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

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

[Jan Beulich]

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;

[tip:x86/asm] x86: Use REP BSF unconditionally

[tip-bot for Jan Beulich]

Commit-ID:  e26a44a2d618a491d5c6a2a8aaf66ee03a94739f
Gitweb:     http://git.kernel.org/tip/e26a44a2d618a491d5c6a2a8aaf66ee03a94739f
Author:     Jan Beulich <JBeulich@suse.com>
AuthorDate: Tue, 18 Sep 2012 12:16:14 +0100
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Wed, 19 Sep 2012 17:26:08 +0200

x86: Use REP BSF unconditionally

Make "REP BSF" unconditional, as per the suggestion of hpa
and Linus, this removes the insane BSF_PREFIX conditional
and simplifies the logic.

Suggested-by: "H. Peter Anvin" <hpa@zytor.com>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Link: http://lkml.kernel.org/r/5058741E020000780009C014@nat28.tlf.novell.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 arch/x86/include/asm/bitops.h |   19 ++-----------------
 1 files changed, 2 insertions(+), 17 deletions(-)

diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h
index b2af664..6dfd019 100644
--- a/arch/x86/include/asm/bitops.h
+++ b/arch/x86/include/asm/bitops.h
@@ -347,19 +347,6 @@ static int test_bit(int nr, const volatile unsigned long *addr);
 	 ? constant_test_bit((nr), (addr))	\
 	 : variable_test_bit((nr), (addr)))
 
-#if (defined(CONFIG_X86_GENERIC) || defined(CONFIG_GENERIC_CPU)) \
-    && !defined(CONFIG_CC_OPTIMIZE_FOR_SIZE)
-/*
- * Since BSF and TZCNT have sufficiently similar semantics for the purposes
- * for which we use them here, BMI-capable hardware will decode the prefixed
- * variant as 'tzcnt ...' and may execute that faster than 'bsf ...', while
- * older hardware will ignore the REP prefix and decode it as 'bsf ...'.
- */
-# define BSF_PREFIX "rep;"
-#else
-# define BSF_PREFIX
-#endif
-
 /**
  * __ffs - find first set bit in word
  * @word: The word to search
@@ -368,7 +355,7 @@ static int test_bit(int nr, const volatile unsigned long *addr);
  */
 static inline unsigned long __ffs(unsigned long word)
 {
-	asm(BSF_PREFIX "bsf %1,%0"
+	asm("rep; bsf %1,%0"
 		: "=r" (word)
 		: "rm" (word));
 	return word;
@@ -382,14 +369,12 @@ static inline unsigned long __ffs(unsigned long word)
  */
 static inline unsigned long ffz(unsigned long word)
 {
-	asm(BSF_PREFIX "bsf %1,%0"
+	asm("rep; bsf %1,%0"
 		: "=r" (word)
 		: "r" (~word));
 	return word;
 }
 
-#undef BSF_PREFIX
-
 /*
  * __fls: find last set bit in word
  * @word: The word to search