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>
? constant_test_bit((nr), (addr)) \
: variable_test_bit((nr), (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
/**
* __ffs - find first set bit in word
* @word: The word to search
*/
static inline unsigned long __ffs(unsigned long word)
{
*/
static inline unsigned long __ffs(unsigned long word)
{
+ asm(BSF_PREFIX "bsf %1,%0"
: "=r" (word)
: "rm" (word));
return word;
: "=r" (word)
: "rm" (word));
return word;
*/
static inline unsigned long ffz(unsigned long word)
{
*/
static inline unsigned long ffz(unsigned long word)
{
+ asm(BSF_PREFIX "bsf %1,%0"
: "=r" (word)
: "r" (~word));
return word;
}
: "=r" (word)
: "r" (~word));
return word;
}
/*
* __fls: find last set bit in word
* @word: The word to search
/*
* __fls: find last set bit in word
* @word: The word to search