[PING] [PATCH, v6] Fix reverse stepping multiple contiguous PC ranges over the line table

[Carl Love <cel@us.ibm.com>]

GDB developers:

Ping?  Just wondering if anyone has had a chance to look at this patch.

            Carl 
-----------------------------

On Thu, 2022-06-09 at 14:04 +0100, Luis Machado wrote:
> Sending as a clean new thread.
> 
> v6:
> - Fix misc typos and augment the commit message.
> - Updated source file comments.
> 
> v5:
> - Updated test case comments on the purpose of the test.
> - Add test to check system supports record-replay.
> - Removed now unnecessary record 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 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.
> 
> These GCC changes came into effect with commits
> 497b7c47042d542ae48d10badf0c3d0088f6f798 and
> and 0029b929c9719a9794492915206314308fbdf03a.
> 
> 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  |  54 +++++++
>  .../gdb.reverse/map-to-same-line.exp          | 141
> ++++++++++++++++++
>  5 files changed, 281 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 8564986f66d..9625ad7aa8a 100644
> --- a/gdb/symtab.c
> +++ b/gdb/symtab.c
> @@ -3390,6 +3390,55 @@ 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 ac902a4cbbe..500faf7d93b 100644
> --- a/gdb/symtab.h
> +++ b/gdb/symtab.h
> @@ -2286,6 +2286,22 @@ 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..89a1898bdc2
> --- /dev/null
> +++ b/gdb/testsuite/gdb.reverse/map-to-same-line.c
> @@ -0,0 +1,54 @@
> +/* 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 source file provides dummy code that is only meant to force
> the
> +   compiler to generate instructions for the various labels.  The
> TAG markers
> +   are used by the testcase to generate DWARF information and line
> tables.  */
> +
> +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..fd958ba114b
> --- /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
> multiple 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"
> +}