Re: [PATCH] lib/clz_ctz.c: Fix __clzdi2() and __ctzdi2() for 32-bit kernels

[Linus Torvalds]

On Mon, 28 Aug 2023 at 03:53, David Laight <David.Laight@xxxxxxxxxx> wrote:
>
> From: Linus Torvalds
> >
> > We use this:
> >
> >   static __always_inline unsigned long variable__ffs(unsigned long word)
> >   {
> >         asm("rep; bsf %1,%0"
> >                 : "=r" (word)
> >                 : "rm" (word));
> >         return word;
> >   }
> >
> > for the definition, and it looks like clang royally just screws up
> > here. Yes, "m" is _allowed_ in that input set, but it damn well
> > shouldn't be used for something that is already in a register, since
> > "r" is also allowed, and is the first choice.
>
> Why don't we just remove the "m" option?

For this particular case, it would probably be the right thing to do.
It's sad, though, because gcc handles this correctly, and always has.

And in this particular case, it probably matters not at all.

In many other cases where we have 'rm', we may actually be in the
situation that having 'rm' (or other cases like "g" that also allows
immediates) helps because register pressure can be a thing.

It's mostly a thing on 32-bit x86 where you have a lot fewer
registers, and there we've literally run into situations where we have
had internal compiler errors because of complex inline asm statements
running out of registers.

With a simple "one input, one output" case, that just isn't an issue,
so to work around a clang misfeature we could do it - if somebody
finds a case where it actually matters (as opposed to "damn, when
looking at the generted code for a function that we never actually use
on x86, I noticed that code generation is horrendous").

             Linus