diff options
| author | Jakub Jelinek <jakub@redhat.com> | 2020-12-15 20:43:37 -0800 |
|---|---|---|
| committer | Linus Torvalds <torvalds@linux-foundation.org> | 2020-12-15 22:46:16 -0800 |
| commit | 2f78788b55baa3410b1ec91a576286abe1ad4d6a (patch) | |
| tree | b3040cbef4a4ceb60035560e901f76ebe45a813f /include/linux/log2.h | |
| parent | 7546861a8c55f1d704a6cfd699b33a9e2dd8c021 (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.h | 3 |
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) \ |