[Bug c++/127363] New: auto vectorize remove __builtin_assoc_barrier

[z00823823 at outlook dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=127363

            Bug ID: 127363
           Summary: auto vectorize remove __builtin_assoc_barrier
           Product: gcc
           Version: 16.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: z00823823 at outlook dot com
  Target Milestone: ---

```c++
void round_values(float* values) {
    constexpr float big = 0x1p24f;
    for (int i = 0; i < 8; ++i) {
        values[i] = __builtin_assoc_barrier(values[i] + big) - big;
    }
}
```

this code compiles to nothing due to auto vectorize under `-ffast-math -O2`.
adding `-fno-tree-vectorize` fix it.

https://gcc.godbolt.org/z/h44s9nfPW