[PATCH] x86_64: fls in asm for x86_64

Use single instruction for find largest set bit on x86_64.

[Updated by Jan Beulich to fix wrong asm constraints in original
patch -AK]

Cc: jbeulich@novell.com
Signed-off-by: Stephen Hemminger <shemminger@osdl.org>
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 16 insertions, 3 deletions

diff --git a/include/asm-x86_64/bitops.h b/include/asm-x86_64/bitops.h
index 3638ffff87fd..eb4df23e1e41 100644
--- a/include/asm-x86_64/bitops.h
+++ b/include/asm-x86_64/bitops.h
@@ -397,6 +397,22 @@ static __inline__ int fls64(__u64 x)
 }
 
 /**
+ * fls - find last bit set
+ * @x: the word to search
+ *
+ * This is defined the same way as ffs.
+ */
+static __inline__ int fls(int x)
+{
+        int r;
+
+        __asm__("bsrl %1,%0\n\t"
+                "cmovzl %2,%0"
+                : "=&r" (r) : "rm" (x), "rm" (-1));
+        return r+1;
+}
+
+/**
  * hweightN - returns the hamming weight of a N-bit word
  * @x: the word to weigh
  *
@@ -434,9 +450,6 @@ static __inline__ int fls64(__u64 x)
 #define minix_find_first_zero_bit(addr,size) \
         find_first_zero_bit((void*)addr,size)
 
-/* find last set bit */
-#define fls(x) generic_fls(x)
-
 #endif /* __KERNEL__ */
 
 #endif /* _X86_64_BITOPS_H */