[PATCH 3/3] Fix reverse stepping multiple contiguous PC ranges

[Luis Machado <luis.machado@linaro.org>]

When running GDB's testsuite on aarch64-linux/Ubuntu 20.04, 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 1 will land on line 2.
- step from line 2 will land on line 3.

Reverse execution:

- step from line 3 will land on line 2.
- step from line 2 will land on line 2.
- step from line 2 will land on line 1.

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

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

When stepping forward from line 2, we skip both of these ranges and land on
line 42. When stepping backward from line 3, 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->suspend.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.

I'm not particularly happy with how we go back in the ranges (using "pc - 1").
Feedback would be welcome.

Validated on aarch64-linux/Ubuntu 20.04/18.04.

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

gdb/ChangeLog:

YYYY-MM-DD  Luis Machado  <luis.machado@linaro.org>

	* infrun.c (process_event_stop_test): Handle backward stepping
	across multiple ranges for the same line.
	* symtab.c (find_line_range_start): New function.
	* symtab.h (find_line_range_start): New prototype.
---
 gdb/infrun.c | 22 +++++++++++++++++++++-
 gdb/symtab.c | 35 +++++++++++++++++++++++++++++++++++
 gdb/symtab.h | 16 ++++++++++++++++
 3 files changed, 72 insertions(+), 1 deletion(-)

diff --git a/gdb/infrun.c b/gdb/infrun.c
index e070eff33d..5bb268529d 100644
--- a/gdb/infrun.c
+++ b/gdb/infrun.c
@@ -6534,11 +6534,31 @@ 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->suspend.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->suspend.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 7ffb52a943..e8f5301951 100644
--- a/gdb/symtab.c
+++ b/gdb/symtab.c
@@ -3382,6 +3382,41 @@ find_pc_line (CORE_ADDR pc, int notcurrent)
   return sal;
 }
 
+/* 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 (prev_sal.line != current_sal.line)
+    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 (prev_sal.line != current_sal.line)
+	done = true;
+    }
+
+  return prev_pc;
+}
+
 /* See symtab.h.  */
 
 struct symtab *
diff --git a/gdb/symtab.h b/gdb/symtab.h
index f060e0ebc1..659cb46292 100644
--- a/gdb/symtab.h
+++ b/gdb/symtab.h
@@ -1916,6 +1916,22 @@ 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);
-- 
2.25.1