[Bug ipa/99986] 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=99986

            Bug ID: 99986
           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: ---

[599] % 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 20210409 (experimental) [master revision
96292c3e343:4e14cad25b9:019a922063f26784d5a070d9198a1f937b8a8343] (GCC) 
[600] % 
[600] % gcctk -O1 -S -o O1.s small.c
[601] % gcctk -O3 -S -o O3.s small.c
[602] % 
[602] % wc O1.s O3.s
  38   86  669 O1.s
  48  107  845 O3.s
  86  193 1514 total
[603] % 
[603] % grep foo O1.s
[604] % grep foo O3.s
        call    foo
[605] % 
[605] % cat small.c
extern void foo(void);
static int a, *b[8] = {&a}, **c[8];
static void d() {
  int e = 0;
  for (; e < 8; e++)
    c[e] = &b[1];
  if (!c[0])
    foo();
}
int main() {
  if (a)
    d();
  return 0;
}