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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>
hifive-unleashed-5.1
Jan Beulich 2012-09-10 12:24:43 +01:00 committed by Ingo Molnar
parent 1edfbb4153
commit 5870661c09
1 changed files with 17 additions and 2 deletions
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19
arch/x86/include/asm/bitops.h
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@ -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