linux-kernel - [PATCH, v2] x86: use tzcnt instead of bsf

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Date:	Tue, 18 Sep 2012 12:16:14 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	<mingo@...e.hu>, <tglx@...utronix.de>, <hpa@...or.com>
Cc:	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	<linux-kernel@...r.kernel.org>
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@...e.com>
Cc: Linus Torvalds <torvalds@...ux-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;



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