Commit f43804bf authored Mar 23, 2012 by Akinobu Mita Committed by Linus Torvalds Mar 23, 2012

string: memchr_inv() speed improvements



- 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>

parent a403d930

lib/string.c

+16 −4

Original line number	Diff line number	Diff line
		@@ -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;

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