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 <[email protected]>
Cc: Linus Torvalds <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Ingo Molnar <[email protected]>

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