lkml.org 
[lkml]   [2012]   [Sep]   [18]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
Date
From
Subject[PATCH, v2] x86: use tzcnt instead of bsf
Following a relatively recent compiler change, make use of the fact
that for non-zero input BSF and TZCNT produce the same result, and that
CPUs not knowing of TZCNT will treat the instruction as BSF (i.e.
ignore what looks like a REP prefix to them). The assumption here is
that TZCNT would never have worse performance than BSF.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>

---
v2: Do this unconditionally, and change title accordingly.

---
arch/x86/include/asm/bitops.h | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

--- 3.6-rc6/arch/x86/include/asm/bitops.h
+++ 3.6-rc6-x86-bsf-tzcnt/arch/x86/include/asm/bitops.h
@@ -355,7 +355,7 @@ static int test_bit(int nr, const volati
*/
static inline unsigned long __ffs(unsigned long word)
{
- asm("bsf %1,%0"
+ asm("rep; bsf %1,%0"
: "=r" (word)
: "rm" (word));
return word;
@@ -369,7 +369,7 @@ static inline unsigned long __ffs(unsign
*/
static inline unsigned long ffz(unsigned long word)
{
- asm("bsf %1,%0"
+ asm("rep; bsf %1,%0"
: "=r" (word)
: "r" (~word));
return word;




\
 
 \ /
  Last update: 2012-09-18 13:41    [W:0.030 / U:14.296 seconds]
©2003-2017 Jasper Spaans. hosted at Digital OceanAdvertise on this site