[Bug tree-optimization/100112] New: missed optimization for dead code elimination at -O3, -Os (vs. -O1, -O2)

[zhendong.su at inf dot ethz.ch]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100112

            Bug ID: 100112
           Summary: missed optimization for dead code elimination at -O3,
                    -Os (vs. -O1, -O2)
           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: ---

[545] % 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 20210416 (experimental) [master revision
89c863488bc:10ed13839be:76c7e7d6b003a17d183d0571bf9b34c691819d25] (GCC) 
[546] % 
[546] % gcctk -O1 -S -o O1.s small.c
[547] % gcctk -O3 -S -o O3.s small.c
[548] % 
[548] % wc O1.s O3.s
 17  38 365 O1.s
 37  78 633 O3.s
 54 116 998 total
[549] % 
[549] % grep foo O1.s
[550] % grep foo O3.s
        call    foo
[551] % 
[551] % cat small.c
extern void foo(void);
static int e, *a = &e, b, *c = &b;
static int d(int f, int i) {
  if (f ^ i)
    foo();
}
int main() {
  int **g = &c;
  (*a)++;
  *g = c;
  d(1, c != 0);
  return 0;
}