Re: B^HDEAD code generation (AMD64)

[LIU Hao <lh_mouse@126.com>]

[-- Attachment #1.1: Type: text/plain, Size: 966 bytes --]

在 2023/1/9 19:48, Thomas Koenig via Gcc 写道:
> On 09.01.23 12:35, Stefan Kanthak wrote:
>> 20 superfluous instructions of the total 102 instructions!
> 
> The proper place for bug reports is https://gcc.gnu.org/bugzilla/ .
> Feel free to submit these cases there.

Created https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108341, although I'm not sure whether it's 
related to this issue.


Given

    ```
    extern int r;

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

`value` is passed to `__builtin_ctz` and GCC should assume it can't be zero, otherwise the behavior 
is undefined. However GCC fails to optimize it and produces:

    ```
    bz:
      endbr64
      xor	eax, eax
      rep bsf	eax, edi
      mov	DWORD PTR r[rip], eax
      xor	eax, eax
      test	edi, edi
      setne	al
      ret
    ```


-- 
Best regards,
LIU Hao


[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 840 bytes --]