[Bug lto/107030] New: Emitted binary code changes when -g is enabled at -O2 -flto and optimize attribute

[Bug lto/107030] New: Emitted binary code changes when -g is enabled at -O2 -flto and optimize attribute

[tlwang at uwaterloo dot ca]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107030

            Bug ID: 107030
           Summary: Emitted binary code changes when -g is enabled at -O2
                    -flto and optimize attribute
           Product: gcc
           Version: 13.0
            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 when compiling attributes_program0_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/13.0.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /tmp/tmp.O2a2UsOhYi-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 13.0.0 20220924 (experimental) [master -g4afaeaab5] (GCC)
$ cat attributes_program0_preprocessed.c
int a;
union {
  char b;
  long c;
} d = {0};
void e(f) { g(d.b); }
__attribute__((optimize(0))) static short h() {
  e(d.c);
  int i = a;
}
void g(j) {}
int main() {}
$ gcc-trunk -w -flto -O2 attributes_program0_preprocessed.c -o a.out; objdump
--disassemble --section=.text a.out > no_dbg.txt
$ gcc-trunk -w -flto -O2 attributes_program0_preprocessed.c -g -o a.out;
objdump --disassemble --section=.text a.out > dbg.txt
$ diff no_dbg.txt dbg.txt
96c96
<   401124:     8b 05 de 0e 00 00       mov    0xede(%rip),%eax        # 402008
<a>
---
>   401124:	8b 05 da 0e 00 00    	mov    0xeda(%rip),%eax        # 402004 <a>

[Bug lto/107030] Emitted binary code changes when -g is enabled at -O2 -flto and optimize attribute

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107030

--- Comment #1 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
I doubt this is a bug.
The difference is in the got.
If anything the got is slightly different.

[Bug lto/107030] Emitted binary code changes when -g is enabled at -O2 -flto and optimize attribute

[schwab@linux-m68k.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107030

--- Comment #2 from Andreas Schwab <schwab@linux-m68k.org> ---
But -g is supposed to never change the generated code.

[Bug lto/107030] Emitted binary code changes when -g is enabled at -O2 -flto and optimize attribute

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107030

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
   Last reconfirmed|                            |2022-09-26
           Keywords|                            |compare-debug-failure
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW

--- Comment #3 from Richard Biener <rguenth at gcc dot gnu.org> ---
The difference is that with -g 'd' is not emitted:

@@ -46,10 +57,217 @@
        .size   a, 4
 a:
        .zero   4
-       .align 8
-       .type   d, @object
-       .size   d, 8
-d:
-       .zero   8
+       .text
+.Letext0:
+       .section        .debug_info,"",@progbits

with just -fwhole-program but without -flto 'd' is emitted in both cases.