powerpc/64: Align bytes before fall back to .Lshort in powerpc64 memcmp()

powerpc/64: Align bytes before fall back to .Lshort in powerpc64 memcmp()

Currently memcmp() 64bytes version in powerpc will fall back to .Lshort
(compare per byte mode) if either src or dst address is not 8 bytes aligned.
It can be opmitized in 2 situations:

1) if both addresses are with the same offset with 8 bytes boundary:
memcmp() can compare the unaligned bytes within 8 bytes boundary firstly
and then compare the rest 8-bytes-aligned content with .Llong mode.

2)  If src/dst addrs are not with the same offset of 8 bytes boundary:
memcmp() can align src addr with 8 bytes, increment dst addr accordingly,
 then load src with aligned mode and load dst with unaligned mode.

This patch optmizes memcmp() behavior in the above 2 situations.

Tested with both little/big endian. Performance result below is based on
little endian.

Following is the test result with src/dst having the same offset case:
(a similar result was observed when src/dst having different offset):
(1) 256 bytes
Test with the existing tools/testing/selftests/powerpc/stringloops/memcmp:
- without patch
	29.773018302 seconds time elapsed                                          ( +- 0.09% )
- with patch
	16.485568173 seconds time elapsed                                          ( +-  0.02% )
		-> There is ~+80% percent improvement

(2) 32 bytes
To observe performance impact on < 32 bytes, modify
tools/testing/selftests/powerpc/stringloops/memcmp.c with following:
-------
 #include <string.h>
 #include "utils.h"

-#define SIZE 256
+#define SIZE 32
 #define ITERATIONS 10000

 int test_memcmp(const void *s1, const void *s2, size_t n);
--------

- Without patch
	0.244746482 seconds time elapsed                                          ( +-  0.36%)
- with patch
	0.215069477 seconds time elapsed                                          ( +-  0.51%)
		-> There is ～+13% improvement

(3) 0~8 bytes
To observe <8 bytes performance impact, modify
tools/testing/selftests/powerpc/stringloops/memcmp.c with following:
-------
 #include <string.h>
 #include "utils.h"

-#define SIZE 256
-#define ITERATIONS 10000
+#define SIZE 8
+#define ITERATIONS 1000000

 int test_memcmp(const void *s1, const void *s2, size_t n);
-------
- Without patch
       1.845642503 seconds time elapsed                                          ( +- 0.12% )
- With patch
       1.849767135 seconds time elapsed                                          ( +- 0.26% )
		-> They are nearly the same. (-0.2%)

Signed-off-by: Simon Guo <wei.guo.simon@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>