[Bug tree-optimization/108341] argument to `__builtin_ctz` should be assumed non-zero when CTZ_DEFINED_VALUE_AT_ZERO says it is undefined

[lh_mouse at 126 dot com via Gcc-bugs]

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

--- Comment #7 from LIU Hao <lh_mouse at 126 dot com> ---
(In reply to Jakub Jelinek from comment #4)
> I think 0 argument for __builtin_c[lt]z{,l,ll,imax} is always undefined, 0
> argument
> to .C[LT]Z (internal calls) is undefined if C[LT]Z_DEFINED_VALUE_AT_ZERO is
> not 2 and
> 0 argument to C[LT]Z RTL is undefined if C[LT]Z_DEFINED_VALUE_AT_ZERO is not
> non-zero.

I agree with this.


#94801 mentioned the `if(value == 0) __builtin_unreachable();` trick, but it
isn't an option if the argument is really possibly a zero:

(https://gcc.godbolt.org/z/dePvcMhTr)
```
#include <stdint.h>

uint32_t
my_tzcnt(uint32_t value)
  {
    return (value == 0) ? 32 : __builtin_ctz(value);
  }
```

This can be TZCNT if the CPU supports it.