From: Peter Zijlstra <peterz@infradead.org>
To: Andrew Morton <akpm@linux-foundation.org>,
Linus Torvalds <torvalds@linux-foundation.org>
Cc: linux-kernel@vger.kernel.org, christophe.leroy@csgroup.eu,
Jakub Jelinek <jakub@redhat.com>,
rdunlap@infradead.org
Subject: [PATCH] ilog2: Improve ilog2 for constant arguments
Date: Fri, 20 Nov 2020 13:51:54 +0100 [thread overview]
Message-ID: <20201120125154.GB3040@hirez.programming.kicks-ass.net> (raw)
From: Jakub Jelinek <jakub@redhat.com>
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.
Signed-off-by: Jakub Jelinek <jakub@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20201021132718.GB2176@tucnak
---
include/linux/log2.h | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)
--- a/include/linux/log2.h
+++ b/include/linux/log2.h
@@ -156,7 +156,8 @@ unsigned long __rounddown_pow_of_two(uns
#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) \
next reply other threads:[~2020-11-20 12:52 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-11-20 12:51 Peter Zijlstra [this message]
2020-11-21 20:23 ` [PATCH] ilog2: Improve ilog2 for constant arguments Luc Van Oostenryck
2020-11-21 20:29 ` Jakub Jelinek
2020-11-21 20:46 ` Luc Van Oostenryck
2024-04-25 1:59 [PATCH] ilog2: improve " Eric Lin
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