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Subject[PATCH] ilog2: Improve ilog2 for constant arguments

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) \
\
 
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