[Bug lto/104589] New: Emitted binary code changes when -g is enabled at -O0 -flto and optimize attribute

[tlwang at uwaterloo dot ca]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104589

            Bug ID: 104589
           Summary: Emitted binary code changes when -g is enabled at -O0
                    -flto and optimize attribute
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: lto
          Assignee: unassigned at gcc dot gnu.org
          Reporter: tlwang at uwaterloo dot ca
                CC: marxin at gcc dot gnu.org
  Target Milestone: ---

This occurs in preprocessed.c, as shown below:

$ gcc-trunk -v
Using built-in specs.
COLLECT_GCC=gcc-trunk
COLLECT_LTO_WRAPPER=/scratch/software/gcc-trunk/libexec/gcc/x86_64-pc-linux-gnu/12.0.1/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /tmp/tmp.m8P0CFupUR-gcc-builder/gcc/configure
--enable-languages=c,c++,lto --enable-checking-yes --enable-multiarch
--prefix=/scratch/software/gcc-trunk --disable-bootstrap
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 12.0.1 20220217 (experimental) [master -g837eb1262] (GCC)
$
$ cat preprocessed.c
short a, b;
int c, d;
__attribute__((always_inline)) __attribute__((optimize(1))) short e() {
  for (; 0;)
    return 0;
  d = b;
  c = ({
    int g = b;
    g &&a ? g : a;
  });
  short f;
  return 8;
# 8 ""
}
int main() {
  strcmp("");
  e();
}
$
$ gcc-trunk -flto -w -O0 preprocessed.c; objdump --disassemble --section=.text
a.out > no_debug.txt
$ gcc-trunk -flto -w -O0 -g preprocessed.c; objdump --disassemble
--section=.text a.out > debug.txt
$ diff no_debug.txt debug.txt
94c94
<   401126:     75 08                   jne    401130 <main+0x2a>
---
>   401126:	75 09                	jne    401131 <main+0x2b>
97,101c97,102
<   401130:     89 05 fa 2e 00 00       mov    %eax,0x2efa(%rip)        #
404030 <c>
<   401136:     b8 00 00 00 00          mov    $0x0,%eax
<   40113b:     5d                      pop    %rbp
<   40113c:     c3                      retq
<   40113d:     0f 1f 00                nopl   (%rax)
---
>   401130:	90                   	nop
>   401131:	89 05 f9 2e 00 00    	mov    %eax,0x2ef9(%rip)        # 404030 <c>
>   401137:	b8 00 00 00 00       	mov    $0x0,%eax
>   40113c:	5d                   	pop    %rbp
>   40113d:	c3                   	retq
>   40113e:	66 90                	xchg   %ax,%ax