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string: memchr_inv() speed improvements

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- 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>
This commit is contained in:
Akinobu Mita 2012-03-23 15:02:14 -07:00 committed by Linus Torvalds
parent a403d930c5
commit f43804bf5f
1 changed files with 16 additions and 4 deletions
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20
lib/string.c
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@ -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;