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From: "zhendong.su at inf dot ethz.ch" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/100221] New: missed optimization for dead
 code elimination at -O3 (vs. -O1, -Os, -O2)
Date: Thu, 22 Apr 2021 20:07:53 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D100221

            Bug ID: 100221
           Summary: missed optimization for dead code elimination at -O3
                    (vs. -O1, -Os, -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: ---

[551] % gcctk -v
Using built-in specs.
COLLECT_GCC=3Dgcctk
COLLECT_LTO_WRAPPER=3D/local/suz-local/software/local/gcc-trunk/libexec/gcc=
/x86_64-pc-linux-gnu/12.0.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: ../gcc-trunk/configure --disable-bootstrap
--prefix=3D/local/suz-local/software/local/gcc-trunk --enable-languages=3Dc=
,c++
--disable-werror --enable-multilib --with-system-zlib
Thread model: posix
Supported LTO compression algorithms: zlib
gcc version 12.0.0 20210422 (experimental) [master revision
3cf04d1afa8:0e51007a40c:d42088e453042f4f8ba9190a7e29efd937ea2181] (GCC)=20
[552] %=20
[552] % gcctk -O1 -S -o O1.s small.c
[553] % gcctk -O3 -S -o O3.s small.c
[554] %=20
[554] % wc O1.s O3.s
  49  100  693 O1.s
  73  151 1072 O3.s
 122  251 1765 total
[555] %=20
[555] % grep foo O1.s=20
[556] % grep foo O3.s
        call    foo
[557] %=20
[557] % cat small.c
extern void foo(void);
int a, b;
static int c;
static void f() {
  while (a)
    for (; b; b--)
      ;
}
void i() {
  if (c)
    foo();
  int *g =3D &c;
  {
    int **h[1] =3D {&g};
    f();
  }
}
int main() {
  i();
  return 0;
}=