linux-kernel - [PATCH 3/7] powerpc/lib: optimise PPC32 memcmp

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Message-Id: <9a28569116a3ddc89f2c14ebed81dcca74dac7bc.1523950415.git.christophe.leroy@c-s.fr>
Date:   Tue, 17 Apr 2018 09:38:39 +0200 (CEST)
From:   Christophe Leroy <christophe.leroy@....fr>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: [PATCH  3/7] powerpc/lib: optimise PPC32 memcmp

At the time being, memcmp() compares two chunks of memory
byte per byte.

This patch optimised the comparison by comparing word by word.

A small benchmark performed on an 8xx based on the comparison
of two chuncks of 512 bytes performed 100000 times gives:

Before : 5852274 TB ticks
After:   1488638 TB ticks

This is almost 4 times faster

Signed-off-by: Christophe Leroy <christophe.leroy@....fr>
---
 arch/powerpc/lib/string_32.S | 42 +++++++++++++++++++++++++++++++++++-------
 1 file changed, 35 insertions(+), 7 deletions(-)

diff --git a/arch/powerpc/lib/string_32.S b/arch/powerpc/lib/string_32.S
index 94e9c9bc31c3..5b2a73fb07be 100644
--- a/arch/powerpc/lib/string_32.S
+++ b/arch/powerpc/lib/string_32.S
@@ -19,13 +19,41 @@ _GLOBAL(memcmp)
 	PPC_LCMPI 0,r5,0
 	beq-	2f
 #endif
-	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
+	addi	r7, r7, 4
+	subf.	r3, r0, r3
+	bdnzt	eq, 1b
+	bnelr
+	andi.	r5, r5, 3
+	beqlr
+3:	cmplwi	cr1, r5, 2
+	blt-	cr1, 4f
+#ifdef __LITTLE_ENDIAN__
+	lhbrx	r3, r6, r7
+	lhbrx	r0, r4, r7
+#else
+	lhzx	r3, r6, r7
+	lhzx	r0, r4, r7
+#endif
+	addi	r7, r7, 2
+	subf.	r3, r0, r3
+	beqlr	cr1
+	bnelr
+4:	lbzx	r3, r6, r7
+	lbzx	r0, r4, r7
+	subf.	r3, r0, r3
 	blr
 #ifdef CONFIG_FORTIFY_SOURCE
 2:	li	r3,0
-- 
2.13.3