useful,+   but WITHOUT ANY WARRANTY; without even the implied
warranty of+   MERCHANTABILITY or FITNESS FOR A PARTICULAR
PURPOSE.  See the+   GNU General Public License for more
details.++   You should have received a copy of the GNU General Public
License+   along with this program.  If not, see <
http://www.gnu.org/licenses/
 >.  */++/* The purpose of this test is to create a DWARF line table
that contains two+   or more entries for the same line.  When stepping
(forwards or backwards),+   GDB should step over the entire line and
not just a particular entry in the+   line table.  */++int+main
()+{     /* TAG: main prologue */+  asm ("main_label: .globl
main_label");+  int i = 1, j = 2, k;+  float f1 = 2.0, f2 = 4.1,
f3;+  const char *str_1 = "foo", *str_2 = "bar", *str_3;++  asm
("line1: .globl line1");+  k = i; f3 = f1; str_3 = str_1;    /* TAG:
line 1 */++  asm ("line2: .globl line2");+  k = j; f3 = f2; str_3 =
str_2;    /* TAG: line 2 */++  asm ("line3: .globl line3");+  k = i; f3
= f1; str_3 = str_1;    /* TAG: line 3 */++  asm ("line4: .globl
line4");+  k = j; f3 = f2; str_3 = str_2;    /* TAG: line 4 */++  asm
("line5: .globl line5");+  k = i; f3 = f1; str_3 = str_1;    /* TAG:
line 5 */++  asm ("line6: .globl line6");+  k = j; f3 = f2; str_3 =
str_2;    /* TAG: line 6 */++  asm ("line7: .globl line7");+  k = i; f3
= f1; str_3 = str_1;    /* TAG: line 7 */++  asm ("line8: .globl
line8");+  k = j; f3 = f2; str_3 = str_2;    /* TAG: line 8 */++  asm
("main_return: .globl main_return");+  return 0; /* TAG: main return
*/+}diff --git a/gdb/testsuite/gdb.reverse/map-to-same-line.exp
b/gdb/testsuite/gdb.reverse/map-to-same-line.expnew file mode
100644index 00000000000..c3fb859be55--- /dev/null+++
b/gdb/testsuite/gdb.reverse/map-to-same-line.exp@@ -0,0 +1,141 @@+#
Copyright 2022 Free Software Foundation, Inc.++# This program is free
software; you can redistribute it and/or modify+# it under the terms of
the GNU General Public License as published by+# the Free Software
Foundation; either version 3 of the License, or+# (at your option) any
later version.+#+# This program is distributed in the hope that it will
be useful,+# but WITHOUT ANY WARRANTY; without even the implied
warranty of+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See
the+# GNU General Public License for more details.+#+# You should have
received a copy of the GNU General Public License+# along with this
program.  If not, see <
http://www.gnu.org/licenses/
 >.++# When stepping (forwards or backwards), GDB should step over the
entire line+# and not just a particular entry in the line table. This
test was added to+# verify the find_line_range_start function properly
sets the step range for a+# line that consists of multiple statements,
i.e. multiple entries in the line+# table.  This test creates a DWARF
line table that contains two entries for+# the same line to do the
needed testing.++load_lib dwarf.exp++# This test can only be run on
targets which support DWARF-2 and use gas.+if {![dwarf2_support]}
{+    unsupported "dwarf2 support required for this test"+    return
0+}++if [get_compiler_info] {+    return -1+}++# The DWARF assembler
requires the gcc compiler.+if {!$gcc_compiled} {+    unsupported "gcc
is required for this test"+    return 0+}++# This test suitable only
for process record-replay+if ![supports_process_record]
{+    return+}++standard_testfile .c .S++if { [prepare_for_testing
"failed to prepare" ${testfile} ${srcfile}] } {+    return -1+}++set
asm_file [standard_output_file $srcfile2]+Dwarf::assemble $asm_file
{+    global srcdir subdir srcfile+    declare_labels integer_label
L++    # Find start address and length of program+    lassign
[function_range main [list ${srcdir}/${subdir}/$srcfile]] \+	main_st
art main_len+    set main_end "$main_start + $main_len"++    cu {} {+	
compile_unit {+	    {language @DW_LANG_C}+	    {name map-to-same-
line.c}+	    {stmt_list $L DW_FORM_sec_offset}+	    {low_pc 0
addr}+	} {+	    subprogram {+		{external 1 flag}+	
	{name main}+		{low_pc $main_start addr}+		
{high_pc $main_len DW_FORM_data4}+	    }+	}+    }++    lines
{version 2 default_is_stmt 1} L {+	include_dir
"${srcdir}/${subdir}"+	file_name "$srcfile" 1++	# Generate the
line table program with distinct source lines being+	# mapped to the
same line entry. Line 1, 5 and 8 contain 1 statement+	# each.  Line 2
contains 2 statements.  Line 3 contains 3 statements.+	program {+	
    DW_LNE_set_address $main_start+	    line [gdb_get_line_number
"TAG: main prologue"]+	    DW_LNS_copy+	    DW_LNE_set_address
line1+	    line [gdb_get_line_number "TAG: line 1" ]+	    DW_LNS_copy
+	    DW_LNE_set_address line2+	    line [gdb_get_line_number
"TAG: line 2" ]+	    DW_LNS_copy+	    DW_LNE_set_address
line3+	    line [gdb_get_line_number "TAG: line 2" ]+	    DW_LNS_copy
+	    DW_LNE_set_address line4+	    line [gdb_get_line_number
"TAG: line 3" ]+	    DW_LNS_copy+	    DW_LNE_set_address
line5+	    line [gdb_get_line_number "TAG: line 3" ]+	    DW_LNS_copy
+	    DW_LNE_set_address line6+	    line [gdb_get_line_number
"TAG: line 3" ]+	    DW_LNS_copy+	    DW_LNE_set_address
line7+	    line [gdb_get_line_number "TAG: line 5" ]+	    DW_LNS_copy
+	    DW_LNE_set_address line8+	    line [gdb_get_line_number
"TAG: line 8" ]+	    DW_LNS_copy+	    DW_LNE_set_address
main_return+	    line [gdb_get_line_number "TAG: main return"]+	
    DW_LNS_copy+	    DW_LNE_end_sequence+	}+    }+}++if {
[prepare_for_testing "failed to prepare" ${testfile} \+	[list $srcfile
$asm_file] {nodebug} ] } {+    return -1+}++if ![runto_main]
{+    return -1+}++# Activate process record/replay+gdb_test_no_output
"record" "turn on process record"++gdb_test "tbreak main_return"
"Temporary breakpoint .*" "breakpoint at return"+gdb_test "continue"
"Temporary breakpoint .*" "run to end of main"++# At this point, GDB
has already recorded the execution up until the return+#
statement.  Reverse-step and test if GDB transitions between lines in
the+# expected order.  It should reverse-step across lines 8, 5, 3, 2
and 1.+foreach line {8 5 3 2 1} {+    gdb_test "reverse-step" ".*TAG:
line $line.*" "reverse step to line $line"+}-- 2.31.1

[PING][PATCH,v5] Fix reverse stepping multiple contiguous PC ranges over the line table

[Carl Love]

GDB maintainers:

We have addressed the comments from Bruno. Unfortunately, Bruno is not
a maintainer and can't give approval for the patch.  We are hoping a
maintainer can review the patch and provide us feedback.  

Thank you for your time.

                       Carl Love
---------------------------------------------------

On Fri, 2022-05-06 at 09:48 -0700, Carl Love via Gdb-patches wrote:
> Bruno, GDB maintainers:
> 
> The patch has been updated per the comments on the testcase from
> Bruno.
> 
> The patch was retested on Power 10 to ensure the test still works
> correctly.
> 
> Please let us know if there are any additional comments or if the
> patch
> is ready to commit.  We thank you for your help with this patch.
> 
>                    Carl Love
> --------------------------------------------------------
> Fix reverse stepping multiple contiguous PC ranges over the line
> table
> v5:- Updated test case comments on the purpose of the test.- Add test
> to check system supports record-replay.- Removed now unnecessary
> reord
> test when activating record.
> v4:- Updated testcase to make it a bit longer so it can exercise
> reverse-stepping  multiple times.- Cleaned up debugging prints.
> v3:- Updated testcase.  The format for writing the DWARF program body
> in the  testcase expect file changed.  See commit
> gdb/testsuite/dwarf:
> simplify line number program syntax  (commit
> d4c4a2298cad06ca71cfef725f5248f68205f0be)
> v2:- Check if both the line and symtab match for a particular line
> table entry.
> --
> When running GDB's testsuite on aarch64-linux/Ubuntu 20.04 (also
> spotted onthe ppc backend), I noticed some failures in
> gdb.reverse/solib-precsave.expand gdb.reverse/solib-reverse.exp.
> The failure happens around the following code:
> 38  b[1] = shr2(17);		/* middle part two */40  b[0] =
> 6;   b[1] = 9;	/* generic statement, end part two */42  shr1
> ("message
> 1\n");	/* shr1 one */
> Normal execution:
> - step from line 38 will land on line 40.- step from line 40 will
> land
> on line 42.
> Reverse execution:
> - step from line 42 will land on line 40.- step from line 40 will
> land
> on line 40.- step from line 40 will land on line 38.
> The problem here is that line 40 contains two contiguous but
> distinctPC
> ranges in the line table, like so:
> Line 40 - [0x7ec ~ 0x7f4]Line 40 - [0x7f4 ~ 0x7fc]
> The two distinct ranges are generated because GCC started outputting
> sourcecolumn information, which GDB doesn't take into account at the
> moment.
> When stepping forward from line 40, we skip both of these ranges and
> land online 42. When stepping backward from line 42, we stop at the
> start PC of thesecond (or first, going backwards) range of line 40.
> This happens because we have this check in
> infrun.c:process_event_stop_test:
>       /* When stepping backward, stop at beginning of line range	
>  (unless it's the function entry point, in which case	 keep going
> back to the call point).  */      CORE_ADDR stop_pc = ecs-
> > event_thread->stop_pc ();      if (stop_pc == ecs->event_thread-
> > control.step_range_start	  && stop_pc != ecs->stop_func_start	
>   && execution_direction == EXEC_REVERSE)	end_stepping_range
> (ecs);      else	keep_going (ecs);
> Since we've reached ecs->event_thread->control.step_range_start, we
> stopstepping backwards.
> The right thing to do is to look for adjacent PC ranges for the same
> line,until we notice a line change. Then we take that as the start PC
> of therange.
> Another solution I thought about is to merge the contiguous ranges
> whenwe are reading the line tables. Though I'm not sure if we really
> want to processthat data as opposed to keeping it as the compiler
> created, and then workingaround that.
> In any case, the following patch addresses this problem.
> Validated on aarch64-linux and x86_64/Ubuntu 20.04/18.04. Carl Love
> hasverified that it does fix a similar issue on ppc.
> Ubuntu 18.04 doesn't actually run into these failures because the
> compilerdoesn't generate distinct PC ranges for the same line.
> I see similar failures on x86_64 in the gdb.reverse
> tests(gdb.reverse/step-reverse.exp and gdb.reverse/step-reverse.exp).
> Those arealso fixed by this patch.
> The included testcase (based on a test Carl wrote) exercises this
> problem forArm, ppc and x86. It shows full passes with the patch
> applied.
> Co-authored-by: Carl Love <cel@us.ibm.com>---
> gdb/infrun.c                                  |  22 ++-
> gdb/symtab.c                                  |  49 ++++++
> gdb/symtab.h                                  |  16 ++
> gdb/testsuite/gdb.reverse/map-to-same-line.c  |  55 +++++++
> .../gdb.reverse/map-to-same-line.exp          | 141
> ++++++++++++++++++
> 5 files changed, 282 insertions(+), 1 deletion(-) create mode 100644
> gdb/testsuite/gdb.reverse/map-to-same-line.c create mode 100644
> gdb/testsuite/gdb.reverse/map-to-same-line.exp
> diff --git a/gdb/infrun.c b/gdb/infrun.cindex
> 6e5853ef42a..82c28961aeb
> 100644--- a/gdb/infrun.c+++ b/gdb/infrun.c@@ -6955,11 +6955,31 @@ if
> (ecs->event_thread->control.proceed_to_finish 	 have software
> watchpoints).  */       ecs->event_thread->control.may_range_step =
> 1;
> +      /* When we are stepping inside a particular line range, in
> reverse,+	 and we are sitting at the first address of that range,
> we need to+	 check if this address also shows up in another line
> range as the+	 end address.++	 If so, we need to check what line
> such
> a step range points to.+	 If it points to the same line as the
> current step range, that+	 means we need to keep going in order
> to reach the first address+	 of the line range.  We repeat this
> until we eventually get to the+	 first address of a particular
> line
> we're stepping through.  */+      CORE_ADDR range_start = ecs-
> > event_thread->control.step_range_start;+      if
> > (execution_direction
> == EXEC_REVERSE)+	{+	  gdb::optional<CORE_ADDR>
> real_range_start+	    = find_line_range_start (ecs->event_thread-
> > stop_pc ());++	  if (real_range_start.has_value ())+	   
> >  range_start
> = *real_range_start;+	}+       /* When stepping backward, stop at
> beginning of line range 	 (unless it's the function entry point,
> in which case 	 keep going back to the call
> point).  */       CORE_ADDR stop_pc = ecs->event_thread->stop_pc
> ();-      if (stop_pc == ecs->event_thread-
> > control.step_range_start+      if (stop_pc == range_start 	  &&
> stop_pc != ecs->stop_func_start 	  && execution_direction ==
> EXEC_REVERSE) 	end_stepping_range (ecs);diff --git
> a/gdb/symtab.c
> b/gdb/symtab.cindex 4b33d6c91af..de4cb5dd0eb 100644---
> a/gdb/symtab.c+++ b/gdb/symtab.c@@ -3433,6 +3433,55 @@ find_pc_line
> (CORE_ADDR pc, int notcurrent)   return sal; } +/* Compare two
> symtab_and_line entries.  Return true if both have+   the same line
> number and the same symtab pointer.  That means we+   are dealing
> with
> two entries from the same line and from the same+   source
> file.++   Return false otherwise.  */++static
> bool+sal_line_symtab_matches_p (const symtab_and_line &sal1,+		
> 	   const symtab_and_line &sal2)+{+  return (sal1.line ==
> sal2.line && sal1.symtab == sal2.symtab);+}++/* See
> symtah.h.  */++gdb::optional<CORE_ADDR>+find_line_range_start
> (CORE_ADDR pc)+{+  struct symtab_and_line current_sal = find_pc_line
> (pc, 0);++  if (current_sal.line == 0)+    return {};++  struct
> symtab_and_line prev_sal = find_pc_line (current_sal.pc - 1,
> 0);++  /*
> If the previous entry is for a different line, that means we are
> already+     at the entry with the start PC for this line.  */+  if
> (!sal_line_symtab_matches_p (prev_sal, current_sal))+    return
> current_sal.pc;++  /* Otherwise, keep looking for entries for the
> same
> line but with+     smaller PC's.  */+  bool done = false;+  CORE_ADDR
> prev_pc;+  while (!done)+    {+      prev_pc =
> prev_sal.pc;++      prev_sal = find_pc_line (prev_pc - 1,
> 0);++      /*
> Did we notice a line change?  If so, we are done with the
> search.  */+      if (!sal_line_symtab_matches_p (prev_sal,
> current_sal))+	done = true;+    }++  return prev_pc;+}+ /* See
> symtab.h.  */  struct symtab *diff --git a/gdb/symtab.h
> b/gdb/symtab.hindex b1cf84f756f..226fe8803db 100644---
> a/gdb/symtab.h+++ b/gdb/symtab.h@@ -2285,6 +2285,22 @@ extern struct
> symtab_and_line find_pc_line (CORE_ADDR, int); extern struct
> symtab_and_line find_pc_sect_line (CORE_ADDR, 			
> 	
> 		 struct obj_section *, int); +/* Given PC, and assuming
> it is part of a range of addresses that is part of a+   line, go back
> through the linetable and find the starting PC of
> that+   line.++   For
> example, suppose we have 3 PC ranges for line X:++   Line X - [0x0 -
> 0x8]+   Line X - [0x8 - 0x10]+   Line X - [0x10 - 0x18]++   If we
> call
> the function with PC == 0x14, we want to return 0x0, as that
> is+   the
> starting PC of line X, and the ranges are contiguous.+*/++extern
> gdb::optional<CORE_ADDR> find_line_range_start (CORE_ADDR pc);+ /*
> Wrapper around find_pc_line to just return the symtab.  */  extern
> struct symtab *find_pc_line_symtab (CORE_ADDR);diff --git
> a/gdb/testsuite/gdb.reverse/map-to-same-line.c
> b/gdb/testsuite/gdb.reverse/map-to-same-line.cnew file mode
> 100644index
> 00000000000..dd9f9f8a400--- /dev/null+++
> b/gdb/testsuite/gdb.reverse/map-to-same-line.c@@ -0,0 +1,55 @@+/*
> Copyright 2022 Free Software Foundation, Inc.++   This program is
> free
> software; you can redistribute it and/or modify+   it under the terms
> of the GNU General Public License as published by+   the Free
> Software
> Foundation; either version 3 of the License, or+   (at your option)
> any
> later version.++   This program is distributed in the hope that it
> will
> be useful,+   but WITHOUT ANY WARRANTY; without even the implied
> warranty of+   MERCHANTABILITY or FITNESS FOR A PARTICULAR
> PURPOSE.  See the+   GNU General Public License for more
> details.++   You should have received a copy of the GNU General
> Public
> License+   along with this program.  If not, see <
> http://www.gnu.org/licenses/ 
>  >.  */++/* The purpose of this test is to create a DWARF line table
> that contains two+   or more entries for the same line.  When
> stepping
> (forwards or backwards),+   GDB should step over the entire line and
> not just a particular entry in the+   line table.  */++int+main
> ()+{     /* TAG: main prologue */+  asm ("main_label: .globl
> main_label");+  int i = 1, j = 2, k;+  float f1 = 2.0, f2 = 4.1,
> f3;+  const char *str_1 = "foo", *str_2 = "bar", *str_3;++  asm
> ("line1: .globl line1");+  k = i; f3 = f1; str_3 = str_1;    /* TAG:
> line 1 */++  asm ("line2: .globl line2");+  k = j; f3 = f2; str_3 =
> str_2;    /* TAG: line 2 */++  asm ("line3: .globl line3");+  k = i;
> f3
> = f1; str_3 = str_1;    /* TAG: line 3 */++  asm ("line4: .globl
> line4");+  k = j; f3 = f2; str_3 = str_2;    /* TAG: line 4 */++  asm
> ("line5: .globl line5");+  k = i; f3 = f1; str_3 = str_1;    /* TAG:
> line 5 */++  asm ("line6: .globl line6");+  k = j; f3 = f2; str_3 =
> str_2;    /* TAG: line 6 */++  asm ("line7: .globl line7");+  k = i;
> f3
> = f1; str_3 = str_1;    /* TAG: line 7 */++  asm ("line8: .globl
> line8");+  k = j; f3 = f2; str_3 = str_2;    /* TAG: line 8 */++  asm
> ("main_return: .globl main_return");+  return 0; /* TAG: main return
> */+}diff --git a/gdb/testsuite/gdb.reverse/map-to-same-line.exp
> b/gdb/testsuite/gdb.reverse/map-to-same-line.expnew file mode
> 100644index 00000000000..c3fb859be55--- /dev/null+++
> b/gdb/testsuite/gdb.reverse/map-to-same-line.exp@@ -0,0 +1,141 @@+#
> Copyright 2022 Free Software Foundation, Inc.++# This program is free
> software; you can redistribute it and/or modify+# it under the terms
> of
> the GNU General Public License as published by+# the Free Software
> Foundation; either version 3 of the License, or+# (at your option)
> any
> later version.+#+# This program is distributed in the hope that it
> will
> be useful,+# but WITHOUT ANY WARRANTY; without even the implied
> warranty of+# MERCHANTABILITY or FITNESS FOR A PARTICULAR
> PURPOSE.  See
> the+# GNU General Public License for more details.+#+# You should
> have
> received a copy of the GNU General Public License+# along with this
> program.  If not, see <
> http://www.gnu.org/licenses/ 
>  >.++# When stepping (forwards or backwards), GDB should step over
> the
> entire line+# and not just a particular entry in the line table. This
> test was added to+# verify the find_line_range_start function
> properly
> sets the step range for a+# line that consists of multiple
> statements,
> i.e. multiple entries in the line+# table.  This test creates a DWARF
> line table that contains two entries for+# the same line to do the
> needed testing.++load_lib dwarf.exp++# This test can only be run on
> targets which support DWARF-2 and use gas.+if {![dwarf2_support]}
> {+    unsupported "dwarf2 support required for this test"+    return
> 0+}++if [get_compiler_info] {+    return -1+}++# The DWARF assembler
> requires the gcc compiler.+if {!$gcc_compiled} {+    unsupported "gcc
> is required for this test"+    return 0+}++# This test suitable only
> for process record-replay+if ![supports_process_record]
> {+    return+}++standard_testfile .c .S++if { [prepare_for_testing
> "failed to prepare" ${testfile} ${srcfile}] } {+    return -1+}++set
> asm_file [standard_output_file $srcfile2]+Dwarf::assemble $asm_file
> {+    global srcdir subdir srcfile+    declare_labels integer_label
> L++    # Find start address and length of program+    lassign
> [function_range main [list ${srcdir}/${subdir}/$srcfile]] \+	main_st
> art main_len+    set main_end "$main_start + $main_len"++    cu {} {+
> 	
> compile_unit {+	    {language @DW_LANG_C}+	    {name
> map-to-same-
> line.c}+	    {stmt_list $L DW_FORM_sec_offset}+	    {low_pc 0
> addr}+	} {+	    subprogram {+		{external 1
> flag}+	
> 	{name main}+		{low_pc $main_start addr}+		
> {high_pc $main_len DW_FORM_data4}+	    }+	}+    }++    lines
> {version 2 default_is_stmt 1} L {+	include_dir
> "${srcdir}/${subdir}"+	file_name "$srcfile" 1++	# Generate
> the
> line table program with distinct source lines being+	# mapped to the
> same line entry. Line 1, 5 and 8 contain 1 statement+	# each.  Line 2
> contains 2 statements.  Line 3 contains 3 statements.+	program
> {+	
>     DW_LNE_set_address $main_start+	    line [gdb_get_line_number
> "TAG: main prologue"]+	    DW_LNS_copy+	    DW_LNE_set_addres
> s
> line1+	    line [gdb_get_line_number "TAG: line 1" ]+	    D
> W_LNS_copy
> +	    DW_LNE_set_address line2+	    line [gdb_get_line_number
> "TAG: line 2" ]+	    DW_LNS_copy+	    DW_LNE_set_address
> line3+	    line [gdb_get_line_number "TAG: line 2" ]+	    D
> W_LNS_copy
> +	    DW_LNE_set_address line4+	    line [gdb_get_line_number
> "TAG: line 3" ]+	    DW_LNS_copy+	    DW_LNE_set_address
> line5+	    line [gdb_get_line_number "TAG: line 3" ]+	    D
> W_LNS_copy
> +	    DW_LNE_set_address line6+	    line [gdb_get_line_number
> "TAG: line 3" ]+	    DW_LNS_copy+	    DW_LNE_set_address
> line7+	    line [gdb_get_line_number "TAG: line 5" ]+	    D
> W_LNS_copy
> +	    DW_LNE_set_address line8+	    line [gdb_get_line_number
> "TAG: line 8" ]+	    DW_LNS_copy+	    DW_LNE_set_address
> main_return+	    line [gdb_get_line_number "TAG: main return"]+	
>     DW_LNS_copy+	    DW_LNE_end_sequence+	}+    }+}++if {
> [prepare_for_testing "failed to prepare" ${testfile} \+	[list
> $srcfile
> $asm_file] {nodebug} ] } {+    return -1+}++if ![runto_main]
> {+    return -1+}++# Activate process
> record/replay+gdb_test_no_output
> "record" "turn on process record"++gdb_test "tbreak main_return"
> "Temporary breakpoint .*" "breakpoint at return"+gdb_test "continue"
> "Temporary breakpoint .*" "run to end of main"++# At this point, GDB
> has already recorded the execution up until the return+#
> statement.  Reverse-step and test if GDB transitions between lines in
> the+# expected order.  It should reverse-step across lines 8, 5, 3, 2
> and 1.+foreach line {8 5 3 2 1} {+    gdb_test "reverse-step" ".*TAG:
> line $line.*" "reverse step to line $line"+}-- 2.31.1
> 
> 

Re: [PING][PATCH,v5] Fix reverse stepping multiple contiguous PC ranges over the line table

[Luis Machado]

Ping?

On 5/13/22 18:00, Carl Love wrote:
> GDB maintainers:
> 
> We have addressed the comments from Bruno. Unfortunately, Bruno is not
> a maintainer and can't give approval for the patch.  We are hoping a
> maintainer can review the patch and provide us feedback.
> 
> Thank you for your time.
> 
>                         Carl Love
> ---------------------------------------------------
> 
> On Fri, 2022-05-06 at 09:48 -0700, Carl Love via Gdb-patches wrote:
>> Bruno, GDB maintainers:
>>
>> The patch has been updated per the comments on the testcase from
>> Bruno.
>>
>> The patch was retested on Power 10 to ensure the test still works
>> correctly.
>>
>> Please let us know if there are any additional comments or if the
>> patch
>> is ready to commit.  We thank you for your help with this patch.
>>
>>                     Carl Love
>> --------------------------------------------------------
>> Fix reverse stepping multiple contiguous PC ranges over the line
>> table
>> v5:- Updated test case comments on the purpose of the test.- Add test
>> to check system supports record-replay.- Removed now unnecessary
>> reord
>> test when activating record.
>> v4:- Updated testcase to make it a bit longer so it can exercise
>> reverse-stepping  multiple times.- Cleaned up debugging prints.
>> v3:- Updated testcase.  The format for writing the DWARF program body
>> in the  testcase expect file changed.  See commit
>> gdb/testsuite/dwarf:
>> simplify line number program syntax  (commit
>> d4c4a2298cad06ca71cfef725f5248f68205f0be)
>> v2:- Check if both the line and symtab match for a particular line
>> table entry.
>> --
>> When running GDB's testsuite on aarch64-linux/Ubuntu 20.04 (also
>> spotted onthe ppc backend), I noticed some failures in
>> gdb.reverse/solib-precsave.expand gdb.reverse/solib-reverse.exp.
>> The failure happens around the following code:
>> 38  b[1] = shr2(17);		/* middle part two */40  b[0] =
>> 6;   b[1] = 9;	/* generic statement, end part two */42  shr1
>> ("message
>> 1\n");	/* shr1 one */
>> Normal execution:
>> - step from line 38 will land on line 40.- step from line 40 will
>> land
>> on line 42.
>> Reverse execution:
>> - step from line 42 will land on line 40.- step from line 40 will
>> land
>> on line 40.- step from line 40 will land on line 38.
>> The problem here is that line 40 contains two contiguous but
>> distinctPC
>> ranges in the line table, like so:
>> Line 40 - [0x7ec ~ 0x7f4]Line 40 - [0x7f4 ~ 0x7fc]
>> The two distinct ranges are generated because GCC started outputting
>> sourcecolumn information, which GDB doesn't take into account at the
>> moment.
>> When stepping forward from line 40, we skip both of these ranges and
>> land online 42. When stepping backward from line 42, we stop at the
>> start PC of thesecond (or first, going backwards) range of line 40.
>> This happens because we have this check in
>> infrun.c:process_event_stop_test:
>>        /* When stepping backward, stop at beginning of line range	
>>   (unless it's the function entry point, in which case	 keep going
>> back to the call point).  */      CORE_ADDR stop_pc = ecs-
>>> event_thread->stop_pc ();      if (stop_pc == ecs->event_thread-
>>> control.step_range_start	  && stop_pc != ecs->stop_func_start	
>>    && execution_direction == EXEC_REVERSE)	end_stepping_range
>> (ecs);      else	keep_going (ecs);
>> Since we've reached ecs->event_thread->control.step_range_start, we
>> stopstepping backwards.
>> The right thing to do is to look for adjacent PC ranges for the same
>> line,until we notice a line change. Then we take that as the start PC
>> of therange.
>> Another solution I thought about is to merge the contiguous ranges
>> whenwe are reading the line tables. Though I'm not sure if we really
>> want to processthat data as opposed to keeping it as the compiler
>> created, and then workingaround that.
>> In any case, the following patch addresses this problem.
>> Validated on aarch64-linux and x86_64/Ubuntu 20.04/18.04. Carl Love
>> hasverified that it does fix a similar issue on ppc.
>> Ubuntu 18.04 doesn't actually run into these failures because the
>> compilerdoesn't generate distinct PC ranges for the same line.
>> I see similar failures on x86_64 in the gdb.reverse
>> tests(gdb.reverse/step-reverse.exp and gdb.reverse/step-reverse.exp).
>> Those arealso fixed by this patch.
>> The included testcase (based on a test Carl wrote) exercises this
>> problem forArm, ppc and x86. It shows full passes with the patch
>> applied.
>> Co-authored-by: Carl Love <cel@us.ibm.com>---
>> gdb/infrun.c                                  |  22 ++-
>> gdb/symtab.c                                  |  49 ++++++
>> gdb/symtab.h                                  |  16 ++
>> gdb/testsuite/gdb.reverse/map-to-same-line.c  |  55 +++++++
>> .../gdb.reverse/map-to-same-line.exp          | 141
>> ++++++++++++++++++
>> 5 files changed, 282 insertions(+), 1 deletion(-) create mode 100644
>> gdb/testsuite/gdb.reverse/map-to-same-line.c create mode 100644
>> gdb/testsuite/gdb.reverse/map-to-same-line.exp
>> diff --git a/gdb/infrun.c b/gdb/infrun.cindex
>> 6e5853ef42a..82c28961aeb
>> 100644--- a/gdb/infrun.c+++ b/gdb/infrun.c@@ -6955,11 +6955,31 @@ if
>> (ecs->event_thread->control.proceed_to_finish 	 have software
>> watchpoints).  */       ecs->event_thread->control.may_range_step =
>> 1;
>> +      /* When we are stepping inside a particular line range, in
>> reverse,+	 and we are sitting at the first address of that range,
>> we need to+	 check if this address also shows up in another line
>> range as the+	 end address.++	 If so, we need to check what line
>> such
>> a step range points to.+	 If it points to the same line as the
>> current step range, that+	 means we need to keep going in order
>> to reach the first address+	 of the line range.  We repeat this
>> until we eventually get to the+	 first address of a particular
>> line
>> we're stepping through.  */+      CORE_ADDR range_start = ecs-
>>> event_thread->control.step_range_start;+      if
>>> (execution_direction
>> == EXEC_REVERSE)+	{+	  gdb::optional<CORE_ADDR>
>> real_range_start+	    = find_line_range_start (ecs->event_thread-
>>> stop_pc ());++	  if (real_range_start.has_value ())+	
>>>   range_start
>> = *real_range_start;+	}+       /* When stepping backward, stop at
>> beginning of line range 	 (unless it's the function entry point,
>> in which case 	 keep going back to the call
>> point).  */       CORE_ADDR stop_pc = ecs->event_thread->stop_pc
>> ();-      if (stop_pc == ecs->event_thread-
>>> control.step_range_start+      if (stop_pc == range_start 	  &&
>> stop_pc != ecs->stop_func_start 	  && execution_direction ==
>> EXEC_REVERSE) 	end_stepping_range (ecs);diff --git
>> a/gdb/symtab.c
>> b/gdb/symtab.cindex 4b33d6c91af..de4cb5dd0eb 100644---
>> a/gdb/symtab.c+++ b/gdb/symtab.c@@ -3433,6 +3433,55 @@ find_pc_line
>> (CORE_ADDR pc, int notcurrent)   return sal; } +/* Compare two
>> symtab_and_line entries.  Return true if both have+   the same line
>> number and the same symtab pointer.  That means we+   are dealing
>> with
>> two entries from the same line and from the same+   source
>> file.++   Return false otherwise.  */++static
>> bool+sal_line_symtab_matches_p (const symtab_and_line &sal1,+		
>> 	   const symtab_and_line &sal2)+{+  return (sal1.line ==
>> sal2.line && sal1.symtab == sal2.symtab);+}++/* See
>> symtah.h.  */++gdb::optional<CORE_ADDR>+find_line_range_start
>> (CORE_ADDR pc)+{+  struct symtab_and_line current_sal = find_pc_line
>> (pc, 0);++  if (current_sal.line == 0)+    return {};++  struct
>> symtab_and_line prev_sal = find_pc_line (current_sal.pc - 1,
>> 0);++  /*
>> If the previous entry is for a different line, that means we are
>> already+     at the entry with the start PC for this line.  */+  if
>> (!sal_line_symtab_matches_p (prev_sal, current_sal))+    return
>> current_sal.pc;++  /* Otherwise, keep looking for entries for the
>> same
>> line but with+     smaller PC's.  */+  bool done = false;+  CORE_ADDR
>> prev_pc;+  while (!done)+    {+      prev_pc =
>> prev_sal.pc;++      prev_sal = find_pc_line (prev_pc - 1,
>> 0);++      /*
>> Did we notice a line change?  If so, we are done with the
>> search.  */+      if (!sal_line_symtab_matches_p (prev_sal,
>> current_sal))+	done = true;+    }++  return prev_pc;+}+ /* See
>> symtab.h.  */  struct symtab *diff --git a/gdb/symtab.h
>> b/gdb/symtab.hindex b1cf84f756f..226fe8803db 100644---
>> a/gdb/symtab.h+++ b/gdb/symtab.h@@ -2285,6 +2285,22 @@ extern struct
>> symtab_and_line find_pc_line (CORE_ADDR, int); extern struct
>> symtab_and_line find_pc_sect_line (CORE_ADDR, 			
>> 	
>> 		 struct obj_section *, int); +/* Given PC, and assuming
>> it is part of a range of addresses that is part of a+   line, go back
>> through the linetable and find the starting PC of
>> that+   line.++   For
>> example, suppose we have 3 PC ranges for line X:++   Line X - [0x0 -
>> 0x8]+   Line X - [0x8 - 0x10]+   Line X - [0x10 - 0x18]++   If we
>> call
>> the function with PC == 0x14, we want to return 0x0, as that
>> is+   the
>> starting PC of line X, and the ranges are contiguous.+*/++extern
>> gdb::optional<CORE_ADDR> find_line_range_start (CORE_ADDR pc);+ /*
>> Wrapper around find_pc_line to just return the symtab.  */  extern
>> struct symtab *find_pc_line_symtab (CORE_ADDR);diff --git
>> a/gdb/testsuite/gdb.reverse/map-to-same-line.c
>> b/gdb/testsuite/gdb.reverse/map-to-same-line.cnew file mode
>> 100644index
>> 00000000000..dd9f9f8a400--- /dev/null+++
>> b/gdb/testsuite/gdb.reverse/map-to-same-line.c@@ -0,0 +1,55 @@+/*
>> Copyright 2022 Free Software Foundation, Inc.++   This program is
>> free
>> software; you can redistribute it and/or modify+   it under the terms
>> of the GNU General Public License as published by+   the Free
>> Software
>> Foundation; either version 3 of the License, or+   (at your option)
>> any
>> later version.++   This program is distributed in the hope that it
>> will
>> be useful,+   but WITHOUT ANY WARRANTY; without even the implied
>> warranty of+   MERCHANTABILITY or FITNESS FOR A PARTICULAR
>> PURPOSE.  See the+   GNU General Public License for more
>> details.++   You should have received a copy of the GNU General
>> Public
>> License+   along with this program.  If not, see <
>> http://www.gnu.org/licenses/
>>   >.  */++/* The purpose of this test is to create a DWARF line table
>> that contains two+   or more entries for the same line.  When
>> stepping
>> (forwards or backwards),+   GDB should step over the entire line and
>> not just a particular entry in the+   line table.  */++int+main
>> ()+{     /* TAG: main prologue */+  asm ("main_label: .globl
>> main_label");+  int i = 1, j = 2, k;+  float f1 = 2.0, f2 = 4.1,
>> f3;+  const char *str_1 = "foo", *str_2 = "bar", *str_3;++  asm
>> ("line1: .globl line1");+  k = i; f3 = f1; str_3 = str_1;    /* TAG:
>> line 1 */++  asm ("line2: .globl line2");+  k = j; f3 = f2; str_3 =
>> str_2;    /* TAG: line 2 */++  asm ("line3: .globl line3");+  k = i;
>> f3
>> = f1; str_3 = str_1;    /* TAG: line 3 */++  asm ("line4: .globl
>> line4");+  k = j; f3 = f2; str_3 = str_2;    /* TAG: line 4 */++  asm
>> ("line5: .globl line5");+  k = i; f3 = f1; str_3 = str_1;    /* TAG:
>> line 5 */++  asm ("line6: .globl line6");+  k = j; f3 = f2; str_3 =
>> str_2;    /* TAG: line 6 */++  asm ("line7: .globl line7");+  k = i;
>> f3
>> = f1; str_3 = str_1;    /* TAG: line 7 */++  asm ("line8: .globl
>> line8");+  k = j; f3 = f2; str_3 = str_2;    /* TAG: line 8 */++  asm
>> ("main_return: .globl main_return");+  return 0; /* TAG: main return
>> */+}diff --git a/gdb/testsuite/gdb.reverse/map-to-same-line.exp
>> b/gdb/testsuite/gdb.reverse/map-to-same-line.expnew file mode
>> 100644index 00000000000..c3fb859be55--- /dev/null+++
>> b/gdb/testsuite/gdb.reverse/map-to-same-line.exp@@ -0,0 +1,141 @@+#
>> Copyright 2022 Free Software Foundation, Inc.++# This program is free
>> software; you can redistribute it and/or modify+# it under the terms
>> of
>> the GNU General Public License as published by+# the Free Software
>> Foundation; either version 3 of the License, or+# (at your option)
>> any
>> later version.+#+# This program is distributed in the hope that it
>> will
>> be useful,+# but WITHOUT ANY WARRANTY; without even the implied
>> warranty of+# MERCHANTABILITY or FITNESS FOR A PARTICULAR
>> PURPOSE.  See
>> the+# GNU General Public License for more details.+#+# You should
>> have
>> received a copy of the GNU General Public License+# along with this
>> program.  If not, see <
>> http://www.gnu.org/licenses/
>>   >.++# When stepping (forwards or backwards), GDB should step over
>> the
>> entire line+# and not just a particular entry in the line table. This
>> test was added to+# verify the find_line_range_start function
>> properly
>> sets the step range for a+# line that consists of multiple
>> statements,
>> i.e. multiple entries in the line+# table.  This test creates a DWARF
>> line table that contains two entries for+# the same line to do the
>> needed testing.++load_lib dwarf.exp++# This test can only be run on
>> targets which support DWARF-2 and use gas.+if {![dwarf2_support]}
>> {+    unsupported "dwarf2 support required for this test"+    return
>> 0+}++if [get_compiler_info] {+    return -1+}++# The DWARF assembler
>> requires the gcc compiler.+if {!$gcc_compiled} {+    unsupported "gcc
>> is required for this test"+    return 0+}++# This test suitable only
>> for process record-replay+if ![supports_process_record]
>> {+    return+}++standard_testfile .c .S++if { [prepare_for_testing
>> "failed to prepare" ${testfile} ${srcfile}] } {+    return -1+}++set
>> asm_file [standard_output_file $srcfile2]+Dwarf::assemble $asm_file
>> {+    global srcdir subdir srcfile+    declare_labels integer_label
>> L++    # Find start address and length of program+    lassign
>> [function_range main [list ${srcdir}/${subdir}/$srcfile]] \+	main_st
>> art main_len+    set main_end "$main_start + $main_len"++    cu {} {+
>> 	
>> compile_unit {+	    {language @DW_LANG_C}+	    {name
>> map-to-same-
>> line.c}+	    {stmt_list $L DW_FORM_sec_offset}+	    {low_pc 0
>> addr}+	} {+	    subprogram {+		{external 1
>> flag}+	
>> 	{name main}+		{low_pc $main_start addr}+		
>> {high_pc $main_len DW_FORM_data4}+	    }+	}+    }++    lines
>> {version 2 default_is_stmt 1} L {+	include_dir
>> "${srcdir}/${subdir}"+	file_name "$srcfile" 1++	# Generate
>> the
>> line table program with distinct source lines being+	# mapped to the
>> same line entry. Line 1, 5 and 8 contain 1 statement+	# each.  Line 2
>> contains 2 statements.  Line 3 contains 3 statements.+	program
>> {+	
>>      DW_LNE_set_address $main_start+	    line [gdb_get_line_number
>> "TAG: main prologue"]+	    DW_LNS_copy+	    DW_LNE_set_addres
>> s
>> line1+	    line [gdb_get_line_number "TAG: line 1" ]+	    D
>> W_LNS_copy
>> +	    DW_LNE_set_address line2+	    line [gdb_get_line_number
>> "TAG: line 2" ]+	    DW_LNS_copy+	    DW_LNE_set_address
>> line3+	    line [gdb_get_line_number "TAG: line 2" ]+	    D
>> W_LNS_copy
>> +	    DW_LNE_set_address line4+	    line [gdb_get_line_number
>> "TAG: line 3" ]+	    DW_LNS_copy+	    DW_LNE_set_address
>> line5+	    line [gdb_get_line_number "TAG: line 3" ]+	    D
>> W_LNS_copy
>> +	    DW_LNE_set_address line6+	    line [gdb_get_line_number
>> "TAG: line 3" ]+	    DW_LNS_copy+	    DW_LNE_set_address
>> line7+	    line [gdb_get_line_number "TAG: line 5" ]+	    D
>> W_LNS_copy
>> +	    DW_LNE_set_address line8+	    line [gdb_get_line_number
>> "TAG: line 8" ]+	    DW_LNS_copy+	    DW_LNE_set_address
>> main_return+	    line [gdb_get_line_number "TAG: main return"]+	
>>      DW_LNS_copy+	    DW_LNE_end_sequence+	}+    }+}++if {
>> [prepare_for_testing "failed to prepare" ${testfile} \+	[list
>> $srcfile
>> $asm_file] {nodebug} ] } {+    return -1+}++if ![runto_main]
>> {+    return -1+}++# Activate process
>> record/replay+gdb_test_no_output
>> "record" "turn on process record"++gdb_test "tbreak main_return"
>> "Temporary breakpoint .*" "breakpoint at return"+gdb_test "continue"
>> "Temporary breakpoint .*" "run to end of main"++# At this point, GDB
>> has already recorded the execution up until the return+#
>> statement.  Reverse-step and test if GDB transitions between lines in
>> the+# expected order.  It should reverse-step across lines 8, 5, 3, 2
>> and 1.+foreach line {8 5 3 2 1} {+    gdb_test "reverse-step" ".*TAG:
>> line $line.*" "reverse step to line $line"+}-- 2.31.1
>>
>>
> 

RE: [PING 2][PATCH, v5] Fix reverse stepping multiple contiguous PC ranges over the line table

[Carl Love]

Ping?

On Mon, 2022-05-23 at 11:12 +0100, Luis Machado wrote:
> Ping?
> 
> On 5/13/22 18:00, Carl Love wrote:
> > GDB maintainers:
> > 
> > We have addressed the comments from Bruno. Unfortunately, Bruno is
> > not
> > a maintainer and can't give approval for the patch.  We are hoping
> > a
> > maintainer can review the patch and provide us feedback.
> > 
> > Thank you for your time.
> > 
> >                         Carl Love
> > ---------------------------------------------------
> > 
> > On Fri, 2022-05-06 at 09:48 -0700, Carl Love via Gdb-patches wrote:
> > > Bruno, GDB maintainers:
> > > 
> > > The patch has been updated per the comments on the testcase from
> > > Bruno.
> > > 
> > > The patch was retested on Power 10 to ensure the test still works
> > > correctly.
> > > 
> > > Please let us know if there are any additional comments or if the
> > > patch
> > > is ready to commit.  We thank you for your help with this patch.
> > > 
> > >                     Carl Love
> > > --------------------------------------------------------
> > > Fix reverse stepping multiple contiguous PC ranges over the line
> > > table
> > > v5:- Updated test case comments on the purpose of the test.- Add
> > > test
> > > to check system supports record-replay.- Removed now unnecessary
> > > reord
> > > test when activating record.
> > > v4:- Updated testcase to make it a bit longer so it can exercise
> > > reverse-stepping  multiple times.- Cleaned up debugging prints.
> > > v3:- Updated testcase.  The format for writing the DWARF program
> > > body
> > > in the  testcase expect file changed.  See commit
> > > gdb/testsuite/dwarf:
> > > simplify line number program syntax  (commit
> > > d4c4a2298cad06ca71cfef725f5248f68205f0be)
> > > v2:- Check if both the line and symtab match for a particular
> > > line
> > > table entry.
> > > --
> > > When running GDB's testsuite on aarch64-linux/Ubuntu 20.04 (also
> > > spotted onthe ppc backend), I noticed some failures in
> > > gdb.reverse/solib-precsave.expand gdb.reverse/solib-reverse.exp.
> > > The failure happens around the following code:
> > > 38  b[1] = shr2(17);		/* middle part two */40  b[0] =
> > > 6;   b[1] = 9;	/* generic statement, end part two */42  shr1
> > > ("message
> > > 1\n");	/* shr1 one */
> > > Normal execution:
> > > - step from line 38 will land on line 40.- step from line 40 will
> > > land
> > > on line 42.
> > > Reverse execution:
> > > - step from line 42 will land on line 40.- step from line 40 will
> > > land
> > > on line 40.- step from line 40 will land on line 38.
> > > The problem here is that line 40 contains two contiguous but
> > > distinctPC
> > > ranges in the line table, like so:
> > > Line 40 - [0x7ec ~ 0x7f4]Line 40 - [0x7f4 ~ 0x7fc]
> > > The two distinct ranges are generated because GCC started
> > > outputting
> > > sourcecolumn information, which GDB doesn't take into account at
> > > the
> > > moment.
> > > When stepping forward from line 40, we skip both of these ranges
> > > and
> > > land online 42. When stepping backward from line 42, we stop at
> > > the
> > > start PC of thesecond (or first, going backwards) range of line
> > > 40.
> > > This happens because we have this check in
> > > infrun.c:process_event_stop_test:
> > >        /* When stepping backward, stop at beginning of line range
> > > 	
> > >   (unless it's the function entry point, in which case	 keep
> > > going
> > > back to the call point).  */      CORE_ADDR stop_pc = ecs-
> > > > event_thread->stop_pc ();      if (stop_pc == ecs-
> > > > >event_thread-
> > > > control.step_range_start	  && stop_pc != ecs-
> > > > >stop_func_start	
> > >    && execution_direction == EXEC_REVERSE)	end_stepping_ra
> > > nge
> > > (ecs);      else	keep_going (ecs);
> > > Since we've reached ecs->event_thread->control.step_range_start,
> > > we
> > > stopstepping backwards.
> > > The right thing to do is to look for adjacent PC ranges for the
> > > same
> > > line,until we notice a line change. Then we take that as the
> > > start PC
> > > of therange.
> > > Another solution I thought about is to merge the contiguous
> > > ranges
> > > whenwe are reading the line tables. Though I'm not sure if we
> > > really
> > > want to processthat data as opposed to keeping it as the compiler
> > > created, and then workingaround that.
> > > In any case, the following patch addresses this problem.
> > > Validated on aarch64-linux and x86_64/Ubuntu 20.04/18.04. Carl
> > > Love
> > > hasverified that it does fix a similar issue on ppc.
> > > Ubuntu 18.04 doesn't actually run into these failures because the
> > > compilerdoesn't generate distinct PC ranges for the same line.
> > > I see similar failures on x86_64 in the gdb.reverse
> > > tests(gdb.reverse/step-reverse.exp and gdb.reverse/step-
> > > reverse.exp).
> > > Those arealso fixed by this patch.
> > > The included testcase (based on a test Carl wrote) exercises this
> > > problem forArm, ppc and x86. It shows full passes with the patch
> > > applied.
> > > Co-authored-by: Carl Love <cel@us.ibm.com>---
> > > gdb/infrun.c                                  |  22 ++-
> > > gdb/symtab.c                                  |  49 ++++++
> > > gdb/symtab.h                                  |  16 ++
> > > gdb/testsuite/gdb.reverse/map-to-same-line.c  |  55 +++++++
> > > .../gdb.reverse/map-to-same-line.exp          | 141
> > > ++++++++++++++++++
> > > 5 files changed, 282 insertions(+), 1 deletion(-) create mode
> > > 100644
> > > gdb/testsuite/gdb.reverse/map-to-same-line.c create mode 100644
> > > gdb/testsuite/gdb.reverse/map-to-same-line.exp
> > > diff --git a/gdb/infrun.c b/gdb/infrun.cindex
> > > 6e5853ef42a..82c28961aeb
> > > 100644--- a/gdb/infrun.c+++ b/gdb/infrun.c@@ -6955,11 +6955,31 @@
> > > if
> > > (ecs->event_thread->control.proceed_to_finish 	 have software
> > > watchpoints).  */       ecs->event_thread->control.may_range_step 
> > > =
> > > 1;
> > > +      /* When we are stepping inside a particular line range, in
> > > reverse,+	 and we are sitting at the first address of that range,
> > > we need to+	 check if this address also shows up in another
> > > line
> > > range as the+	 end address.++	 If so, we need to check
> > > what line
> > > such
> > > a step range points to.+	 If it points to the same line as the
> > > current step range, that+	 means we need to keep going in order
> > > to reach the first address+	 of the line range.  We repeat
> > > this
> > > until we eventually get to the+	 first address of a particular
> > > line
> > > we're stepping through.  */+      CORE_ADDR range_start = ecs-
> > > > event_thread->control.step_range_start;+      if
> > > > (execution_direction
> > > == EXEC_REVERSE)+	{+	  gdb::optional<CORE_ADDR>
> > > real_range_start+	    = find_line_range_start (ecs->event_thread-
> > > > stop_pc ());++	  if (real_range_start.has_value ())+	
> > > >   range_start
> > > = *real_range_start;+	}+       /* When stepping backward,
> > > stop at
> > > beginning of line range 	 (unless it's the function entry point,
> > > in which case 	 keep going back to the call
> > > point).  */       CORE_ADDR stop_pc = ecs->event_thread->stop_pc
> > > ();-      if (stop_pc == ecs->event_thread-
> > > > control.step_range_start+      if (stop_pc == range_start 	
> > > >   &&
> > > stop_pc != ecs->stop_func_start 	  && execution_direction ==
> > > EXEC_REVERSE) 	end_stepping_range (ecs);diff --git
> > > a/gdb/symtab.c
> > > b/gdb/symtab.cindex 4b33d6c91af..de4cb5dd0eb 100644---
> > > a/gdb/symtab.c+++ b/gdb/symtab.c@@ -3433,6 +3433,55 @@
> > > find_pc_line
> > > (CORE_ADDR pc, int notcurrent)   return sal; } +/* Compare two
> > > symtab_and_line entries.  Return true if both have+   the same
> > > line
> > > number and the same symtab pointer.  That means we+   are dealing
> > > with
> > > two entries from the same line and from the same+   source
> > > file.++   Return false otherwise.  */++static
> > > bool+sal_line_symtab_matches_p (const symtab_and_line &sal1,+	
> > > 	
> > > 	   const symtab_and_line &sal2)+{+  return (sal1.line ==
> > > sal2.line && sal1.symtab == sal2.symtab);+}++/* See
> > > symtah.h.  */++gdb::optional<CORE_ADDR>+find_line_range_start
> > > (CORE_ADDR pc)+{+  struct symtab_and_line current_sal =
> > > find_pc_line
> > > (pc, 0);++  if (current_sal.line == 0)+    return {};++  struct
> > > symtab_and_line prev_sal = find_pc_line (current_sal.pc - 1,
> > > 0);++  /*
> > > If the previous entry is for a different line, that means we are
> > > already+     at the entry with the start PC for this
> > > line.  */+  if
> > > (!sal_line_symtab_matches_p (prev_sal, current_sal))+    return
> > > current_sal.pc;++  /* Otherwise, keep looking for entries for the
> > > same
> > > line but with+     smaller PC's.  */+  bool done =
> > > false;+  CORE_ADDR
> > > prev_pc;+  while (!done)+    {+      prev_pc =
> > > prev_sal.pc;++      prev_sal = find_pc_line (prev_pc - 1,
> > > 0);++      /*
> > > Did we notice a line change?  If so, we are done with the
> > > search.  */+      if (!sal_line_symtab_matches_p (prev_sal,
> > > current_sal))+	done = true;+    }++  return prev_pc;+}+ /* See
> > > symtab.h.  */  struct symtab *diff --git a/gdb/symtab.h
> > > b/gdb/symtab.hindex b1cf84f756f..226fe8803db 100644---
> > > a/gdb/symtab.h+++ b/gdb/symtab.h@@ -2285,6 +2285,22 @@ extern
> > > struct
> > > symtab_and_line find_pc_line (CORE_ADDR, int); extern struct
> > > symtab_and_line find_pc_sect_line (CORE_ADDR, 			
> > > 	
> > > 		 struct obj_section *, int); +/* Given PC, and assuming
> > > it is part of a range of addresses that is part of a+   line, go
> > > back
> > > through the linetable and find the starting PC of
> > > that+   line.++   For
> > > example, suppose we have 3 PC ranges for line X:++   Line X -
> > > [0x0 -
> > > 0x8]+   Line X - [0x8 - 0x10]+   Line X - [0x10 - 0x18]++   If we
> > > call
> > > the function with PC == 0x14, we want to return 0x0, as that
> > > is+   the
> > > starting PC of line X, and the ranges are contiguous.+*/++extern
> > > gdb::optional<CORE_ADDR> find_line_range_start (CORE_ADDR pc);+
> > > /*
> > > Wrapper around find_pc_line to just return the
> > > symtab.  */  extern
> > > struct symtab *find_pc_line_symtab (CORE_ADDR);diff --git
> > > a/gdb/testsuite/gdb.reverse/map-to-same-line.c
> > > b/gdb/testsuite/gdb.reverse/map-to-same-line.cnew file mode
> > > 100644index
> > > 00000000000..dd9f9f8a400--- /dev/null+++
> > > b/gdb/testsuite/gdb.reverse/map-to-same-line.c@@ -0,0 +1,55 @@+/*
> > > Copyright 2022 Free Software Foundation, Inc.++   This program is
> > > free
> > > software; you can redistribute it and/or modify+   it under the
> > > terms
> > > of the GNU General Public License as published by+   the Free
> > > Software
> > > Foundation; either version 3 of the License, or+   (at your
> > > option)
> > > any
> > > later version.++   This program is distributed in the hope that
> > > it
> > > will
> > > be useful,+   but WITHOUT ANY WARRANTY; without even the implied
> > > warranty of+   MERCHANTABILITY or FITNESS FOR A PARTICULAR
> > > PURPOSE.  See the+   GNU General Public License for more
> > > details.++   You should have received a copy of the GNU General
> > > Public
> > > License+   along with this program.  If not, see <
> > > http://www.gnu.org/licenses/ 
> > >   >.  */++/* The purpose of this test is to create a DWARF line
> > > table
> > > that contains two+   or more entries for the same line.  When
> > > stepping
> > > (forwards or backwards),+   GDB should step over the entire line
> > > and
> > > not just a particular entry in the+   line table.  */++int+main
> > > ()+{     /* TAG: main prologue */+  asm ("main_label: .globl
> > > main_label");+  int i = 1, j = 2, k;+  float f1 = 2.0, f2 = 4.1,
> > > f3;+  const char *str_1 = "foo", *str_2 = "bar", *str_3;++  asm
> > > ("line1: .globl line1");+  k = i; f3 = f1; str_3 = str_1;    /*
> > > TAG:
> > > line 1 */++  asm ("line2: .globl line2");+  k = j; f3 = f2; str_3
> > > =
> > > str_2;    /* TAG: line 2 */++  asm ("line3: .globl line3");+  k =
> > > i;
> > > f3
> > > = f1; str_3 = str_1;    /* TAG: line 3 */++  asm ("line4: .globl
> > > line4");+  k = j; f3 = f2; str_3 = str_2;    /* TAG: line 4
> > > */++  asm
> > > ("line5: .globl line5");+  k = i; f3 = f1; str_3 = str_1;    /*
> > > TAG:
> > > line 5 */++  asm ("line6: .globl line6");+  k = j; f3 = f2; str_3
> > > =
> > > str_2;    /* TAG: line 6 */++  asm ("line7: .globl line7");+  k =
> > > i;
> > > f3
> > > = f1; str_3 = str_1;    /* TAG: line 7 */++  asm ("line8: .globl
> > > line8");+  k = j; f3 = f2; str_3 = str_2;    /* TAG: line 8
> > > */++  asm
> > > ("main_return: .globl main_return");+  return 0; /* TAG: main
> > > return
> > > */+}diff --git a/gdb/testsuite/gdb.reverse/map-to-same-line.exp
> > > b/gdb/testsuite/gdb.reverse/map-to-same-line.expnew file mode
> > > 100644index 00000000000..c3fb859be55--- /dev/null+++
> > > b/gdb/testsuite/gdb.reverse/map-to-same-line.exp@@ -0,0 +1,141
> > > @@+#
> > > Copyright 2022 Free Software Foundation, Inc.++# This program is
> > > free
> > > software; you can redistribute it and/or modify+# it under the
> > > terms
> > > of
> > > the GNU General Public License as published by+# the Free
> > > Software
> > > Foundation; either version 3 of the License, or+# (at your
> > > option)
> > > any
> > > later version.+#+# This program is distributed in the hope that
> > > it
> > > will
> > > be useful,+# but WITHOUT ANY WARRANTY; without even the implied
> > > warranty of+# MERCHANTABILITY or FITNESS FOR A PARTICULAR
> > > PURPOSE.  See
> > > the+# GNU General Public License for more details.+#+# You should
> > > have
> > > received a copy of the GNU General Public License+# along with
> > > this
> > > program.  If not, see <
> > > http://www.gnu.org/licenses/ 
> > >   >.++# When stepping (forwards or backwards), GDB should step
> > > over
> > > the
> > > entire line+# and not just a particular entry in the line table.
> > > This
> > > test was added to+# verify the find_line_range_start function
> > > properly
> > > sets the step range for a+# line that consists of multiple
> > > statements,
> > > i.e. multiple entries in the line+# table.  This test creates a
> > > DWARF
> > > line table that contains two entries for+# the same line to do
> > > the
> > > needed testing.++load_lib dwarf.exp++# This test can only be run
> > > on
> > > targets which support DWARF-2 and use gas.+if {![dwarf2_support]}
> > > {+    unsupported "dwarf2 support required for this
> > > test"+    return
> > > 0+}++if [get_compiler_info] {+    return -1+}++# The DWARF
> > > assembler
> > > requires the gcc compiler.+if {!$gcc_compiled} {+    unsupported
> > > "gcc
> > > is required for this test"+    return 0+}++# This test suitable
> > > only
> > > for process record-replay+if ![supports_process_record]
> > > {+    return+}++standard_testfile .c .S++if {
> > > [prepare_for_testing
> > > "failed to prepare" ${testfile} ${srcfile}] } {+    return
> > > -1+}++set
> > > asm_file [standard_output_file $srcfile2]+Dwarf::assemble
> > > $asm_file
> > > {+    global srcdir subdir srcfile+    declare_labels
> > > integer_label
> > > L++    # Find start address and length of program+    lassign
> > > [function_range main [list ${srcdir}/${subdir}/$srcfile]] \+	
> > > main_st
> > > art main_len+    set main_end "$main_start + $main_len"++    cu
> > > {} {+
> > > 	
> > > compile_unit {+	    {language @DW_LANG_C}+	    {name
> > > map-to-same-
> > > line.c}+	    {stmt_list $L DW_FORM_sec_offset}+	    {low_
> > > pc 0
> > > addr}+	} {+	    subprogram {+		{external 1
> > > flag}+	
> > > 	{name main}+		{low_pc $main_start addr}+		
> > > {high_pc $main_len DW_FORM_data4}+	    }+	}+    }++
> > >     lines
> > > {version 2 default_is_stmt 1} L {+	include_dir
> > > "${srcdir}/${subdir}"+	file_name "$srcfile" 1++	#
> > > Generate
> > > the
> > > line table program with distinct source lines being+	#
> > > mapped to the
> > > same line entry. Line 1, 5 and 8 contain 1 statement+	#
> > > each.  Line 2
> > > contains 2 statements.  Line 3 contains 3 statements.+	program
> > > {+	
> > >      DW_LNE_set_address $main_start+	    line
> > > [gdb_get_line_number
> > > "TAG: main prologue"]+	    DW_LNS_copy+	    DW_LNE_set_ad
> > > dres
> > > s
> > > line1+	    line [gdb_get_line_number "TAG: line 1" ]+	 
> > >    D
> > > W_LNS_copy
> > > +	    DW_LNE_set_address line2+	    line [gdb_get_line_number
> > > "TAG: line 2" ]+	    DW_LNS_copy+	    DW_LNE_set_address
> > > line3+	    line [gdb_get_line_number "TAG: line 2" ]+	 
> > >    D
> > > W_LNS_copy
> > > +	    DW_LNE_set_address line4+	    line [gdb_get_line_number
> > > "TAG: line 3" ]+	    DW_LNS_copy+	    DW_LNE_set_address
> > > line5+	    line [gdb_get_line_number "TAG: line 3" ]+	 
> > >    D
> > > W_LNS_copy
> > > +	    DW_LNE_set_address line6+	    line [gdb_get_line_number
> > > "TAG: line 3" ]+	    DW_LNS_copy+	    DW_LNE_set_address
> > > line7+	    line [gdb_get_line_number "TAG: line 5" ]+	 
> > >    D
> > > W_LNS_copy
> > > +	    DW_LNE_set_address line8+	    line [gdb_get_line_number
> > > "TAG: line 8" ]+	    DW_LNS_copy+	    DW_LNE_set_address
> > > main_return+	    line [gdb_get_line_number "TAG: main
> > > return"]+	
> > >      DW_LNS_copy+	    DW_LNE_end_sequence+	}+    }+}++if {
> > > [prepare_for_testing "failed to prepare" ${testfile} \+	[list
> > > $srcfile
> > > $asm_file] {nodebug} ] } {+    return -1+}++if ![runto_main]
> > > {+    return -1+}++# Activate process
> > > record/replay+gdb_test_no_output
> > > "record" "turn on process record"++gdb_test "tbreak main_return"
> > > "Temporary breakpoint .*" "breakpoint at return"+gdb_test
> > > "continue"
> > > "Temporary breakpoint .*" "run to end of main"++# At this point,
> > > GDB
> > > has already recorded the execution up until the return+#
> > > statement.  Reverse-step and test if GDB transitions between
> > > lines in
> > > the+# expected order.  It should reverse-step across lines 8, 5,
> > > 3, 2
> > > and 1.+foreach line {8 5 3 2 1} {+    gdb_test "reverse-step"
> > > ".*TAG:
> > > line $line.*" "reverse step to line $line"+}-- 2.31.1
> > > 
> > > 

Re: [PATCH,v5] Fix reverse stepping multiple contiguous PC ranges over the line table

[will schmidt]

On Fri, 2022-05-06 at 09:48 -0700, Carl Love wrote:
> Bruno, GDB maintainers:
> 
> The patch has been updated per the comments on the testcase from
> Bruno.
> 
> The patch was retested on Power 10 to ensure the test still works
> correctly.
> 
> Please let us know if there are any additional comments or if the
> patch is ready to commit. We thank you for your help with this patch.
> 
> 

Assorted nits below, but unless my comments reveal something, LGTM. 




> 
> Carl Love
> --------------------------------------------------------
> Fix reverse stepping multiple contiguous PC ranges over the line
> table
> 
> v5:
> - Updated test case comments on the purpose of the test.
> - Add test to check system supports record-replay.
> - Removed now unnecessary reord test when activating record.


s/reord/record/  :-)

> v4:
> - Updated testcase to make it a bit longer so it can exercise
> reverse-stepping
>   multiple times.
> - Cleaned up debugging prints.
> 
> v3:
> - Updated testcase.  The format for writing the DWARF program body in
> the
>   testcase expect file changed.
>   See commit gdb/testsuite/dwarf: simplify line number program syntax
>   (commit d4c4a2298cad06ca71cfef725f5248f68205f0be)
> 
> v2:
> - Check if both the line and symtab match for a particular line table
> entry.
> 
> --
> 
> When running GDB's testsuite on aarch64-linux/Ubuntu 20.04 (also
> spotted on
> the ppc backend), I noticed some failures in gdb.reverse/solib-
> precsave.exp
> and gdb.reverse/solib-reverse.exp.
> 
> The failure happens around the following code:
> 
> 38  b[1] = shr2(17);		/* middle part two */
> 40  b[0] = 6;   b[1] = 9;	/* generic statement, end part two */
> 42  shr1 ("message 1\n");	/* shr1 one */
> 
> Normal execution:
> 
> - step from line 38 will land on line 40.
> - step from line 40 will land on line 42.
> 
> Reverse execution:
> 
> - step from line 42 will land on line 40.
> - step from line 40 will land on line 40.
> - step from line 40 will land on line 38.
> 
> The problem here is that line 40 contains two contiguous but distinct
> PC ranges in the line table, like so:
> 
> Line 40 - [0x7ec ~ 0x7f4]
> Line 40 - [0x7f4 ~ 0x7fc]


Curious, what incantation is used to collect the PC range info?


> The two distinct ranges are generated because GCC started outputting
> source
> column information, which GDB doesn't take into account at the
> moment.


Not critical for this, but is there a general sense for when GCC
started generating the source column info?


> When stepping forward from line 40, we skip both of these ranges and
> land on
> line 42. When stepping backward from line 42, we stop at the start PC
> of the
> second (or first, going backwards) range of line 40.
> 
> This happens because we have this check in
> infrun.c:process_event_stop_test:
> 
>       /* When stepping backward, stop at beginning of line range
> 	 (unless it's the function entry point, in which case
> 	 keep going back to the call point).  */
>       CORE_ADDR stop_pc = ecs->event_thread->stop_pc ();
>       if (stop_pc == ecs->event_thread->control.step_range_start
> 	  && stop_pc != ecs->stop_func_start
> 	  && execution_direction == EXEC_REVERSE)
> 	end_stepping_range (ecs);
>       else
> 	keep_going (ecs);
> 
> Since we've reached ecs->event_thread->control.step_range_start, we
> stop
> stepping backwards.
> 
> The right thing to do is to look for adjacent PC ranges for the same
> line,
> until we notice a line change. Then we take that as the start PC of
> the
> range.
> 
> Another solution I thought about is to merge the contiguous ranges
> when
> we are reading the line tables. Though I'm not sure if we really want
> to process
> that data as opposed to keeping it as the compiler created, and then
> working
> around that.
> 
> In any case, the following patch addresses this problem.
> 
> Validated on aarch64-linux and x86_64/Ubuntu 20.04/18.04. Carl Love
> has
> verified that it does fix a similar issue on ppc.
> 
> Ubuntu 18.04 doesn't actually run into these failures because the
> compiler
> doesn't generate distinct PC ranges for the same line.
> 
> I see similar failures on x86_64 in the gdb.reverse tests
> (gdb.reverse/step-reverse.exp and gdb.reverse/step-reverse.exp).
> Those are
> also fixed by this patch.
> 
> The included testcase (based on a test Carl wrote) exercises this
> problem for
> Arm, ppc and x86. It shows full passes with the patch applied.
> 
> Co-authored-by: Carl Love <
> cel@us.ibm.com
> >


> ---
>  gdb/infrun.c                                  |  22 ++-
>  gdb/symtab.c                                  |  49 ++++++
>  gdb/symtab.h                                  |  16 ++
>  gdb/testsuite/gdb.reverse/map-to-same-line.c  |  55 +++++++
>  .../gdb.reverse/map-to-same-line.exp          | 141
> ++++++++++++++++++
>  5 files changed, 282 insertions(+), 1 deletion(-)
>  create mode 100644 gdb/testsuite/gdb.reverse/map-to-same-line.c
>  create mode 100644 gdb/testsuite/gdb.reverse/map-to-same-line.exp
> 
> diff --git a/gdb/infrun.c b/gdb/infrun.c
> index 6e5853ef42a..82c28961aeb 100644
> --- a/gdb/infrun.c
> +++ b/gdb/infrun.c
> @@ -6955,11 +6955,31 @@ if (ecs->event_thread-
> >control.proceed_to_finish
>  	 have software watchpoints).  */
>        ecs->event_thread->control.may_range_step = 1;
>  
> +      /* When we are stepping inside a particular line range, in
> reverse,

Nit: Could be rephrased as "When we are reverse-stepping inside a
particular line range"


> +	 and we are sitting at the first address of that range, we need
> to
> +	 check if this address also shows up in another line range as
> the
> +	 end address.
> +
> +	 If so, we need to check what line such a step range points to.
> +	 If it points to the same line as the current step range, that
> +	 means we need to keep going in order to reach the first
> address
> +	 of the line range.  We repeat this until we eventually get to
> the
> +	 first address of a particular line we're stepping through.  */
> +      CORE_ADDR range_start = ecs->event_thread-
> >control.step_range_start;
> +      if (execution_direction == EXEC_REVERSE)
> +	{
> +	  gdb::optional<CORE_ADDR> real_range_start
> +	    = find_line_range_start (ecs->event_thread->stop_pc ());
> +
> +	  if (real_range_start.has_value ())
> +	    range_start = *real_range_start;
> +	}
> +
>        /* When stepping backward, stop at beginning of line range
>  	 (unless it's the function entry point, in which case
>  	 keep going back to the call point).  */
>        CORE_ADDR stop_pc = ecs->event_thread->stop_pc ();
> -      if (stop_pc == ecs->event_thread->control.step_range_start
> +      if (stop_pc == range_start
>  	  && stop_pc != ecs->stop_func_start
>  	  && execution_direction == EXEC_REVERSE)
>  	end_stepping_range (ecs);

ok



> diff --git a/gdb/symtab.c b/gdb/symtab.c
> index 4b33d6c91af..de4cb5dd0eb 100644
> --- a/gdb/symtab.c
> +++ b/gdb/symtab.c
> @@ -3433,6 +3433,55 @@ find_pc_line (CORE_ADDR pc, int notcurrent)
>    return sal;
>  }
>  
> +/* Compare two symtab_and_line entries.  Return true if both have
> +   the same line number and the same symtab pointer.  That means we
> +   are dealing with two entries from the same line and from the same
> +   source file.
> +
> +   Return false otherwise.  */
> +
> +static bool
> +sal_line_symtab_matches_p (const symtab_and_line &sal1,
> +			   const symtab_and_line &sal2)
> +{
> +  return (sal1.line == sal2.line && sal1.symtab == sal2.symtab);
> +}

ok


> +
> +/* See symtah.h.  */
> +
> +gdb::optional<CORE_ADDR>
> +find_line_range_start (CORE_ADDR pc)
> +{
> +  struct symtab_and_line current_sal = find_pc_line (pc, 0);
> +
> +  if (current_sal.line == 0)
> +    return {};

I assume our lines start at 1 so this will never false positive?

> +
> +  struct symtab_and_line prev_sal = find_pc_line (current_sal.pc -
> 1, 0);
> +
> +  /* If the previous entry is for a different line, that means we
> are already
> +     at the entry with the start PC for this line.  */
> +  if (!sal_line_symtab_matches_p (prev_sal, current_sal))
> +    return current_sal.pc;

ok


> +
> +  /* Otherwise, keep looking for entries for the same line but with
> +     smaller PC's.  */
> +  bool done = false;
> +  CORE_ADDR prev_pc;
> +  while (!done)
> +    {
> +      prev_pc = prev_sal.pc;
> +
> +      prev_sal = find_pc_line (prev_pc - 1, 0);
> +
> +      /* Did we notice a line change?  If so, we are done with the
> search.  */
> +      if (!sal_line_symtab_matches_p (prev_sal, current_sal))
> +	done = true;
> +    }
> +
> +  return prev_pc;

ok

> +}
> +
>  /* See symtab.h.  */
>  
>  struct symtab *
> diff --git a/gdb/symtab.h b/gdb/symtab.h
> index b1cf84f756f..226fe8803db 100644
> --- a/gdb/symtab.h
> +++ b/gdb/symtab.h
> @@ -2285,6 +2285,22 @@ extern struct symtab_and_line find_pc_line
> (CORE_ADDR, int);
>  extern struct symtab_and_line find_pc_sect_line (CORE_ADDR,
>  						 struct obj_section *,
> int);
>  
> +/* Given PC, and assuming it is part of a range of addresses that is
> part of a
> +   line, go back through the linetable and find the starting PC of
> that
> +   line.
> +
> +   For example, suppose we have 3 PC ranges for line X:
> +
> +   Line X - [0x0 - 0x8]
> +   Line X - [0x8 - 0x10]
> +   Line X - [0x10 - 0x18]
> +
> +   If we call the function with PC == 0x14, we want to return 0x0,
> as that is
> +   the starting PC of line X, and the ranges are contiguous.
> +*/

ok

> +
> +extern gdb::optional<CORE_ADDR> find_line_range_start (CORE_ADDR
> pc);
> +
>  /* Wrapper around find_pc_line to just return the symtab.  */
>  
>  extern struct symtab *find_pc_line_symtab (CORE_ADDR);
> diff --git a/gdb/testsuite/gdb.reverse/map-to-same-line.c
> b/gdb/testsuite/gdb.reverse/map-to-same-line.c
> new file mode 100644
> index 00000000000..dd9f9f8a400
> --- /dev/null
> +++ b/gdb/testsuite/gdb.reverse/map-to-same-line.c
> @@ -0,0 +1,55 @@
> +/* Copyright 2022 Free Software Foundation, Inc.
> +
> +   This program is free software; you can redistribute it and/or
> modify
> +   it under the terms of the GNU General Public License as published
> by
> +   the Free Software Foundation; either version 3 of the License, or
> +   (at your option) any later version.
> +
> +   This program is distributed in the hope that it will be useful,
> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> +   GNU General Public License for more details.
> +
> +   You should have received a copy of the GNU General Public License
> +   along with this program.  If not, see <
> http://www.gnu.org/licenses/
>  >.  */
> +

This URL will need to be fixed up.  Likely a side effect of the IBM
email infrastructure that does not exist in the patch itself.   :-)



> +/* The purpose of this test is to create a DWARF line table that
> contains two
> +   or more entries for the same line.  When stepping (forwards or
> backwards),
> +   GDB should step over the entire line and not just a particular
> entry in the
> +   line table.  */
> +
> +int
> +main ()
> +{     /* TAG: main prologue */
> +  asm ("main_label: .globl main_label");
> +  int i = 1, j = 2, k;
> +  float f1 = 2.0, f2 = 4.1, f3;
> +  const char *str_1 = "foo", *str_2 = "bar", *str_3;
> +
> +  asm ("line1: .globl line1");
> +  k = i; f3 = f1; str_3 = str_1;    /* TAG: line 1 */
> +
> +  asm ("line2: .globl line2");
> +  k = j; f3 = f2; str_3 = str_2;    /* TAG: line 2 */
> +
> +  asm ("line3: .globl line3");
> +  k = i; f3 = f1; str_3 = str_1;    /* TAG: line 3 */
> +
> +  asm ("line4: .globl line4");
> +  k = j; f3 = f2; str_3 = str_2;    /* TAG: line 4 */
> +
> +  asm ("line5: .globl line5");
> +  k = i; f3 = f1; str_3 = str_1;    /* TAG: line 5 */
> +
> +  asm ("line6: .globl line6");
> +  k = j; f3 = f2; str_3 = str_2;    /* TAG: line 6 */
> +
> +  asm ("line7: .globl line7");
> +  k = i; f3 = f1; str_3 = str_1;    /* TAG: line 7 */
> +
> +  asm ("line8: .globl line8");
> +  k = j; f3 = f2; str_3 = str_2;    /* TAG: line 8 */
> +
> +  asm ("main_return: .globl main_return");
> +  return 0; /* TAG: main return */
> +}
> diff --git a/gdb/testsuite/gdb.reverse/map-to-same-line.exp
> b/gdb/testsuite/gdb.reverse/map-to-same-line.exp
> new file mode 100644
> index 00000000000..c3fb859be55
> --- /dev/null
> +++ b/gdb/testsuite/gdb.reverse/map-to-same-line.exp
> @@ -0,0 +1,141 @@
> +# Copyright 2022 Free Software Foundation, Inc.
> +
> +# This program is free software; you can redistribute it and/or
> modify
> +# it under the terms of the GNU General Public License as published
> by
> +# the Free Software Foundation; either version 3 of the License, or
> +# (at your option) any later version.
> +#
> +# This program is distributed in the hope that it will be useful,
> +# but WITHOUT ANY WARRANTY; without even the implied warranty of
> +# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> +# GNU General Public License for more details.
> +#
> +# You should have received a copy of the GNU General Public License
> +# along with this program.  If not, see <
> http://www.gnu.org/licenses/
>  >.
> +


Same.



> +# When stepping (forwards or backwards), GDB should step over the
> entire line
> +# and not just a particular entry in the line table. This test was
> added to
> +# verify the find_line_range_start function properly sets the step
> range for a
> +# line that consists of multiple statements, i.e. multiple entries
> in the line
> +# table.  This test creates a DWARF line table that contains two
> entries for
> +# the same line to do the needed testing.
> +
> +load_lib dwarf.exp
> +
> +# This test can only be run on targets which support DWARF-2 and use
> gas.
> +if {![dwarf2_support]} {
> +    unsupported "dwarf2 support required for this test"
> +    return 0
> +}
> +
> +if [get_compiler_info] {
> +    return -1
> +}
> +
> +# The DWARF assembler requires the gcc compiler.
> +if {!$gcc_compiled} {
> +    unsupported "gcc is required for this test"
> +    return 0
> +}
> +
> +# This test suitable only for process record-replay
> +if ![supports_process_record] {
> +    return
> +}
> +
> +standard_testfile .c .S
> +
> +if { [prepare_for_testing "failed to prepare" ${testfile}
> ${srcfile}] } {
> +    return -1
> +}
> +
> +set asm_file [standard_output_file $srcfile2]
> +Dwarf::assemble $asm_file {
> +    global srcdir subdir srcfile
> +    declare_labels integer_label L
> +
> +    # Find start address and length of program
> +    lassign [function_range main [list
> ${srcdir}/${subdir}/$srcfile]] \
> +	main_start main_len
> +    set main_end "$main_start + $main_len"
> +
> +    cu {} {
> +	compile_unit {
> +	    {language @DW_LANG_C}
> +	    {name map-to-same-line.c}
> +	    {stmt_list $L DW_FORM_sec_offset}
> +	    {low_pc 0 addr}
> +	} {
> +	    subprogram {
> +		{external 1 flag}
> +		{name main}
> +		{low_pc $main_start addr}
> +		{high_pc $main_len DW_FORM_data4}
> +	    }
> +	}
> +    }
> +
> +    lines {version 2 default_is_stmt 1} L {
> +	include_dir "${srcdir}/${subdir}"
> +	file_name "$srcfile" 1
> +
> +	# Generate the line table program with distinct source lines
> being
> +	# mapped to the same line entry. Line 1, 5 and 8 contain 1
> statement
> +	# each.  Line 2 contains 2 statements.  Line 3 contains 3
> statements.
> +	program {
> +	    DW_LNE_set_address $main_start
> +	    line [gdb_get_line_number "TAG: main prologue"]
> +	    DW_LNS_copy
> +	    DW_LNE_set_address line1
> +	    line [gdb_get_line_number "TAG: line 1" ]
> +	    DW_LNS_copy
> +	    DW_LNE_set_address line2
> +	    line [gdb_get_line_number "TAG: line 2" ]
> +	    DW_LNS_copy
> +	    DW_LNE_set_address line3
> +	    line [gdb_get_line_number "TAG: line 2" ]
> +	    DW_LNS_copy
> +	    DW_LNE_set_address line4
> +	    line [gdb_get_line_number "TAG: line 3" ]
> +	    DW_LNS_copy
> +	    DW_LNE_set_address line5
> +	    line [gdb_get_line_number "TAG: line 3" ]
> +	    DW_LNS_copy
> +	    DW_LNE_set_address line6
> +	    line [gdb_get_line_number "TAG: line 3" ]
> +	    DW_LNS_copy
> +	    DW_LNE_set_address line7
> +	    line [gdb_get_line_number "TAG: line 5" ]
> +	    DW_LNS_copy
> +	    DW_LNE_set_address line8
> +	    line [gdb_get_line_number "TAG: line 8" ]
> +	    DW_LNS_copy
> +	    DW_LNE_set_address main_return
> +	    line [gdb_get_line_number "TAG: main return"]
> +	    DW_LNS_copy
> +	    DW_LNE_end_sequence
> +	}
> +    }
> +}
> +
> +if { [prepare_for_testing "failed to prepare" ${testfile} \
> +	[list $srcfile $asm_file] {nodebug} ] } {
> +    return -1
> +}
> +
> +if ![runto_main] {
> +    return -1
> +}
> +
> +# Activate process record/replay
> +gdb_test_no_output "record" "turn on process record"
> +
> +gdb_test "tbreak main_return" "Temporary breakpoint .*" "breakpoint
> at return"
> +gdb_test "continue" "Temporary breakpoint .*" "run to end of main"
> +
> +# At this point, GDB has already recorded the execution up until the
> return
> +# statement.  Reverse-step and test if GDB transitions between lines
> in the
> +# expected order.  It should reverse-step across lines 8, 5, 3, 2
> and 1.
> +foreach line {8 5 3 2 1} {
> +    gdb_test "reverse-step" ".*TAG: line $line.*" "reverse step to
> line $line"
> +}
> -- 
> 2.31.1
> 

ok.

Re: [PATCH,v5] Fix reverse stepping multiple contiguous PC ranges over the line table

[Luis Machado]

Hi Will,

On 6/7/22 18:11, will schmidt wrote:
> On Fri, 2022-05-06 at 09:48 -0700, Carl Love wrote:
>> Bruno, GDB maintainers:
>>
>> The patch has been updated per the comments on the testcase from
>> Bruno.
>>
>> The patch was retested on Power 10 to ensure the test still works
>> correctly.
>>
>> Please let us know if there are any additional comments or if the
>> patch is ready to commit. We thank you for your help with this patch.
>>
>>
> 
> Assorted nits below, but unless my comments reveal something, LGTM.
> 
> 
> 
> 
>>
>> Carl Love
>> --------------------------------------------------------
>> Fix reverse stepping multiple contiguous PC ranges over the line
>> table
>>
>> v5:
>> - Updated test case comments on the purpose of the test.
>> - Add test to check system supports record-replay.
>> - Removed now unnecessary reord test when activating record.
> 
> 
> s/reord/record/  :-)
> 
>> v4:
>> - Updated testcase to make it a bit longer so it can exercise
>> reverse-stepping
>>    multiple times.
>> - Cleaned up debugging prints.
>>
>> v3:
>> - Updated testcase.  The format for writing the DWARF program body in
>> the
>>    testcase expect file changed.
>>    See commit gdb/testsuite/dwarf: simplify line number program syntax
>>    (commit d4c4a2298cad06ca71cfef725f5248f68205f0be)
>>
>> v2:
>> - Check if both the line and symtab match for a particular line table
>> entry.
>>
>> --
>>
>> When running GDB's testsuite on aarch64-linux/Ubuntu 20.04 (also
>> spotted on
>> the ppc backend), I noticed some failures in gdb.reverse/solib-
>> precsave.exp
>> and gdb.reverse/solib-reverse.exp.
>>
>> The failure happens around the following code:
>>
>> 38  b[1] = shr2(17);		/* middle part two */
>> 40  b[0] = 6;   b[1] = 9;	/* generic statement, end part two */
>> 42  shr1 ("message 1\n");	/* shr1 one */
>>
>> Normal execution:
>>
>> - step from line 38 will land on line 40.
>> - step from line 40 will land on line 42.
>>
>> Reverse execution:
>>
>> - step from line 42 will land on line 40.
>> - step from line 40 will land on line 40.
>> - step from line 40 will land on line 38.
>>
>> The problem here is that line 40 contains two contiguous but distinct
>> PC ranges in the line table, like so:
>>
>> Line 40 - [0x7ec ~ 0x7f4]
>> Line 40 - [0x7f4 ~ 0x7fc]
> 
> 
> Curious, what incantation is used to collect the PC range info?
> 
> 

You can use the maintenance info line-table.

>> The two distinct ranges are generated because GCC started outputting
>> source
>> column information, which GDB doesn't take into account at the
>> moment.
> 
> 
> Not critical for this, but is there a general sense for when GCC
> started generating the source column info?
> 
> 

A couple commits, from these patches here:

https://gcc.gnu.org/pipermail/gcc-patches/2017-February/469643.html
https://gcc.gnu.org/pipermail/gcc-patches/2017-February/469644.html

>> When stepping forward from line 40, we skip both of these ranges and
>> land on
>> line 42. When stepping backward from line 42, we stop at the start PC
>> of the
>> second (or first, going backwards) range of line 40.
>>
>> This happens because we have this check in
>> infrun.c:process_event_stop_test:
>>
>>        /* When stepping backward, stop at beginning of line range
>> 	 (unless it's the function entry point, in which case
>> 	 keep going back to the call point).  */
>>        CORE_ADDR stop_pc = ecs->event_thread->stop_pc ();
>>        if (stop_pc == ecs->event_thread->control.step_range_start
>> 	  && stop_pc != ecs->stop_func_start
>> 	  && execution_direction == EXEC_REVERSE)
>> 	end_stepping_range (ecs);
>>        else
>> 	keep_going (ecs);
>>
>> Since we've reached ecs->event_thread->control.step_range_start, we
>> stop
>> stepping backwards.
>>
>> The right thing to do is to look for adjacent PC ranges for the same
>> line,
>> until we notice a line change. Then we take that as the start PC of
>> the
>> range.
>>
>> Another solution I thought about is to merge the contiguous ranges
>> when
>> we are reading the line tables. Though I'm not sure if we really want
>> to process
>> that data as opposed to keeping it as the compiler created, and then
>> working
>> around that.
>>
>> In any case, the following patch addresses this problem.
>>
>> Validated on aarch64-linux and x86_64/Ubuntu 20.04/18.04. Carl Love
>> has
>> verified that it does fix a similar issue on ppc.
>>
>> Ubuntu 18.04 doesn't actually run into these failures because the
>> compiler
>> doesn't generate distinct PC ranges for the same line.
>>
>> I see similar failures on x86_64 in the gdb.reverse tests
>> (gdb.reverse/step-reverse.exp and gdb.reverse/step-reverse.exp).
>> Those are
>> also fixed by this patch.
>>
>> The included testcase (based on a test Carl wrote) exercises this
>> problem for
>> Arm, ppc and x86. It shows full passes with the patch applied.
>>
>> Co-authored-by: Carl Love <
>> cel@us.ibm.com
>>>
> 
> 
>> ---
>>   gdb/infrun.c                                  |  22 ++-
>>   gdb/symtab.c                                  |  49 ++++++
>>   gdb/symtab.h                                  |  16 ++
>>   gdb/testsuite/gdb.reverse/map-to-same-line.c  |  55 +++++++
>>   .../gdb.reverse/map-to-same-line.exp          | 141
>> ++++++++++++++++++
>>   5 files changed, 282 insertions(+), 1 deletion(-)
>>   create mode 100644 gdb/testsuite/gdb.reverse/map-to-same-line.c
>>   create mode 100644 gdb/testsuite/gdb.reverse/map-to-same-line.exp
>>
>> diff --git a/gdb/infrun.c b/gdb/infrun.c
>> index 6e5853ef42a..82c28961aeb 100644
>> --- a/gdb/infrun.c
>> +++ b/gdb/infrun.c
>> @@ -6955,11 +6955,31 @@ if (ecs->event_thread-
>>> control.proceed_to_finish
>>   	 have software watchpoints).  */
>>         ecs->event_thread->control.may_range_step = 1;
>>   
>> +      /* When we are stepping inside a particular line range, in
>> reverse,
> 
> Nit: Could be rephrased as "When we are reverse-stepping inside a
> particular line range"
> 
> 

I agree that looks better in isolation. But if you check the code, you'll notice
it seems to favor "in reverse". I just went with the current "standard".

I don't have a strong opinion for either of those.

>> +	 and we are sitting at the first address of that range, we need
>> to
>> +	 check if this address also shows up in another line range as
>> the
>> +	 end address.
>> +
>> +	 If so, we need to check what line such a step range points to.
>> +	 If it points to the same line as the current step range, that
>> +	 means we need to keep going in order to reach the first
>> address
>> +	 of the line range.  We repeat this until we eventually get to
>> the
>> +	 first address of a particular line we're stepping through.  */
>> +      CORE_ADDR range_start = ecs->event_thread-
>>> control.step_range_start;
>> +      if (execution_direction == EXEC_REVERSE)
>> +	{
>> +	  gdb::optional<CORE_ADDR> real_range_start
>> +	    = find_line_range_start (ecs->event_thread->stop_pc ());
>> +
>> +	  if (real_range_start.has_value ())
>> +	    range_start = *real_range_start;
>> +	}
>> +
>>         /* When stepping backward, stop at beginning of line range
>>   	 (unless it's the function entry point, in which case
>>   	 keep going back to the call point).  */
>>         CORE_ADDR stop_pc = ecs->event_thread->stop_pc ();
>> -      if (stop_pc == ecs->event_thread->control.step_range_start
>> +      if (stop_pc == range_start
>>   	  && stop_pc != ecs->stop_func_start
>>   	  && execution_direction == EXEC_REVERSE)
>>   	end_stepping_range (ecs);
> 
> ok
> 
> 
> 
>> diff --git a/gdb/symtab.c b/gdb/symtab.c
>> index 4b33d6c91af..de4cb5dd0eb 100644
>> --- a/gdb/symtab.c
>> +++ b/gdb/symtab.c
>> @@ -3433,6 +3433,55 @@ find_pc_line (CORE_ADDR pc, int notcurrent)
>>     return sal;
>>   }
>>   
>> +/* Compare two symtab_and_line entries.  Return true if both have
>> +   the same line number and the same symtab pointer.  That means we
>> +   are dealing with two entries from the same line and from the same
>> +   source file.
>> +
>> +   Return false otherwise.  */
>> +
>> +static bool
>> +sal_line_symtab_matches_p (const symtab_and_line &sal1,
>> +			   const symtab_and_line &sal2)
>> +{
>> +  return (sal1.line == sal2.line && sal1.symtab == sal2.symtab);
>> +}
> 
> ok
> 
> 
>> +
>> +/* See symtah.h.  */
>> +
>> +gdb::optional<CORE_ADDR>
>> +find_line_range_start (CORE_ADDR pc)
>> +{
>> +  struct symtab_and_line current_sal = find_pc_line (pc, 0);
>> +
>> +  if (current_sal.line == 0)
>> +    return {};
> 
> I assume our lines start at 1 so this will never false positive?
> 

Right, as documented in gdb/symtab.h:

   /* Line number.  Line numbers start at 1 and proceed through symtab->nlines.
      0 is never a valid line number; it is used to indicate that line number
      information is not available.  */
   int line = 0;


>> +
>> +  struct symtab_and_line prev_sal = find_pc_line (current_sal.pc -
>> 1, 0);
>> +
>> +  /* If the previous entry is for a different line, that means we
>> are already
>> +     at the entry with the start PC for this line.  */
>> +  if (!sal_line_symtab_matches_p (prev_sal, current_sal))
>> +    return current_sal.pc;
> 
> ok
> 
> 
>> +
>> +  /* Otherwise, keep looking for entries for the same line but with
>> +     smaller PC's.  */
>> +  bool done = false;
>> +  CORE_ADDR prev_pc;
>> +  while (!done)
>> +    {
>> +      prev_pc = prev_sal.pc;
>> +
>> +      prev_sal = find_pc_line (prev_pc - 1, 0);
>> +
>> +      /* Did we notice a line change?  If so, we are done with the
>> search.  */
>> +      if (!sal_line_symtab_matches_p (prev_sal, current_sal))
>> +	done = true;
>> +    }
>> +
>> +  return prev_pc;
> 
> ok
> 
>> +}
>> +
>>   /* See symtab.h.  */
>>   
>>   struct symtab *
>> diff --git a/gdb/symtab.h b/gdb/symtab.h
>> index b1cf84f756f..226fe8803db 100644
>> --- a/gdb/symtab.h
>> +++ b/gdb/symtab.h
>> @@ -2285,6 +2285,22 @@ extern struct symtab_and_line find_pc_line
>> (CORE_ADDR, int);
>>   extern struct symtab_and_line find_pc_sect_line (CORE_ADDR,
>>   						 struct obj_section *,
>> int);
>>   
>> +/* Given PC, and assuming it is part of a range of addresses that is
>> part of a
>> +   line, go back through the linetable and find the starting PC of
>> that
>> +   line.
>> +
>> +   For example, suppose we have 3 PC ranges for line X:
>> +
>> +   Line X - [0x0 - 0x8]
>> +   Line X - [0x8 - 0x10]
>> +   Line X - [0x10 - 0x18]
>> +
>> +   If we call the function with PC == 0x14, we want to return 0x0,
>> as that is
>> +   the starting PC of line X, and the ranges are contiguous.
>> +*/
> 
> ok
> 
>> +
>> +extern gdb::optional<CORE_ADDR> find_line_range_start (CORE_ADDR
>> pc);
>> +
>>   /* Wrapper around find_pc_line to just return the symtab.  */
>>   
>>   extern struct symtab *find_pc_line_symtab (CORE_ADDR);
>> diff --git a/gdb/testsuite/gdb.reverse/map-to-same-line.c
>> b/gdb/testsuite/gdb.reverse/map-to-same-line.c
>> new file mode 100644
>> index 00000000000..dd9f9f8a400
>> --- /dev/null
>> +++ b/gdb/testsuite/gdb.reverse/map-to-same-line.c
>> @@ -0,0 +1,55 @@
>> +/* Copyright 2022 Free Software Foundation, Inc.
>> +
>> +   This program is free software; you can redistribute it and/or
>> modify
>> +   it under the terms of the GNU General Public License as published
>> by
>> +   the Free Software Foundation; either version 3 of the License, or
>> +   (at your option) any later version.
>> +
>> +   This program is distributed in the hope that it will be useful,
>> +   but WITHOUT ANY WARRANTY; without even the implied warranty of
>> +   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
>> +   GNU General Public License for more details.
>> +
>> +   You should have received a copy of the GNU General Public License
>> +   along with this program.  If not, see <
>> http://www.gnu.org/licenses/
>>   >.  */
>> +
> 
> This URL will need to be fixed up.  Likely a side effect of the IBM
> email infrastructure that does not exist in the patch itself.   :-)
> 
> 

I have a local copy where this isn't a problem. Pesky e-mail servers.

> 
>> +/* The purpose of this test is to create a DWARF line table that
>> contains two
>> +   or more entries for the same line.  When stepping (forwards or
>> backwards),
>> +   GDB should step over the entire line and not just a particular
>> entry in the
>> +   line table.  */
>> +
>> +int
>> +main ()
>> +{     /* TAG: main prologue */
>> +  asm ("main_label: .globl main_label");
>> +  int i = 1, j = 2, k;
>> +  float f1 = 2.0, f2 = 4.1, f3;
>> +  const char *str_1 = "foo", *str_2 = "bar", *str_3;
>> +
>> +  asm ("line1: .globl line1");
>> +  k = i; f3 = f1; str_3 = str_1;    /* TAG: line 1 */
>> +
>> +  asm ("line2: .globl line2");
>> +  k = j; f3 = f2; str_3 = str_2;    /* TAG: line 2 */
>> +
>> +  asm ("line3: .globl line3");
>> +  k = i; f3 = f1; str_3 = str_1;    /* TAG: line 3 */
>> +
>> +  asm ("line4: .globl line4");
>> +  k = j; f3 = f2; str_3 = str_2;    /* TAG: line 4 */
>> +
>> +  asm ("line5: .globl line5");
>> +  k = i; f3 = f1; str_3 = str_1;    /* TAG: line 5 */
>> +
>> +  asm ("line6: .globl line6");
>> +  k = j; f3 = f2; str_3 = str_2;    /* TAG: line 6 */
>> +
>> +  asm ("line7: .globl line7");
>> +  k = i; f3 = f1; str_3 = str_1;    /* TAG: line 7 */
>> +
>> +  asm ("line8: .globl line8");
>> +  k = j; f3 = f2; str_3 = str_2;    /* TAG: line 8 */
>> +
>> +  asm ("main_return: .globl main_return");
>> +  return 0; /* TAG: main return */
>> +}
>> diff --git a/gdb/testsuite/gdb.reverse/map-to-same-line.exp
>> b/gdb/testsuite/gdb.reverse/map-to-same-line.exp
>> new file mode 100644
>> index 00000000000..c3fb859be55
>> --- /dev/null
>> +++ b/gdb/testsuite/gdb.reverse/map-to-same-line.exp
>> @@ -0,0 +1,141 @@
>> +# Copyright 2022 Free Software Foundation, Inc.
>> +
>> +# This program is free software; you can redistribute it and/or
>> modify
>> +# it under the terms of the GNU General Public License as published
>> by
>> +# the Free Software Foundation; either version 3 of the License, or
>> +# (at your option) any later version.
>> +#
>> +# This program is distributed in the hope that it will be useful,
>> +# but WITHOUT ANY WARRANTY; without even the implied warranty of
>> +# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
>> +# GNU General Public License for more details.
>> +#
>> +# You should have received a copy of the GNU General Public License
>> +# along with this program.  If not, see <
>> http://www.gnu.org/licenses/
>>   >.
>> +
> 
> 
> Same.
> 
> 
> 
>> +# When stepping (forwards or backwards), GDB should step over the
>> entire line
>> +# and not just a particular entry in the line table. This test was
>> added to
>> +# verify the find_line_range_start function properly sets the step
>> range for a
>> +# line that consists of multiple statements, i.e. multiple entries
>> in the line
>> +# table.  This test creates a DWARF line table that contains two
>> entries for
>> +# the same line to do the needed testing.
>> +
>> +load_lib dwarf.exp
>> +
>> +# This test can only be run on targets which support DWARF-2 and use
>> gas.
>> +if {![dwarf2_support]} {
>> +    unsupported "dwarf2 support required for this test"
>> +    return 0
>> +}
>> +
>> +if [get_compiler_info] {
>> +    return -1
>> +}
>> +
>> +# The DWARF assembler requires the gcc compiler.
>> +if {!$gcc_compiled} {
>> +    unsupported "gcc is required for this test"
>> +    return 0
>> +}
>> +
>> +# This test suitable only for process record-replay
>> +if ![supports_process_record] {
>> +    return
>> +}
>> +
>> +standard_testfile .c .S
>> +
>> +if { [prepare_for_testing "failed to prepare" ${testfile}
>> ${srcfile}] } {
>> +    return -1
>> +}
>> +
>> +set asm_file [standard_output_file $srcfile2]
>> +Dwarf::assemble $asm_file {
>> +    global srcdir subdir srcfile
>> +    declare_labels integer_label L
>> +
>> +    # Find start address and length of program
>> +    lassign [function_range main [list
>> ${srcdir}/${subdir}/$srcfile]] \
>> +	main_start main_len
>> +    set main_end "$main_start + $main_len"
>> +
>> +    cu {} {
>> +	compile_unit {
>> +	    {language @DW_LANG_C}
>> +	    {name map-to-same-line.c}
>> +	    {stmt_list $L DW_FORM_sec_offset}
>> +	    {low_pc 0 addr}
>> +	} {
>> +	    subprogram {
>> +		{external 1 flag}
>> +		{name main}
>> +		{low_pc $main_start addr}
>> +		{high_pc $main_len DW_FORM_data4}
>> +	    }
>> +	}
>> +    }
>> +
>> +    lines {version 2 default_is_stmt 1} L {
>> +	include_dir "${srcdir}/${subdir}"
>> +	file_name "$srcfile" 1
>> +
>> +	# Generate the line table program with distinct source lines
>> being
>> +	# mapped to the same line entry. Line 1, 5 and 8 contain 1
>> statement
>> +	# each.  Line 2 contains 2 statements.  Line 3 contains 3
>> statements.
>> +	program {
>> +	    DW_LNE_set_address $main_start
>> +	    line [gdb_get_line_number "TAG: main prologue"]
>> +	    DW_LNS_copy
>> +	    DW_LNE_set_address line1
>> +	    line [gdb_get_line_number "TAG: line 1" ]
>> +	    DW_LNS_copy
>> +	    DW_LNE_set_address line2
>> +	    line [gdb_get_line_number "TAG: line 2" ]
>> +	    DW_LNS_copy
>> +	    DW_LNE_set_address line3
>> +	    line [gdb_get_line_number "TAG: line 2" ]
>> +	    DW_LNS_copy
>> +	    DW_LNE_set_address line4
>> +	    line [gdb_get_line_number "TAG: line 3" ]
>> +	    DW_LNS_copy
>> +	    DW_LNE_set_address line5
>> +	    line [gdb_get_line_number "TAG: line 3" ]
>> +	    DW_LNS_copy
>> +	    DW_LNE_set_address line6
>> +	    line [gdb_get_line_number "TAG: line 3" ]
>> +	    DW_LNS_copy
>> +	    DW_LNE_set_address line7
>> +	    line [gdb_get_line_number "TAG: line 5" ]
>> +	    DW_LNS_copy
>> +	    DW_LNE_set_address line8
>> +	    line [gdb_get_line_number "TAG: line 8" ]
>> +	    DW_LNS_copy
>> +	    DW_LNE_set_address main_return
>> +	    line [gdb_get_line_number "TAG: main return"]
>> +	    DW_LNS_copy
>> +	    DW_LNE_end_sequence
>> +	}
>> +    }
>> +}
>> +
>> +if { [prepare_for_testing "failed to prepare" ${testfile} \
>> +	[list $srcfile $asm_file] {nodebug} ] } {
>> +    return -1
>> +}
>> +
>> +if ![runto_main] {
>> +    return -1
>> +}
>> +
>> +# Activate process record/replay
>> +gdb_test_no_output "record" "turn on process record"
>> +
>> +gdb_test "tbreak main_return" "Temporary breakpoint .*" "breakpoint
>> at return"
>> +gdb_test "continue" "Temporary breakpoint .*" "run to end of main"
>> +
>> +# At this point, GDB has already recorded the execution up until the
>> return
>> +# statement.  Reverse-step and test if GDB transitions between lines
>> in the
>> +# expected order.  It should reverse-step across lines 8, 5, 3, 2
>> and 1.
>> +foreach line {8 5 3 2 1} {
>> +    gdb_test "reverse-step" ".*TAG: line $line.*" "reverse step to
>> line $line"
>> +}
>> -- 
>> 2.31.1
>>
> 
> ok.
> 
> 

I'll send a v6 with the fixups.

Thanks!