[Bug ipa/99788] New: missed optimization for dead code elimination at -O3 (vs. -O1)

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99788

            Bug ID: 99788
           Summary: missed optimization for dead code elimination at -O3
                    (vs. -O1)
           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: ---

[606] % 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) 
[607] % 
[607] % gcctk -O1 -S -o O1.s small.c
[608] % gcctk -O3 -S -o O3.s small.c
[609] % 
[609] % wc O1.s O3.s
  79  162  986 O1.s
 109  229 1433 O3.s
 188  391 2419 total
[610] % 
[610] % grep foo O1.s
[611] % grep foo O3.s
        call    foo
[612] % 
[612] % cat small.c
extern void foo(void);

char a;
int b, *c, g, h = 1, i = 1, j, *k = &i;

static void d();
static int *e() {
  for (a = 1; a; a = a+2)
    ;
  foo();
  h = (g % h) % i;
  *k = -j;
  return 0;
}
static void f() {
  if (b)
    d(e);
}
void d() {
  for (;;)
    c = e();
}
int main() {
  f();
  return 0;
}