[Bug tree-optimization/126760] New: Wrong code at -O2 on x86_64-pc-linux-gnu

[xintong.zhou1 at uwaterloo dot ca via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126760

            Bug ID: 126760
           Summary: Wrong code at -O2 on x86_64-pc-linux-gnu
           Product: gcc
           Version: 17.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: xintong.zhou1 at uwaterloo dot ca
  Target Milestone: ---

Compiler Explorer: https://godbolt.org/z/hnc73djc5

$ gcc-trunk -v                                                                  
Using built-in specs.
COLLECT_GCC=/data/x27zhou/compiler-nightly/install/gcc/bin/gcc
COLLECT_LTO_WRAPPER=/data/x27zhou/compiler-nightly/install/gcc/libexec/gcc/x86_64-pc-linux-gnu/17.0.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /data/x27zhou/compiler-nightly/src/gcc/configure CC='ccache
clang' CXX='ccache clang++' --enable-checking=yes --disable-bootstrap
--disable-multilib --enable-languages=c,c++
--prefix=/data/x27zhou/compiler-nightly/install/gcc
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 17.0.0 20260809 (experimental) (GCC)
$
$ gcc-trunk -O0 small.c && ./a.out
3
$ gcc-trunk -O2 small.c && ./a.out
90
$
$ cat small.c
#include <stdio.h>
short a;
int b;
int main() {
  long c = a;
  unsigned f;
  int d = 0, e;
  for (; d < 6; d++) {
    e = 0;
    while (e < 6) {
      e++;
      f = c;
      switch (f) {
      case 1:
        c = c + d;
      case 3:
        break;
      default:
        c = c + d;
      }
    }
  }
  b = c;
  printf("%d\n", b);
}