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.

[[email protected]: Minor changelog tweaks]
Signed-off-by: Bob Pearson <[email protected]>
Signed-off-by: Darrick J. Wong <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

lib/crc32.c

@@ -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