lists  /  announce  owl-users  owl-dev  john-users  john-dev  passwdqc-users  yescrypt  popa3d-users  /  oss-security  kernel-hardening  musl  sabotage  tlsify  passwords  /  crypt-dev  xvendor  /  Bugtraq  Full-Disclosure  linux-kernel  linux-netdev  linux-ext4  linux-hardening  PHC 
Open Source and information security mailing list archives
Hash Suite: Windows password security audit tool. GUI, reports in PDF.
[<prev] [next>] [thread-next>] [day] [month] [year] [list]
Date:   Fri, 20 Nov 2020 13:51:54 +0100
From:   Peter Zijlstra <>
To:     Andrew Morton <>,
        Linus Torvalds <>
        Jakub Jelinek <>,
Subject: [PATCH] ilog2: Improve ilog2 for constant arguments

From:   Jakub Jelinek <>

As discussed in
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 <>
Signed-off-by: Peter Zijlstra (Intel) <>
 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)			\

Powered by blists - more mailing lists