Builtin for consulting value analysis (better ffs() code gen)

[LIU Hao]

在 2024-03-14 23:33, Andrew Cooper via Gcc 写道:
> And for x86's arch_ffs(),
> 
> unsigned int arch_ffs(unsigned int x)
> {
>      unsigned int res;
> 
>      if ( __builtin_constant_p(x > 0) && x > 0 )
>      {
>          // Well defined when x is known non-zero
>          asm("bsf %1, %0" : "=r"(res) : "rm"(x));

Even if you may assume that the destination operand is always destroyed, the hardware has no 
knowledge about that, so this statement has a dependency on `res`, and shouldn't be declared `=r`.

I think it's better to remove this `if`. The other branch below clearly eliminates the dependency.


>      }
>      else
>      {
>          // The architects say this is safe even for 0.
>          res = -1;
>          asm("bsf %1, %0" : "+r"(res) : "rm"(x));
>      }
> 
>      return res + 1;
> }

-- 
Best regards,
LIU Hao

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