powerpc: 64bit optimised __clear_user

I noticed __clear_user high up in a profile of one of my RAID stress
tests. The testcase was doing a dd from /dev/zero which ends up
calling __clear_user.

__clear_user is basically a loop with a single 4 byte store which
is horribly slow. We can do much better by aligning the desination
and doing 32 bytes of 8 byte stores in a loop.

The following testcase was used to verify the patch:

http://ozlabs.org/~anton/junkcode/stress_clear_user.c

To show the improvement in performance I ran a dd from /dev/zero
to /dev/null on a POWER7 box:

Before:

# dd if=/dev/zero of=/dev/null bs=1M count=10000
10485760000 bytes (10 GB) copied, 3.72379 s, 2.8 GB/s

After:

# time dd if=/dev/zero of=/dev/null bs=1M count=10000
10485760000 bytes (10 GB) copied, 0.728318 s, 14.4 GB/s

Over 5x faster.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

1 files changed, 2 insertions, 0 deletions

diff --git a/arch/powerpc/lib/string.S b/arch/powerpc/lib/string.S
index 093d6316435c..1b5a0a09d609 100644
--- a/arch/powerpc/lib/string.S
+++ b/arch/powerpc/lib/string.S
@@ -119,6 +119,7 @@ _GLOBAL(memchr)
 2:      li      r3,0
         blr
 
+#ifdef CONFIG_PPC32
 _GLOBAL(__clear_user)
         addi    r6,r3,-4
         li      r3,0
@@ -160,3 +161,4 @@ _GLOBAL(__clear_user)
         PPC_LONG        1b,91b
         PPC_LONG        8b,92b
         .text
+#endif