[Bug tree-optimization/100314] New: missed optimization for dead code elimination at -O3 (vs. -O1)

zhendong.su at inf dot ethz.ch gcc-bugzilla@gcc.gnu.org
Wed Apr 28 10:17:26 GMT 2021


https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100314

            Bug ID: 100314
           Summary: missed optimization for dead code elimination at -O3
                    (vs. -O1)
           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: ---

[592] % 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/12.0.0/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 12.0.0 20210428 (experimental) [master revision
852dd866e2f:9b04e5b2651:b81e2d5e76a6bcc71f45b122e8b5538ddb7ebf4c] (GCC) 
[593] % 
[593] % gcctk -O1 -S -o O1.s small.c
[594] % gcctk -O3 -S -o O3.s small.c
[595] % 
[595] % wc O1.s O3.s
  21   45  392 O1.s
  42   86  686 O3.s
  63  131 1078 total
[596] % 
[596] % grep foo O1.s
[597] % grep foo O3.s
        jmp     foo
[598] % 
[598] % cat small.c
extern void foo(void);
static int a, *c, g, **j;
int b;
static void e() {
  int k, *l[5] = {&k, &k, &k, &k, &k};
  while (g) {
    j = &l[0];
    b++;
  }
}
static void d(int m) {
  int **h[30] = {&c}, ***i[1] = {&h[3]};
  if (m)
    foo();
  e();
}
int main() {
  d(a);
  return 0;
}


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