Commit-ID: 5870661c091e827973674cc3469b50c959008c2b Gitweb: http://git.kernel.org/tip/5870661c091e827973674cc3469b50c959008c2b Author: Jan Beulich <jbeul...@suse.com> AuthorDate: Mon, 10 Sep 2012 12:24:43 +0100 Committer: Ingo Molnar <mi...@kernel.org> CommitDate: Thu, 13 Sep 2012 17:44:01 +0200
x86: Prefer TZCNT over BFS 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. For the moment, only do this when the respective generic-CPU option is selected (as there are no specific-CPU options covering the CPUs supporting TZCNT), and don't do that when size optimization was requested. Signed-off-by: Jan Beulich <jbeul...@suse.com> Cc: Linus Torvalds <torva...@linux-foundation.org> Link: http://lkml.kernel.org/r/504dea1b020000780009a...@nat28.tlf.novell.com Signed-off-by: Ingo Molnar <mi...@kernel.org> --- arch/x86/include/asm/bitops.h | 19 +++++++++++++++++-- 1 files changed, 17 insertions(+), 2 deletions(-) diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h index ebaee69..b2af664 100644 --- a/arch/x86/include/asm/bitops.h +++ b/arch/x86/include/asm/bitops.h @@ -347,6 +347,19 @@ 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 @@ -355,7 +368,7 @@ static int test_bit(int nr, const volatile unsigned long *addr); */ static inline unsigned long __ffs(unsigned long word) { - asm("bsf %1,%0" + asm(BSF_PREFIX "bsf %1,%0" : "=r" (word) : "rm" (word)); return word; @@ -369,12 +382,14 @@ static inline unsigned long __ffs(unsigned long word) */ static inline unsigned long ffz(unsigned long word) { - asm("bsf %1,%0" + asm(BSF_PREFIX "bsf %1,%0" : "=r" (word) : "r" (~word)); return word; } +#undef BSF_PREFIX + /* * __fls: find last set bit in word * @word: The word to search -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
