Messages in this thread | | | From | Vincent MAILHOL <> | Date | Wed, 24 Aug 2022 21:10:59 +0900 | Subject | Re: [PATCH v5 2/2] x86/asm/bitops: __ffs,ffz: use __builtin_ctzl to evaluate constant expressions |
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On Wed. 24 Aug 2022 at 17:43, Borislav Petkov <bp@alien8.de> wrote: > On Wed, Aug 24, 2022 at 05:31:20AM +0900, Vincent MAILHOL wrote: > > If the fact that __ffs(0) is undefined is a concern, > > So what is of concern is I'm looking at those *ffs things and they look > like a real mess:
I agree that the thing is a mess. Especially the naming: adding underscores when the behaviour is different is misleading. I think that ctzl() would have been a better name than __ffs().
> * Undefined if no bit exists, so code should check against 0 first. > */ > static __always_inline unsigned long __ffs(unsigned long word) > { > asm("rep; bsf %1,%0" > > and that's TZCNT.
Not exactly, this is TZCNT for x86_64 but for x86, it will be BSF…
> And nowhere in TZCNT's description does it talk about undefined behavior > - it is all defined. > > So I have no clue what that comment is supposed to mean?
It means that __ffs() is not a x86_64 specific function. Each architecture is free to provide an optimized implementation and are free to ignore __ffs(0) because this is undefined. For ffs(0) to be defined, every architecture would have to produce the same result, and this is not the case.
> Then: > > * ffs - find first set bit in word > * @x: the word to search > * > * This is defined the same way as the libc and compiler builtin ffs > * routines, therefore differs in spirit from the other bitops. > * > * ffs(value) returns 0 if value is 0 or the position of the first > * set bit if value is nonzero. The first (least significant) bit > * is at position 1. > > while > > "Built-in Function: int __builtin_ctz (unsigned int x) > > Returns the number of trailing 0-bits in x, starting at the least significant bit position. If x is 0, the result is undefined." > > as previously pasted. > > So ffs() doesn't have undefined behavior either. > > I guess it wants to say, it is undefined in the *respective* libc or > compiler helper implementation. And that should be explained. > > > I can add a safety net: > > Nah, no need. It seems this "behavior" has been the case a long time so > callers should know better (or have burned themselves properly :)). > > > There is an index issue. __ffs() starts at 0 but ffs() starts at one. > > i.e.: __ffs(0x01) is 0 but ffs(0x01) is 1. > > Aside from the zero edge case, ffs(x) equals __ffs(x) + 1. This > > explains why __fss(0) is undefined. > > I'd love to drop the undefined thing and start counting at 1 while > keeping the 0 case the special one. > > But that ship has sailed a long time ago - look at all the __ffs() and > ffs() callers.
ACK. I do not believe that this is something which can be changed now. At least, I am not willing to start such a crusade.
> 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 x86_64 and BSF instruction for x86). Even if x86_64 and x86 had the same behaviour that would still not be OK as it may fool developers into believing that __ffs(0) is defined kernel wide and would result in non portable code.
> but since it needs to adhere to the libc variants' API, it treats 0 > differently. They surely can handle 0 as input. > > I.e., I'd like to see a comment there explaining the whole difference > between ffs() and __ffs() so that people are aware.
This would be helpful but the priority would then be to modify asm-generic: https://elixir.bootlin.com/linux/latest/source/include/asm-generic/bitops/__ffs.h#L11
Regardless, I do not think that the comment of __ffs() and ffs() is related to this patch series.
> Btw, pls do > > s/variable___ffs/variable__ffs/g > > Two underscores are just fine.
OK for me. The rationale was to name it variable_<function_name>() thus the three underscores. But I will also be happy with two underscores.
Yours sincerely, Vincent Mailhol
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