From: Linus Torvalds
> Sent: 25 August 2023 21:43
....
> Clang turns this:
>
> return __ffs64(val);
>
> into this horror:
>
> pushq %rax
> movq %rdi, (%rsp)
> #APP
> rep
> bsfq (%rsp), %rax
> #NO_APP
> popq %rcx
>
> which is just incredibly broken on so many levels. It *should* be a
> single instruction, like gcc does:
>
> rep; bsf %rdi,%rax # tmp87, word
>
> but clang decides that it really wants to put the argument on the
> stack, and apparently also wants to do that nonsensical stack
> alignment thing to make things even worse.
>
> 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?
Pretty much the only time it will be worse is it the value
is in memory and loading it into a register causes a spill
to stack.
While it is possible to generate code where that happens it
is pretty unlikely.
David
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