[Bug debug/101011] New: Inconsistent debug info for "while (1);"

[vries at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101011

            Bug ID: 101011
           Summary: Inconsistent debug info for "while (1);"
           Product: gcc
           Version: 11.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: debug
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vries at gcc dot gnu.org
  Target Milestone: ---

I.

Consider test-cases:
...
$ cat -n test.c
     1  int
     2  main (void)
     3  {
     4    while (1);
     5
     6    return 0;
     7  }
$ cat -n test2.c
     1  int
     2  main (void)
     3  {
     4    int spin = 1;
     5    while (spin);
     6
     7    return 0;
     8  }
...

Compiled with gcc-10:
...
$ gcc-10 test.c -g -o 1
$ gcc-10 test2.c -g -o 2
...
we get the same behaviour when setting the breakpoint on the loop line 4/5.

We stop twice here:
...
$ gdb -q -batch 1 -ex "break 4" -ex run -ex "continue"
Breakpoint 1 at 0x40049a: file test.c, line 4.

Breakpoint 1, main () at test.c:4
4         while (1);

Breakpoint 1, main () at test.c:4
4         while (1);
$
...
and here:
...
$ gdb -q -batch 2 -ex "break 5" -ex run -ex "continue"
Breakpoint 1 at 0x4004a1: file test2.c, line 5.

Breakpoint 1, main () at test2.c:5
5         while (spin);

Breakpoint 1, main () at test2.c:5
5         while (spin);
$
...

Now, with gcc-11:
...
$ gcc-11 test.c -g -o 1
$ gcc-11 test2.c -g -o 2
...
we have the same behaviour for test.c:
...
$ gdb -q -batch 1 -ex "break 4" -ex run -ex "continue"
Breakpoint 1 at 0x40049a: file test.c, line 4.

Breakpoint 1, main () at test.c:4
4         while (1);

Breakpoint 1, main () at test.c:4
4         while (1);
$
...
but for test2.c:
...
$ gdb -q -batch 2 -ex "break 5" -ex run -ex "continue"
Breakpoint 1 at 0x4004a1: file test2.c, line 5.

Breakpoint 1, main () at test2.c:5
5         while (spin);
<HANGS>
...


II.

The difference can be seen in the .s file for test2.c.

With gcc-10, we have:
...
        .loc 1 4 7
        movl    $1, -4(%rbp)
.L2:
        .loc 1 5 9 discriminator 1
        cmpl    $0, -4(%rbp)
        jne     .L2
        .loc 1 7 10
        movl    $0, %eax
...
but with gcc-11 there's an extra nop, with a corresponding .loc for line 5:
...
        .loc 1 4 7
        movl    $1, -4(%rbp)
        .loc 1 5 9
        nop
.L2:
        .loc 1 5 10 discriminator 1
        cmpl    $0, -4(%rbp)
        jne     .L2
        .loc 1 7 10
        movl    $0, %eax
...


III.

OTOH, the behaviour has changed, so it's tempting to say that this is a
regression.

However, my feeling is that this is actually a progression: we stop at the
start of the line.

Either way:
- if this is a regression is needs to be fixed.
- if this is a progression, then can we have the same for test.c?


IV.

FWIW, I tracked down where the nop is inserted.  It happens during
outof_cfglayout, in fixup_reorder_chain, here:
...
              if (!INSN_P (BB_END (nb)))
                BB_END (nb) = emit_insn_after_noloc (gen_nop (), BB_END (nb),
                                                         nb);
              INSN_LOCATION (BB_END (nb)) = e->goto_locus;
...


V.

Funnily enough, with clang the opposite seems to be the case
(https://bugs.llvm.org/show_bug.cgi?id=49614 ): the example with test.c hangs,
and the example with test2.c finishes.