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| author | Jan Beulich <JBeulich@suse.com> | 2012-09-10 12:24:43 +0100 |
|---|---|---|
| committer | Ingo Molnar <mingo@kernel.org> | 2012-09-13 17:44:01 +0200 |
| commit | 5870661c091e827973674cc3469b50c959008c2b (patch) | |
| tree | 4043f28631c98fbc03babd7b8b32af788f6f362e /arch/x86/include/asm | |
| parent | 1edfbb4153bd29bcf8d2236676238d5237972be1 (diff) | |
| download | blackbird-op-linux-5870661c091e827973674cc3469b50c959008c2b.tar.gz blackbird-op-linux-5870661c091e827973674cc3469b50c959008c2b.zip | |
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 <jbeulich@suse.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/504DEA1B020000780009A277@nat28.tlf.novell.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Diffstat (limited to 'arch/x86/include/asm')
| -rw-r--r-- | 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 ebaee695394e..b2af6645ea7e 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 |
