powerpc: Add 64bit optimised memcmp

I noticed ksm spending quite a lot of time in memcmp on a large
KVM box. The current memcmp loop is very unoptimised - byte at a
time compares with no loop unrolling. We can do much much better.

Optimise the loop in a few ways:

- Unroll the byte at a time loop

- For large (at least 32 byte) comparisons that are also 8 byte
  aligned, use an unrolled modulo scheduled loop using 8 byte
  loads. This is similar to our glibc memcmp.

A simple microbenchmark testing 10000000 iterations of an 8192 byte
memcmp was used to measure the performance:

baseline:	29.93 s

modified:	 1.70 s

Just over 17x faster.

v2: Incorporated some suggestions from Segher:

- Use andi. instead of rdlicl.

- Convert bdnzt eq, to bdnz. It's just duplicating the earlier compare
  and was a relic from a previous version.

- Don't use cr5, we have plans to use that CR field for fast local
  atomics.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/lib/Makefile

@@ -15,7 +15,8 @@ obj-$(CONFIG_PPC32)	+= div64.o copy_32.o
 
 obj-$(CONFIG_PPC64)	+= copypage_64.o copyuser_64.o \
 			   usercopy_64.o mem_64.o hweight_64.o \
-			   copyuser_power7.o string_64.o copypage_power7.o
+			   copyuser_power7.o string_64.o copypage_power7.o \
+			   memcmp_64.o
 ifeq ($(CONFIG_GENERIC_CSUM),)
 obj-y			+= checksum_$(CONFIG_WORD_SIZE).o
 obj-$(CONFIG_PPC64)	+= checksum_wrappers_64.o