[Bug ipa/99987] New: missed optimization for dead code elimination at -O3 (vs. -O2)

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99987

            Bug ID: 99987
           Summary: missed optimization for dead code elimination at -O3
                    (vs. -O2)
           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: ---

[748] % 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) 
[749] % 
[749] % 
[749] % gcctk -O2 -S -o O2.s small.c
[750] % gcctk -O3 -S -o O3.s small.c
[751] % 
[751] % wc O2.s O3.s
  68  150  961 O2.s
  89  195 1278 O3.s
 157  345 2239 total
[752] % 
[752] % grep foo O2.s
[753] % grep foo O3.s
        call    foo
[754] % 
[754] % cat small.c
extern void foo(void);
static int a[8] = {0,0,0,0,0,0,0,0};
int b, c, j;
static int *e(int k) {
  if (k && b)
    j = 0;
  while (j)
    if (!k)
      foo();
  return 0;
}
static void d() {
  long g;
  unsigned h = 8;
  if (b)
    for (g = 2; 1; g--) {
      int i = 0;
      h--;
      c = (a[g] == (&i != e(h)));
    }
}
void f() { d(); }