string: memchr_inv() speed improvements
Akinobu Mita [Fri, 23 Mar 2012 22:02:14 +0000 (15:02 -0700)]
- Generate a 64-bit pattern more efficiently

memchr_inv needs to generate a 64-bit pattern filled with a target
character.  The operation can be done by more efficient way.

- Don't call the slow check_bytes() if the memory area is 64-bit aligned

memchr_inv compares contiguous 64-bit words with the 64-bit pattern as
much as possible.  The outside of the region is checked by check_bytes()
that scans for each byte.  Unfortunately, the first 64-bit word is
unexpectedly scanned by check_bytes() even if the memory area is aligned
to a 64-bit boundary.

Both changes were originally suggested by Eric Dumazet.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Suggested-by: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

lib/string.c

index dc4a863..3a03782 100644 (file)
@@ -785,12 +785,24 @@ void *memchr_inv(const void *start, int c, size_t bytes)
        if (bytes <= 16)
                return check_bytes8(start, value, bytes);
 
-       value64 = value | value << 8 | value << 16 | value << 24;
-       value64 = (value64 & 0xffffffff) | value64 << 32;
-       prefix = 8 - ((unsigned long)start) % 8;
+       value64 = value;
+#if defined(ARCH_HAS_FAST_MULTIPLIER) && BITS_PER_LONG == 64
+       value64 *= 0x0101010101010101;
+#elif defined(ARCH_HAS_FAST_MULTIPLIER)
+       value64 *= 0x01010101;
+       value64 |= value64 << 32;
+#else
+       value64 |= value64 << 8;
+       value64 |= value64 << 16;
+       value64 |= value64 << 32;
+#endif
 
+       prefix = (unsigned long)start % 8;
        if (prefix) {
-               u8 *r = check_bytes8(start, value, prefix);
+               u8 *r;
+
+               prefix = 8 - prefix;
+               r = check_bytes8(start, value, prefix);
                if (r)
                        return r;
                start += prefix;