[Bug tree-optimization/108341] New: argument to `__builtin_ctz` should be assumed non-zero

[lh_mouse at 126 dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108341

            Bug ID: 108341
           Summary: argument to `__builtin_ctz` should be assumed non-zero
           Product: gcc
           Version: 12.2.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: lh_mouse at 126 dot com
  Target Milestone: ---

Godbolt: https://gcc.godbolt.org/z/PrPP4v9z1


```
extern int r;

int
bz(int value)
  {
    r = __builtin_ctz(value);
    return value != 0;  // always true
  }
```


According to GCC manual, if the argument to `__builtin_ctz()` is zero then the
behavior is undefined, but GCC fails to assume that this function always
returns `1`. 

But I have read https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94801, not sure
whether it's related.