Le 24/05/2018 à 19:24, Segher Boessenkool a écrit :
On Wed, May 23, 2018 at 09:47:32AM +0200, Christophe Leroy wrote:
At the time being, memcmp() compares two chunks of memory
byte per byte.
This patch optimises the comparison by comparing word by word.
A small benchmark performed on an 8xx comparing two chuncks
of 512 bytes performed 100000 times gives:
Before : 5852274 TB ticks
After: 1488638 TB ticks
diff --git a/arch/powerpc/lib/string_32.S b/arch/powerpc/lib/string_32.S
index 40a576d56ac7..542e6cecbcaf 100644
--- a/arch/powerpc/lib/string_32.S
+++ b/arch/powerpc/lib/string_32.S
@@ -16,17 +16,45 @@
.text
_GLOBAL(memcmp)
- cmpwi cr0, r5, 0
- beq- 2f
- mtctr r5
- addi r6,r3,-1
- addi r4,r4,-1
-1: lbzu r3,1(r6)
- lbzu r0,1(r4)
- subf. r3,r0,r3
- bdnzt 2,1b
+ srawi. r7, r5, 2 /* Divide len by 4 */
+ mr r6, r3
+ beq- 3f
+ mtctr r7
+ li r7, 0
+1:
+#ifdef __LITTLE_ENDIAN__
+ lwbrx r3, r6, r7
+ lwbrx r0, r4, r7
+#else
+ lwzx r3, r6, r7
+ lwzx r0, r4, r7
+#endif
You don't test whether the pointers are word-aligned. Does that work?
copy_tofrom_user() word-aligns the store address and doesn't take care
of the load address, so I believe it works.
Now, I just read in the MPC885 Ref Manual that unaligned access
generates alignment exception when the processor is running in LE mode.
Ref. made to the discussion on patch "powerpc/32be: use stmw/lmw for
registers save/restore in asm"
(https://patchwork.ozlabs.org/patch/899465/), I will drop the handling
for LE mode.
Christophe
Say, when a load is crossing a page boundary, or segment boundary.
Segher
