[Bug ipa/99776] New: missed optimization for dead code elimination at -O3 (vs. -O1)

[zhendong.su at inf dot ethz.ch via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99776

            Bug ID: 99776
           Summary: missed optimization for dead code elimination at -O3
                    (vs. -O1)
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: ipa
          Assignee: unassigned at gcc dot gnu.org
          Reporter: zhendong.su at inf dot ethz.ch
                CC: marxin at gcc dot gnu.org
  Target Milestone: ---

[687] % gcctk -v
Using built-in specs.
COLLECT_GCC=gcctk
COLLECT_LTO_WRAPPER=/local/suz-local/software/local/gcc-trunk/libexec/gcc/x86_64-pc-linux-gnu/11.0.1/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../gcc-trunk/configure --disable-bootstrap
--prefix=/local/suz-local/software/local/gcc-trunk --enable-languages=c,c++
--disable-werror --enable-multilib --with-system-zlib
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 11.0.1 20210325 (experimental) [master revision
08103e4d6ad:4e4f8ee0bf5:a29124d28253cdf603ba1977db2f09c9f233fea5] (GCC) 
[688] % 
[688] % gcctk -O1 -S -o small_O1.s small.c
[689] % gcctk -O3 -S -o small_O3.s small.c
[690] % 
[690] % wc small_O1.s small_O3.s 
  23   52  455 small_O1.s
  37   82  682 small_O3.s
  60  134 1137 total
[691] % 
[691] % grep foo small_O1.s 
[692] % grep foo small_O3.s 
        call    foo
[693] % 
[693] % cat small.c
extern void foo(void);

static int a[2], b, *c[2];

int main() {
  for (b = 0; b < 2; b++)
    c[b] = &a[1];
  if (!c[0])
    foo();
  return 0;
}