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>
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1 changed files with 17 additions and 2 deletions
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@ -347,6 +347,19 @@ static int test_bit(int nr, const volatile unsigned long *addr);
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? constant_test_bit((nr), (addr)) \
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: variable_test_bit((nr), (addr)))
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#if (defined(CONFIG_X86_GENERIC) || defined(CONFIG_GENERIC_CPU)) \
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&& !defined(CONFIG_CC_OPTIMIZE_FOR_SIZE)
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/*
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* Since BSF and TZCNT have sufficiently similar semantics for the purposes
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* for which we use them here, BMI-capable hardware will decode the prefixed
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* variant as 'tzcnt ...' and may execute that faster than 'bsf ...', while
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* older hardware will ignore the REP prefix and decode it as 'bsf ...'.
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*/
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# define BSF_PREFIX "rep;"
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#else
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# define BSF_PREFIX
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#endif
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/**
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* __ffs - find first set bit in word
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* @word: The word to search
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@ -355,7 +368,7 @@ static int test_bit(int nr, const volatile unsigned long *addr);
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*/
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static inline unsigned long __ffs(unsigned long word)
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{
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asm("bsf %1,%0"
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asm(BSF_PREFIX "bsf %1,%0"
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: "=r" (word)
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: "rm" (word));
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return word;
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@ -369,12 +382,14 @@ static inline unsigned long __ffs(unsigned long word)
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*/
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static inline unsigned long ffz(unsigned long word)
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{
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asm("bsf %1,%0"
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asm(BSF_PREFIX "bsf %1,%0"
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: "=r" (word)
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: "r" (~word));
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return word;
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}
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#undef BSF_PREFIX
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/*
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* __fls: find last set bit in word
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* @word: The word to search
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