From: Jakub Jelinek <jakub@redhat.com>
To: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: linux-toolchains@vger.kernel.org
Subject: Re: ilog2 vs. GCC inlining heuristics
Date: Wed, 21 Oct 2020 15:45:13 +0200 [thread overview]
Message-ID: <20201021134513.GD2176@tucnak> (raw)
In-Reply-To: <14215e71-cd43-81aa-1965-0b0a6fb667fa@csgroup.eu>
On Wed, Oct 21, 2020 at 03:36:14PM +0200, Christophe Leroy wrote:
> > --- a/include/linux/log2.h
> > +++ b/include/linux/log2.h
> > @@ -156,7 +156,8 @@ unsigned long __rounddown_pow_of_two(unsigned long n)
> > #define ilog2(n) \
> > ( \
> > __builtin_constant_p(n) ? \
> > - const_ilog2(n) : \
> > + ((n) < 2 ? 0 : \
> > + 63 - __builtin_clzll (n)) : \
>
> That is likely to work only on 64 bits targets.
No, it should work on all. __builtin_clzll argument is long long, which
should be 64-bit on all architectures Linux kernel supports (and all
architectures GCC supports - ok, avr with -mint8 doesn't, but it isn't C
requirements compatible in that case).
const_ilog2 also uses long long and will work with any long long or
unsigned long long value (but e.g. not with __uint128_t values on 64-bit
architectures, guess the kernel doesn't need that though).
And similarly, for the non-constant operands the sizeof below ensures
that __ilog2_u64 is used (even on 32-bit arches) and will handle that.
> On 32 bits targets I guess it needs to be 31 - __builtin_clz (n), doesn't it ?
>
> > (sizeof(n) <= 4) ? \
> > __ilog2_u32(n) : \
> > __ilog2_u64(n) \
> >
> > Jakub
> >
Jakub
next prev parent reply other threads:[~2020-10-21 13:45 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-10-21 13:27 ilog2 vs. GCC inlining heuristics Jakub Jelinek
2020-10-21 13:36 ` Christophe Leroy
2020-10-21 13:45 ` Jakub Jelinek [this message]
2020-10-21 15:19 ` Peter Zijlstra
2020-10-21 18:40 ` Christophe Leroy
2020-10-22 4:01 ` Randy Dunlap
2020-10-22 7:12 ` Jakub Jelinek
2020-11-20 12:33 ` [tip: core/core] " tip-bot2 for Jakub Jelinek
2020-11-20 12:41 ` Peter Zijlstra
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