Messages in this thread | | | From | Vincent MAILHOL <> | Date | Wed, 7 Sep 2022 14:35:41 +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. 7 Sep 2022 at 13:06, Borislav Petkov <bp@alien8.de> wrote: > On Sat, Aug 27, 2022 at 06:32:05AM +0900, Vincent MAILHOL wrote: > > 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? > > Sure we can. > > But then you can't start your commit message with: > > "__ffs(x) is equivalent to (unsigned long)__builtin_ctzl(x) and ffz(x) > is equivalent to (unsigned long)__builtin_ctzl(~x)." > > which will bring unenlightened readers like me into the very same mess. > > So at least mention that there's a difference between the kernel > implementation using hw insns which are well defined on some machines > and what the glibc API does. So that at least people are aware that > there's something dangerous to be cautious about. > > Ok?
OK.
I rephrased the beginning of the commit message as below:
If x is not 0, __ffs(x) is equivalent to: (unsigned long)__builtin_ctzl(x) And if x is not ~0UL, ffz(x) is equivalent to: (unsigned long)__builtin_ctzl(~x) Because __builting_ctzl() returns an int, a cast to (unsigned long) is necessary to avoid potential warnings on implicit casts.
Concerning the edge cases, __builtin_ctzl(0) is always undefined, whereas __ffs(0) and ffz(~0UL) may or may not be defined, depending on the processor. Regardless, for both functions, developers are asked to check against 0 or ~0UL so replacing __ffs() or ffz() by __builting_ctzl() is safe.
Does this solve the issue? If yes, I will prepare the v8 right away.
Yours sincerely, Vincent Mailhol
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