linux-kernel - [PATCH] mbcache: LLVMLinux: Remove double calculation from mbcache

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Date:	Mon, 21 Apr 2014 22:19:53 -0700
From:	behanw@...verseincode.com
To:	viro@...iv.linux.org.uk
Cc:	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	dwmw2@...radead.org, pageexec@...email.hu,
	Mark Charlebois <charlebm@...il.com>,
	Behan Webster <behanw@...verseincode.com>
Subject: [PATCH] mbcache: LLVMLinux: Remove double calculation from mbcache

From: Mark Charlebois <charlebm@...il.com>

The call to __builtin_log2 presumes there is a
double log2(double x) function defined in the kernel.

The call to hash_log is a call to hash_64 which is
defined in include/linux/hash.h

static __always_inline u64 hash_64(u64 val, unsigned int bits)

That means that __builtin_log2(NR_BG_LOCKS) is converting
NR_BG_LOCKS to a double and returning a double and then that
is converted to an unsigned int.

Using ilog2 is much more appropriate and efficient.

Another side effect of using __builtin_log2 is that is uses
__aeabi_* functions for ARM that require linking with libgcc.a.

Author: Mark Charlebois <charlebm@...il.com>
Signed-off-by: Mark Charlebois <charlebm@...il.com>
Signed-off-by: Behan Webster <behanw@...verseincode.com>
---
 fs/mbcache.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/mbcache.c b/fs/mbcache.c
index bf166e3..2c0752b 100644
--- a/fs/mbcache.c
+++ b/fs/mbcache.c
@@ -93,7 +93,7 @@
 
 #define MB_CACHE_WRITER ((unsigned short)~0U >> 1)
 
-#define MB_CACHE_ENTRY_LOCK_BITS	__builtin_log2(NR_BG_LOCKS)
+#define MB_CACHE_ENTRY_LOCK_BITS	ilog2(NR_BG_LOCKS)
 #define	MB_CACHE_ENTRY_LOCK_INDEX(ce)			\
 	(hash_long((unsigned long)ce, MB_CACHE_ENTRY_LOCK_BITS))
 
-- 
1.8.3.2

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