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From: "zhendong.su at inf dot ethz.ch" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug ipa/99834] New: missed optimization for dead code elimination
 at -O3 (vs. -O2)
Date: Tue, 30 Mar 2021 17:20:07 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99834

            Bug ID: 99834
           Summary: missed optimization for dead code elimination at -O3
                    (vs. -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: ---

[590] % 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/11.0.1/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 11.0.1 20210330 (experimental) [master revision
8aac913adfc:ff4ebc2f17c:65374af219f9c5c594951a07e766fe70c1136a1f] (GCC)=20
[591] %=20
[591] % gcctk -O2 -S -o O2.s small.c
[592] % gcctk -O3 -S -o O3.s small.c
[593] %=20
[593] % wc O2.s O3.s
  43   90  704 O2.s
  61  126  950 O3.s
 104  216 1654 total
[594] %=20
[594] % grep foo O2.s
[595] % grep foo O3.s
        call    foo
        call    foo
[596] %=20
[596] % cat small.c
extern void foo(void);
static int a, b, c;
static int d() {
  for (a =3D 0; a < 1; a++)
    b =3D 1;
  for (; b; b--)
    for (; c; c--)
      ;
  return 0;
}
static void e() {
  if (d())
    foo();
}
int main() {
  e();
  e();
  return 0;
}=