[Bug ipa/100034] New: missed optimization for dead code elimination at -O3 (vs. -O1, -Os, -O2)

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100034

            Bug ID: 100034
           Summary: missed optimization for dead code elimination at -O3
                    (vs. -O1, -Os, -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: ---

[641] % 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 20210411 (experimental) [master revision
1d54b138417:e83b9cf4549:936d500dfc17f58f2507ecd0f7f26e4f197052ee] (GCC) 
[642] % 
[642] % gcctk -O1 -S -o O1.s small.c
[643] % gcctk -O3 -S -o O3.s small.c
[644] % 
[644] % wc O1.s O3.s
  23   45  420 O1.s
  41   78  697 O3.s
  64  123 1117 total
[645] % 
[645] % grep foo O1.s
[646] % grep foo O3.s
        call    foo
[647] % 
[647] % cat small.c
extern void foo(void);
static int a, b, f, g;
static int d() {
  while (g)
    f = 0;
  while (1)
    foo();
}
static void c() { d (); }
void e() {
  while (b) {
    if (!a)
      continue;
    c();
  }
}
int main() {
  e();
  return 0;
}