fls(N), ffs(N) and fls64(N) can be optimised on x86/x86_64. Currently they
perform checks against N being 0 before invoking the BSR/BSF instruction, or
use a CMOV instruction afterwards. Either the check involves a conditional
jump which we'd like to avoid, or a CMOV, which we'd also quite like to avoid.
Instead, we can make use of the fact that BSR/BSF doesn't modify its output
register if its input is 0. By preloading the output with -1 and incrementing
the result, we achieve the desired result without the need for a conditional
check.
The 32-bit version of fls64() can also have all its conditional checks removed
by chaining BSRs with a subtraction in the middle. However, this may be
suboptimal on CPUs for which BSR/BSF is really slow (i486 and below for
example).
Signed-off-by: David Howells <dhowells@redhat.com>
---
arch/x86/include/asm/bitops.h | 79 ++++++++++++++++++++++++++---------------
1 files changed, 50 insertions(+), 29 deletions(-)
diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h
index 02b47a6..a8e85ab 100644
--- a/arch/x86/include/asm/bitops.h
+++ b/arch/x86/include/asm/bitops.h
@@ -394,18 +394,12 @@ static inline unsigned long __fls(unsigned long word)
*/
static inline int ffs(int x)
{
- int r;
-#ifdef CONFIG_X86_CMOV
- asm("bsfl %1,%0\n\t"
- "cmovzl %2,%0"
- : "=r" (r) : "rm" (x), "r" (-1));
-#else
- asm("bsfl %1,%0\n\t"
- "jnz 1f\n\t"
- "movl $-1,%0\n"
- "1:" : "=r" (r) : "rm" (x));
-#endif
- return r + 1;
+ int bitpos = -1;
+
+ asm("bsfl %1,%0"
+ : "+r" (bitpos)
+ : "rm" (x));
+ return bitpos + 1;
}
/**
@@ -421,19 +415,52 @@ static inline int ffs(int x)
*/
static inline int fls(int x)
{
- int r;
-#ifdef CONFIG_X86_CMOV
- asm("bsrl %1,%0\n\t"
- "cmovzl %2,%0"
- : "=&r" (r) : "rm" (x), "rm" (-1));
+ int bitpos = -1;
+
+ asm("bsrl %1,%0"
+ : "+r" (bitpos)
+ : "rm" (x));
+ return bitpos + 1;
+}
+
+/**
+ * fls64 - find last set bit in a 64-bit word
+ * @x: the word to search
+ *
+ * This is defined in a similar way as the libc and compiler builtin
+ * ffsll, but returns the position of the most significant set bit.
+ *
+ * fls64(value) returns 0 if value is 0 or the position of the last
+ * set bit if value is nonzero. The last (most significant) bit is
+ * at position 64.
+ */
+#if BITS_PER_LONG == 32
+static __always_inline int fls64(__u64 x)
+{
+ __u32 h = x >> 32;
+ int bitpos = -1;
+
+ asm("bsrl %1,%0 \n"
+ "subl %2,%0 \n"
+ "bsrl %3,%0 \n"
+ : "+r" (bitpos)
+ : "rm" (x), "i"(32), "rm" (h));
+
+ return bitpos + 33;
+}
+#elif BITS_PER_LONG == 64
+static __always_inline int fls64(__u64 x)
+{
+ long bitpos = -1;
+
+ asm("bsrq %1,%0"
+ : "+r" (bitpos)
+ : "rm" (x));
+ return bitpos + 1;
+}
#else
- asm("bsrl %1,%0\n\t"
- "jnz 1f\n\t"
- "movl $-1,%0\n"
- "1:" : "=r" (r) : "rm" (x));
+#error BITS_PER_LONG not 32 or 64
#endif
- return r + 1;
-}
#endif /* __KERNEL__ */
#undef ADDR
@@ -446,12 +473,6 @@ static inline int fls(int x)
#include <asm-generic/bitops/hweight.h>
-#endif /* __KERNEL__ */
-
-#include <asm-generic/bitops/fls64.h>
-
-#ifdef __KERNEL__
-
#include <asm-generic/bitops/ext2-non-atomic.h>
#define ext2_set_bit_atomic(lock, nr, addr) \
--
