[PATCH v6 09/10] crc32-optimize-loops-for-x86.diff

Bob Pearson
2011-08-31T22:30:46Z

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.

Signed-off-by: Bob Pearson <rpearson@systemfabricworks.com>

---
lib/crc32.c | 13 +++++++++++++
1 file changed, 13 insertions(+)

Index: for-next/lib/crc32.c
===================================================================
--- for-next.orig/lib/crc32.c
+++ for-next/lib/crc32.c
@@ -66,6 +66,9 @@ crc32_body(u32 crc, unsigned char const
# 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
# 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
/* 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