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authorJakub Jelinek <jakub@redhat.com>2020-12-15 20:43:37 -0800
committerLinus Torvalds <torvalds@linux-foundation.org>2020-12-15 22:46:16 -0800
commit2f78788b55baa3410b1ec91a576286abe1ad4d6a (patch)
treeb3040cbef4a4ceb60035560e901f76ebe45a813f /include/linux/log2.h
parent7546861a8c55f1d704a6cfd699b33a9e2dd8c021 (diff)
ilog2: improve ilog2 for constant arguments
As discussed in https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97445 the const_ilog2 macro generates a lot of code which interferes badly with GCC inlining heuristics, until it can be proven that the ilog2 argument can or can't be simplified into a constant. It can be expressed using __builtin_clzll builtin which is supported by GCC 3.4 and later and when used only in the __builtin_constant_p guarded code it ought to always fold back to a constant. Other compilers support the same builtin for many years too. Other option would be to change the const_ilog2 macro, though as the description says it is meant to be used also in C constant expressions, and while GCC will fold it to constant with constant argument even in those, perhaps it is better to avoid using extensions in that case. [akpm@linux-foundation.org: coding style fixes] Link: https://lkml.kernel.org/r/20201120125154.GB3040@hirez.programming.kicks-ass.net Link: https://lkml.kernel.org/r/20201021132718.GB2176@tucnak Signed-off-by: Jakub Jelinek <jakub@redhat.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Christophe Leroy <christophe.leroy@csgroup.eu> Cc: Randy Dunlap <rdunlap@infradead.org> Signed-off-by: Andrew Morton <akpm@linux-foundation.org> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'include/linux/log2.h')
-rw-r--r--include/linux/log2.h3
1 files changed, 2 insertions, 1 deletions
diff --git a/include/linux/log2.h b/include/linux/log2.h
index c619ec6eff4a..df0b155c2141 100644
--- 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)) : \
(sizeof(n) <= 4) ? \
__ilog2_u32(n) : \
__ilog2_u64(n) \