runtime: optimize string comparison on amd64

Use AVX2 if possible.
Results below (haswell):

name                            old time/op    new time/op     delta
CompareStringEqual-6              8.77ns ± 0%     8.63ns ± 1%   -1.58%        (p=0.000 n=20+19)
CompareStringIdentical-6          5.02ns ± 0%     5.02ns ± 0%     ~     (all samples are equal)
CompareStringSameLength-6         7.51ns ± 0%     7.51ns ± 0%     ~     (all samples are equal)
CompareStringDifferentLength-6    1.56ns ± 0%     1.56ns ± 0%     ~     (all samples are equal)
CompareStringBigUnaligned-6        124µs ± 1%      105µs ± 5%  -14.99%        (p=0.000 n=20+18)
CompareStringBig-6                 112µs ± 1%      103µs ± 0%   -7.87%        (p=0.000 n=20+17)

name                            old speed      new speed       delta
CompareStringBigUnaligned-6     8.48GB/s ± 1%   9.98GB/s ± 5%  +17.67%        (p=0.000 n=20+18)
CompareStringBig-6              9.37GB/s ± 1%  10.17GB/s ± 0%   +8.54%        (p=0.000 n=20+17)

Change-Id: I1c949626dd2aaf9f633e3c888a9df71c82eed7e1
Reviewed-on: https://go-review.googlesource.com/16481Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Klaus Post <klauspost@gmail.com>

src/cmd/internal/obj/x86/a.out.go

@@ -748,6 +748,7 @@ const (
 	AMOVHDA
 	AVPCMPEQB
 	AVPMOVMSKB
+	AVPAND
 
 	// from 386
 	AJCXZW

src/cmd/internal/obj/x86/anames.go

@@ -689,6 +689,7 @@ var Anames = []string{
 	"MOVHDA",
 	"VPCMPEQB",
 	"VPMOVMSKB",
+	"VPAND",
 	"JCXZW",
 	"FCMOVCC",
 	"FCMOVCS",

src/cmd/internal/obj/x86/asm6.go

@@ -1509,6 +1509,7 @@ var optab =
 	{AMOVHDA, yxmov_vex, Pvex1, [23]uint8{0x6f, 0x7f}},
 	{AVPCMPEQB, yxm_xm_xm, Pvex1, [23]uint8{0x74, 0x74}},
 	{AVPMOVMSKB, ymskb_vex, Pvex1, [23]uint8{0xd7}},
+	{AVPAND, yxm_xm_xm, Pvex1, [23]uint8{0xdb, 0xdb}},
 	{obj.AUSEFIELD, ynop, Px, [23]uint8{0, 0}},
 	{obj.ATYPE, nil, 0, [23]uint8{}},
 	{obj.AFUNCDATA, yfuncdata, Px, [23]uint8{0, 0}},

src/runtime/asm_amd64.s

@@ -1416,6 +1416,10 @@ eq:
 TEXT runtime·memeqbody(SB),NOSPLIT,$0-0
 	CMPQ	BX, $8
 	JB	small
+	CMPQ	BX, $64
+	JB	bigloop
+	CMPB    runtime·support_avx2(SB), $1
+	JE	hugeloop_avx2
 	
 	// 64 bytes at a time using xmm registers
 hugeloop:
@@ -1445,6 +1449,30 @@ hugeloop:
 	MOVB	$0, (AX)
 	RET
 
+	// 64 bytes at a time using ymm registers
+hugeloop_avx2:
+	CMPQ	BX, $64
+	JB	bigloop_avx2
+	MOVHDU	(SI), X0
+	MOVHDU	(DI), X1
+	MOVHDU	32(SI), X2
+	MOVHDU	32(DI), X3
+	VPCMPEQB	X1, X0, X4
+	VPCMPEQB	X2, X3, X5
+	VPAND	X4, X5, X6
+	VPMOVMSKB X6, DX
+	ADDQ	$64, SI
+	ADDQ	$64, DI
+	SUBQ	$64, BX
+	CMPL	DX, $0xffffffff
+	JEQ	hugeloop_avx2
+	VZEROUPPER
+	MOVB	$0, (AX)
+	RET
+
+bigloop_avx2:
+	VZEROUPPER
+
 	// 8 bytes at a time using 64-bit register
 bigloop:
 	CMPQ	BX, $8