On Sat, 26 Apr 2025 at 12:24, Linus Torvalds
<torva...@linux-foundation.org> 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
