Messages in this thread | | | Date | Wed, 24 Aug 2022 10:43:37 +0200 | From | Borislav Petkov <> | Subject | Re: [PATCH v5 2/2] x86/asm/bitops: __ffs,ffz: use __builtin_ctzl to evaluate constant expressions |
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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:
* 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.
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?
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.
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 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.
Btw, pls do
s/variable___ffs/variable__ffs/g
Two underscores are just fine.
Thx.
-- Regards/Gruss, Boris.
https://people.kernel.org/tglx/notes-about-netiquette
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