[gdb/breakpoint] Fix stepping past non-stmt line-table entries

Consider the test-case small.c:
...
$ cat -n small.c
     1  __attribute__ ((noinline, noclone))
     2  int foo (char *c)
     3  {
     4    asm volatile ("" : : "r" (c) : "memory");
     5    return 1;
     6  }
     7
     8  int main ()
     9  {
    10    char tpl1[20] = "/tmp/test.XXX";
    11    char tpl2[20] = "/tmp/test.XXX";
    12    int fd1 = foo (tpl1);
    13    int fd2 = foo (tpl2);
    14    if (fd1 == -1) {
    15      return 1;
    16    }
    17
    18    return 0;
    19  }
...

Compiled with gcc-8 and optimization:
...
$ gcc-8 -O2 -g small.c
...

We step through the calls to foo, but fail to visit line 13:
...
12	  int fd1 = foo (tpl1);
(gdb) step
foo (c=c@entry=0x7fffffffdea0 "/tmp/test.XXX") at small.c:5
5	  return 1;
(gdb) step
foo (c=c@entry=0x7fffffffdec0 "/tmp/test.XXX") at small.c:5
5	  return 1;
(gdb) step
main () at small.c:14
14	  if (fd1 == -1) {
(gdb)
...

This is caused by the following.  The calls to foo are implemented by these
insns:
....
  4003df:       0f 29 04 24             movaps %xmm0,(%rsp)
  4003e3:       0f 29 44 24 20          movaps %xmm0,0x20(%rsp)
  4003e8:       e8 03 01 00 00          callq  4004f0 <foo>
  4003ed:       48 8d 7c 24 20          lea    0x20(%rsp),%rdi
  4003f2:       89 c2                   mov    %eax,%edx
  4003f4:       e8 f7 00 00 00          callq  4004f0 <foo>
  4003f9:       31 c0                   xor    %eax,%eax
...
with corresponding line table entries:
...
INDEX  LINE   ADDRESS            IS-STMT
8      12     0x00000000004003df Y
9      10     0x00000000004003df
10     11     0x00000000004003e3
11     12     0x00000000004003e8
12     13     0x00000000004003ed
13     12     0x00000000004003f2
14     13     0x00000000004003f4 Y
15     13     0x00000000004003f4
16     14     0x00000000004003f9 Y
17     14     0x00000000004003f9
...

Once we step out of the call to foo at 4003e8, we land at 4003ed, and gdb
enters process_event_stop_test to figure out what to do.

That entry has is-stmt=n, so it's not the start of a line, so we don't stop
there.  However, we do update ecs->event_thread->current_line to line 13,
because the frame has changed (because we stepped out of the function).

Next we land at 4003f2.  Again the entry has is-stmt=n, so it's not the start
of a line, so we don't stop there.  However, because the frame hasn't changed,
we don't update update ecs->event_thread->current_line, so it stays 13.

Next we land at 4003f4.  Now is-stmt=y, so it's the start of a line, and we'd
like to stop here.

But we don't stop because this test fails:
...
  if ((ecs->event_thread->suspend.stop_pc == stop_pc_sal.pc)
      && (ecs->event_thread->current_line != stop_pc_sal.line
          || ecs->event_thread->current_symtab != stop_pc_sal.symtab))
    {
...
because ecs->event_thread->current_line == 13 and stop_pc_sal.line == 13.

Fix this by resetting ecs->event_thread->current_line to 0 if is-stmt=n and
the frame has changed, such that we have:
...
12        int fd1 = foo (tpl1);
(gdb) step
foo (c=c@entry=0x7fffffffdbc0 "/tmp/test.XXX") at small.c:5
5         return 1;
(gdb) step
main () at small.c:13
13        int fd2 = foo (tpl2);
(gdb)
...

Tested on x86_64-linux, with gcc-7 and gcc-8.

gdb/ChangeLog:

2021-01-29  Tom de Vries  <tdevries@suse.de>

	PR breakpoints/26063
	* infrun.c (process_event_stop_test): Reset
	ecs->event_thread->current_line to 0 if is-stmt=n and frame has
	changed.

gdb/testsuite/ChangeLog:

2021-01-29  Tom de Vries  <tdevries@suse.de>

	PR breakpoints/26063
	* gdb.dwarf2/dw2-step-out-of-function-no-stmt.c: New test.
	* gdb.dwarf2/dw2-step-out-of-function-no-stmt.exp: New file.

1 files changed, 25 insertions, 8 deletions

diff --git a/gdb/infrun.c b/gdb/infrun.c
index 3f14397..e070eff 100644
--- a/gdb/infrun.c
+++ b/gdb/infrun.c
@@ -7000,27 +7000,44 @@ process_event_stop_test (struct execution_control_state *ecs)
       && (ecs->event_thread->current_line != stop_pc_sal.line
  	  || ecs->event_thread->current_symtab != stop_pc_sal.symtab))
     {
+      /* We are at a different line.  */
+
       if (stop_pc_sal.is_stmt)
 	{
-	  /* We are at the start of a different line.  So stop.  Note that
-	     we don't stop if we step into the middle of a different line.
-	     That is said to make things like for (;;) statements work
-	     better.  */
+	  /* We are at the start of a statement.
+
+	     So stop.  Note that we don't stop if we step into the middle of a
+	     statement.  That is said to make things like for (;;) statements
+	     work better.  */
 	  infrun_debug_printf ("stepped to a different line");
 	  end_stepping_range (ecs);
 	  return;
 	}
       else if (frame_id_eq (get_frame_id (get_current_frame ()),
-			    ecs->event_thread->control.step_frame_id))
+                           ecs->event_thread->control.step_frame_id))
 	{
-	  /* We are at the start of a different line, however, this line is
-	     not marked as a statement, and we have not changed frame.  We
-	     ignore this line table entry, and continue stepping forward,
+	  /* We are not at the start of a statement, and we have not changed
+	     frame.
+
+	     We ignore this line table entry, and continue stepping forward,
 	     looking for a better place to stop.  */
 	  refresh_step_info = false;
 	  infrun_debug_printf ("stepped to a different line, but "
 			       "it's not the start of a statement");
 	}
+      else
+	{
+	  /* We are not the start of a statement, and we have changed frame.
+
+	     We ignore this line table entry, and continue stepping forward,
+	     looking for a better place to stop.  Keep refresh_step_info at
+	     true to note that the frame has changed, but ignore the line
+	     number to make sure we don't ignore a subsequent entry with the
+	     same line number.  */
+	  stop_pc_sal.line = 0;
+	  infrun_debug_printf ("stepped to a different frame, but "
+			       "it's not the start of a statement");
+	}
     }
 
   /* We aren't done stepping.