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Subject[PATCH] string: memchr_inv speed improvements
Date
- 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>
---
lib/string.c | 20 ++++++++++++++++----
1 files changed, 16 insertions(+), 4 deletions(-)
diff --git a/lib/string.c b/lib/string.c
index dc4a863..3a03782 100644
--- a/lib/string.c
+++ b/lib/string.c
@@ -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;
--
1.7.4.4


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