Commit 0292c497 authored Mar 23, 2012 by Bob Pearson Committed by Linus Torvalds Mar 23, 2012

crc32: optimize loop counter for x86



Add two changes that improve the performance of x86 systems

1. replace main loop with incrementing counter this change improves
   the performance of the selftest by about 5-6% on Nehalem CPUs.  The
   apparent reason is that the compiler can use the loop index to perform
   an indexed memory access.  This is reported to make the performance of
   PowerPC CPUs to get worse.

2. replace the rem_len loop with incrementing counter this change
   improves the performance of the selftest, which has more than the usual
   number of occurances, by about 1-2% on x86 CPUs.  In actual work loads
   the length is most often a multiple of 4 bytes and this code does not
   get executed as often if at all.  Again this change is reported to make
   the performance of PowerPC get worse.

[djwong@us.ibm.com: Minor changelog tweaks]
Signed-off-by: Bob Pearson <rpearson@systemfabricworks.com>
Signed-off-by: Darrick J. Wong <djwong@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 324eb0f1

lib/crc32.c

+13 −0

Original line number	Diff line number	Diff line
		@@ -66,6 +66,9 @@ crc32_body(u32 crc, unsigned char const buf, size_t len, const u32 (tab)[256])
		# endif
		const u32 *b;
		size_t rem_len;
		# ifdef CONFIG_X86
		size_t i;
		# endif
		const u32 t0=tab[0], t1=tab[1], t2=tab[2], t3=tab[3];
		const u32 t4 = tab[4], t5 = tab[5], t6 = tab[6], t7 = tab[7];
		u32 q;
		@@ -86,7 +89,12 @@ crc32_body(u32 crc, unsigned char const buf, size_t len, const u32 (tab)[256])
		# endif

		b = (const u32 *)buf;
		# ifdef CONFIG_X86
		--b;
		for (i = 0; i < len; i++) {
		# else
		for (--b; len; --len) {
		# endif
		q = crc ^ ++b; / use pre increment for speed */
		# if CRC_LE_BITS == 32
		crc = DO_CRC4;
		@@ -100,9 +108,14 @@ crc32_body(u32 crc, unsigned char const buf, size_t len, const u32 (tab)[256])
		/* And the last few bytes */
		if (len) {
		u8 p = (u8 )(b + 1) - 1;
		# ifdef CONFIG_X86
		for (i = 0; i < len; i++)
		DO_CRC(++p); / use pre increment for speed */
		# else
		do {
		DO_CRC(++p); / use pre increment for speed */
		} while (--len);
		# endif
		}
		return crc;
		#undef DO_CRC

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