Re: [PATCH v5 2/2] x86/asm/bitops: __ffs,ffz: use __builtin_ctzl to evaluate constant expressions

Vincent MAILHOL
2022-08-26T21:32:25Z

On Wed. 24 Aug. 2022 at 22:24, Borislav Petkov <bp@alien8.de> wrote:
> On Wed, Aug 24, 2022 at 09:10:59PM +0900, Vincent MAILHOL wrote:
> > Not exactly, this is TZCNT for x86_64 but for x86, it will be BSF…
>
> Not x86 - some old models which do not understand TZCNT, I'm being told.

ACK.

> And I'm being also told, "Intel and AMD disagree on what BSF does when
> passed 0". So this is more mess.

ACK.

> > It means that __ffs() is not a x86_64 specific function. Each
>
> No, not that. The comment "Undefined if no bit exists".
>
> On my machine, __ffs(0) - the way it is implemented:
>
> rep; bsf %1,%0
>
> is well-defined:
>
> "If the input operand is zero, CF is set to 1 and the size (in bits) of
> the input operand is written to the destination register. Otherwise, CF
> is cleared."

It is well defined on *your* machine.

On some other machines, it might be undefined:
"If the content of the source operand is 0, the content of the
destination operand is undefined."
https://www.felixcloutier.com/x86/bsf

> Leading to
>
> __ffs(0): 0x40
>
> i.e., input operand of 64 bits.
>
> So on this particular x86 implementation, TZCNT(0) is well defined.

It is here where I do not follow you. OK that on most of the recent
machines, the compiler will emit a TZCNT and that this instruction is
well defined for zero. But on some older machines, it will emit BSF,
and on a subset of those machines, BSF(0) might be undefined.

> So I'd like for that "undefined" thing to be expanded upon and
> explained. Something along the lines of "the libc/compiler primitives'
> *ffs(0) is undefined. Our inline asm helpers adhere to that behavior
> even if the result they return for input operand of 0 is very well
> defined."
>
> Now, if there are some machines which do not adhere to the current hw
> behavior, then they should be ALTERNATIVEd.
>
> Better?
>
> > > Back to your patch: I think the text should be fixed to say that both
> > > ffs() and __ffs()'s kernel implementation doesn't have undefined results
> >
> > NACK. __ffs(0) is an undefined behaviour (c.f. TZCNT instruction for
>
> NACK, SCHMACK. Read my mail again: "I think the text should be fixed".
> The *text* - not __ffs(0) itself. The *text* should be fixed to explain
> what undefined means. See above too.
>
> IOW, to start explaining this humongous mess I've scratched the surface
> of.

Agree that this is only the surface. But, my patch series is about
constant folding, not about the text of *ffs(). Here, I just *move*
the existing text, I did not modify anything.
Can we agree that this is a separate topic? I do not think I am the
good person to fix that mess (and in all honesty, I am not a domain
expert in this domain and I am afraid I would just make you lose your
time if I had to work on this).

Yours sincerely,
Vincent Mailhol