Re: [PATCH] [RFC] x86/cpu: rework instruction set selection

[Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>]

On Sat, 26 Apr 2025 at 12:24, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> (And yes, one use in a x86 header file that is pretty questionable
> too: I think the reason for the cmov is actually i486-only behavior
> and we could probably unify the 32-bit and 64-bit implementation)

Actually, what we *should* do is to remove that manual use of 'cmov'
entirely - even if we decide that yes, that undefined zero case is
actually real.

We should probably change it to use CC_SET(), and the compiler will do
a much better job - and probably never use cmov anyway.

And yes, that will generate worse code if you have an old compiler
that doesn't do ASM_FLAG_OUTPUTS, but hey, that's true in general. If
you want good code, you need a good compiler.

And clang needs to learn the CC_SET() pattern anyway.

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.

And I also bet it doesn't actually matter, of course.

           Linus