Re: [PATCH v5 2/4] powerpc/64: enhance memcmp() with VMX instruction for long bytes comparision

Hi Michael,
On Thu, May 24, 2018 at 05:44:33PM +1000, Michael Ellerman wrote:

Hi Simon,

wei.guo.simon@gmail.com writes:

quoted

From: Simon Guo <redacted>

This patch add VMX primitives to do memcmp() in case the compare size
exceeds 4K bytes. KSM feature can benefit from this.

You say "exceeds 4K" here.

it should be >= 4k. I will correct the message.

quoted

diff --git a/arch/powerpc/lib/memcmp_64.S b/arch/powerpc/lib/memcmp_64.S
index f20e883..6303bbf 100644
--- a/arch/powerpc/lib/memcmp_64.S
+++ b/arch/powerpc/lib/memcmp_64.S

@@ -27,12 +27,73 @@
 #define LH	lhbrx
 #define LW	lwbrx
 #define LD	ldbrx
+#define LVS	lvsr
+#define VPERM(_VRT,_VRA,_VRB,_VRC) \
+	vperm _VRT,_VRB,_VRA,_VRC
 #else
 #define LH	lhzx
 #define LW	lwzx
 #define LD	ldx
+#define LVS	lvsl
+#define VPERM(_VRT,_VRA,_VRB,_VRC) \
+	vperm _VRT,_VRA,_VRB,_VRC
 #endif
 
+#define VMX_OPS_THRES 4096

THRES == 4096

BTW, can we call it VMX_THRESH ?

Sure. I will update it.

quoted

+#define ENTER_VMX_OPS	\
+	mflr    r0;	\
+	std     r3,-STACKFRAMESIZE+STK_REG(R31)(r1); \
+	std     r4,-STACKFRAMESIZE+STK_REG(R30)(r1); \
+	std     r5,-STACKFRAMESIZE+STK_REG(R29)(r1); \
+	std     r0,16(r1); \
+	stdu    r1,-STACKFRAMESIZE(r1); \
+	bl      enter_vmx_ops; \
+	cmpwi   cr1,r3,0; \
+	ld      r0,STACKFRAMESIZE+16(r1); \
+	ld      r3,STK_REG(R31)(r1); \
+	ld      r4,STK_REG(R30)(r1); \
+	ld      r5,STK_REG(R29)(r1); \
+	addi	r1,r1,STACKFRAMESIZE; \
+	mtlr    r0
+
+#define EXIT_VMX_OPS \
+	mflr    r0; \
+	std     r3,-STACKFRAMESIZE+STK_REG(R31)(r1); \
+	std     r4,-STACKFRAMESIZE+STK_REG(R30)(r1); \
+	std     r5,-STACKFRAMESIZE+STK_REG(R29)(r1); \
+	std     r0,16(r1); \
+	stdu    r1,-STACKFRAMESIZE(r1); \
+	bl      exit_vmx_ops; \
+	ld      r0,STACKFRAMESIZE+16(r1); \
+	ld      r3,STK_REG(R31)(r1); \
+	ld      r4,STK_REG(R30)(r1); \
+	ld      r5,STK_REG(R29)(r1); \
+	addi	r1,r1,STACKFRAMESIZE; \
+	mtlr    r0
+
+/*
+ * LD_VSR_CROSS16B load the 2nd 16 bytes for _vaddr which is unaligned with
+ * 16 bytes boundary and permute the result with the 1st 16 bytes.
+
+ *    |  y y y y y y y y y y y y y 0 1 2 | 3 4 5 6 7 8 9 a b c d e f z z z |
+ *    ^                                  ^                                 ^
+ * 0xbbbb10                          0xbbbb20                          0xbbb30
+ *                                 ^
+ *                                _vaddr
+ *
+ *
+ * _vmask is the mask generated by LVS
+ * _v1st_qw is the 1st aligned QW of current addr which is already loaded.
+ *   for example: 0xyyyyyyyyyyyyy012 for big endian
+ * _v2nd_qw is the 2nd aligned QW of cur _vaddr to be loaded.
+ *   for example: 0x3456789abcdefzzz for big endian
+ * The permute result is saved in _v_res.
+ *   for example: 0x0123456789abcdef for big endian.
+ */
+#define LD_VSR_CROSS16B(_vaddr,_vmask,_v1st_qw,_v2nd_qw,_v_res) \
+        lvx     _v2nd_qw,_vaddr,off16; \
+        VPERM(_v_res,_v1st_qw,_v2nd_qw,_vmask)
+
 /*
  * There are 2 categories for memcmp:
  * 1) src/dst has the same offset to the 8 bytes boundary. The handlers

@@ -174,6 +235,13 @@ _GLOBAL(memcmp)
 	blr
 
 .Llong:
+#ifdef CONFIG_ALTIVEC
+	/* Try to use vmx loop if length is larger than 4K */
+	cmpldi  cr6,r5,VMX_OPS_THRES
+	bge	cr6,.Lsameoffset_vmx_cmp

Here we compare the length to 4K and if it's greater *or equal* then we
go to the VMX case. Or am I reading it backward?

So we should say "if the size is 4K or more we do VMX" shouldn't we?

Yes. Again I need reword the comment to "equal or greater than 4K"
here.

Thanks,
- Simon