[Bug tree-optimization/100051] 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=100051

            Bug ID: 100051
           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: ---

[719] % 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 20210412 (experimental) [master revision
46c47420a5f:66634f1d5eb:8f17d44ad986e0ae7e81c81179463000ee9d7f43] (GCC) 
[720] % 
[720] % gcctk -O1 -S -o O1.s small.c
[721] % gcctk -O3 -S -o O3.s small.c
[722] % 
[722] % wc O1.s O3.s
  56  121  784 O1.s
  68  148  996 O3.s
 124  269 1780 total
[723] % 
[723] % grep foo O1.s
[724] % grep foo O3.s
        call    foo
[725] % 
[725] % cat small.c
extern void foo(void);
int a, c, *f, **d = &f;
char b;
static void e() {
  if ((2 ^ b) == 0)
    foo();
}
int main() {
  if (a) {
    b = 0;
    int *g = &c;
    *g = 0;
    f = *d;
    *d = f;
    e();
  }
  return 0;
}