[Bug tree-optimization/99793] New: missed optimization for dead code elimination at -Os, -O2 and -O3 (vs. -O1)

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99793

            Bug ID: 99793
           Summary: missed optimization for dead code elimination at -Os,
                    -O2 and -O3 (vs. -O1)
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: zhendong.su at inf dot ethz.ch
  Target Milestone: ---

[610] % 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 20210326 (experimental) [master revision
9d45e848d02:ca344bbd24f:6081d8994ed1a0aef6b7f5fb34f091faa3580416] (GCC) 
[611] % 
[611] % gcctk -O1 -S -o O1.s small.c
[612] % gcctk -O3 -S -o O3.s small.c
[613] % 
[613] % wc O1.s O3.s
 17  38 365 O1.s
 37  78 633 O3.s
 54 116 998 total
[614] % 
[614] % grep foo O1.s
[615] % grep foo O3.s
        call    foo
[616] % 
[616] % cat small.c
extern void foo(void);
static int a, *b = &a, c, *d = &c;
int main() {
  int **e = &d;
  if (!((unsigned)((*e = d) == 0) - (*b = 1)))
    foo();
  return 0;
}