On 26/04/2025 8:55 pm, Linus Torvalds wrote:
> So I think that manual cmov pattern for x86-32 should be replaced with
>
> bool zero;
>
> asm("bsfl %[in],%[out]"
> CC_SET(z)
> : CC_OUT(z) (zero),
> [out]"=r" (r)
> : [in] "rm" (x));
>
> return zero ? 0 : r+1;
>
> instead (that's ffs(), and fls() would need the same thing except with
> bsrl insteadm, of course).
>
> I bet that would actually improve code generation.
It is possible to do better still.
ffs/fls are commonly found inside loops where x is the loop condition
too. Therefore, using statically_true() to provide a form without the
zero compatibility turns out to be a win.
> And I also bet it doesn't actually matter, of course.
Something that neither Linux nor Xen had, which makes a reasonable
difference, is a for_each_set_bit() optimised over a scalar value. The
existing APIs make it all too easy to spill the loop condition to memory.
~Andrew
