On 24.05.2024 22:03, Andrew Cooper wrote:
> --- a/xen/arch/x86/include/asm/bitops.h
> +++ b/xen/arch/x86/include/asm/bitops.h
> @@ -432,12 +432,28 @@ static inline int ffsl(unsigned long x)
>
> static always_inline unsigned int arch_ffs(unsigned int x)
> {
> - int r;
> + unsigned int r;
> +
> + if ( __builtin_constant_p(x > 0) && x > 0 )
> + {
> + /* Safe, when the compiler knows that x is nonzero. */
> + asm ( "bsf %[val], %[res]"
> + : [res] "=r" (r)
> + : [val] "rm" (x) );
> + }
In patch 11 relevant things are all in a single patch, making it easier
to spot that this is dead code: The sole caller already has a
__builtin_constant_p(), hence I don't see how the one here could ever
return true. With that the respective part of the description is then
questionable, too, I'm afraid: Where did you observe any actual effect
from this? Or if you did - what am I missing?
Jan
