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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/100095] New: missed optimization for dead
 code elimination at -O3 (vs. -O2)
Date: Thu, 15 Apr 2021 11:07:13 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D100095

            Bug ID: 100095
           Summary: missed optimization for dead code elimination at -O3
                    (vs. -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: ---

[654] % 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 20210415 (experimental) [master revision
4dd9e1c541e:7315804b0a0:b5f644a98b3f3543d3a8d2dfea7785c22879013f] (GCC)=20
[655] %=20
[655] % gcctk -O2 -S -c -o O2.s small.c
[656] % gcctk -O3 -S -c -o O3.s small.c
[657] %=20
[657] % wc O2.s O3.s
  75  164  982 O2.s
 121  269 1617 O3.s
 196  433 2599 total
[658] %=20
[658] % grep foo O2.s
[659] % grep foo O3.s
        jmp     foo
[660] %=20
[660] % cat small.c
extern void foo(void);
int a, b, c, d, g, h;
static int *e =3D &d;
volatile int f;
void i() {
  for (d =3D 5; d >=3D 0; d--)
    for (c =3D 0; c < 4; c++)
      while (1) {
        h =3D b ? a % b : 0;
        if (g)
          f;
        if (*e)
          break;
      }
  foo();
}=