strings: add asm version of Index() for short strings on amd64

Currently we have special case for 1-byte strings,
This extends this to strings shorter than 32 bytes on amd64.
Results (broadwell):

name                 old time/op  new time/op  delta
IndexRune-4          57.4ns ± 0%  57.5ns ± 0%   +0.10%        (p=0.000 n=20+19)
IndexRuneFastPath-4  20.4ns ± 0%  20.4ns ± 0%     ~     (all samples are equal)
Index-4              21.0ns ± 0%  21.8ns ± 0%   +3.81%        (p=0.000 n=20+20)
LastIndex-4          7.07ns ± 1%  6.98ns ± 0%   -1.21%        (p=0.000 n=20+16)
IndexByte-4          18.3ns ± 0%  18.3ns ± 0%     ~     (all samples are equal)
IndexHard1-4         1.46ms ± 0%  0.39ms ± 0%  -73.06%        (p=0.000 n=16+16)
IndexHard2-4         1.46ms ± 0%  0.30ms ± 0%  -79.55%        (p=0.000 n=18+18)
IndexHard3-4         1.46ms ± 0%  0.66ms ± 0%  -54.68%        (p=0.000 n=19+19)
LastIndexHard1-4     1.46ms ± 0%  1.46ms ± 0%   -0.01%        (p=0.036 n=18+20)
LastIndexHard2-4     1.46ms ± 0%  1.46ms ± 0%     ~           (p=0.588 n=19+19)
LastIndexHard3-4     1.46ms ± 0%  1.46ms ± 0%     ~           (p=0.283 n=17+20)
IndexTorture-4       11.1µs ± 0%  11.1µs ± 0%   +0.01%        (p=0.000 n=18+17)

Change-Id: I892781549f558f698be4e41f9f568e3d0611efb5
Reviewed-on: https://go-review.googlesource.com/16430
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>

src/runtime/asm_amd64.s

@@ -1725,6 +1725,168 @@ big_loop_avx2_exit:
 	JMP loop
 
 
+// TODO: Also use this in bytes.Index
+TEXT strings·indexShortStr(SB),NOSPLIT,$0-40
+	MOVQ s+0(FP), DI
+	MOVQ s_len+8(FP), CX
+	MOVQ c+16(FP), AX
+	MOVQ c_len+24(FP), BX
+	CMPQ BX, CX
+	JA fail
+	CMPQ BX, $2
+	JA   _3_or_more
+	MOVW (AX), AX
+	LEAQ -1(DI)(CX*1), CX
+loop2:
+	MOVW (DI), SI
+	CMPW SI,AX
+	JZ success
+	ADDQ $1,DI
+	CMPQ DI,CX
+	JB loop2
+	JMP fail
+_3_or_more:
+	CMPQ BX, $3
+	JA   _4_or_more
+	MOVW 1(AX), DX
+	MOVW (AX), AX
+	LEAQ -2(DI)(CX*1), CX
+loop3:
+	MOVW (DI), SI
+	CMPW SI,AX
+	JZ   partial_success3
+	ADDQ $1,DI
+	CMPQ DI,CX
+	JB loop3
+	JMP fail
+partial_success3:
+	MOVW 1(DI), SI
+	CMPW SI,DX
+	JZ success
+	ADDQ $1,DI
+	CMPQ DI,CX
+	JB loop3
+	JMP fail
+_4_or_more:
+	CMPQ BX, $4
+	JA   _5_or_more
+	MOVL (AX), AX
+	LEAQ -3(DI)(CX*1), CX
+loop4:
+	MOVL (DI), SI
+	CMPL SI,AX
+	JZ   success
+	ADDQ $1,DI
+	CMPQ DI,CX
+	JB loop4
+	JMP fail
+_5_or_more:
+	CMPQ BX, $7
+	JA   _8_or_more
+	LEAQ 1(DI)(CX*1), CX
+	SUBQ BX, CX
+	MOVL -4(AX)(BX*1), DX
+	MOVL (AX), AX
+loop5to7:
+	MOVL (DI), SI
+	CMPL SI,AX
+	JZ   partial_success5to7
+	ADDQ $1,DI
+	CMPQ DI,CX
+	JB loop5to7
+	JMP fail
+partial_success5to7:
+	MOVL -4(BX)(DI*1), SI
+	CMPL SI,DX
+	JZ success
+	ADDQ $1,DI
+	CMPQ DI,CX
+	JB loop5to7
+	JMP fail
+_8_or_more:
+	CMPQ BX, $8
+	JA   _9_or_more
+	MOVQ (AX), AX
+	LEAQ -7(DI)(CX*1), CX
+loop8:
+	MOVQ (DI), SI
+	CMPQ SI,AX
+	JZ   success
+	ADDQ $1,DI
+	CMPQ DI,CX
+	JB loop8
+	JMP fail
+_9_or_more:
+	CMPQ BX, $16
+	JA   _16_or_more
+	LEAQ 1(DI)(CX*1), CX
+	SUBQ BX, CX
+	MOVQ -8(AX)(BX*1), DX
+	MOVQ (AX), AX
+loop9to15:
+	MOVQ (DI), SI
+	CMPQ SI,AX
+	JZ   partial_success9to15
+	ADDQ $1,DI
+	CMPQ DI,CX
+	JB loop9to15
+	JMP fail
+partial_success9to15:
+	MOVQ -8(BX)(DI*1), SI
+	CMPQ SI,DX
+	JZ success
+	ADDQ $1,DI
+	CMPQ DI,CX
+	JB loop9to15
+	JMP fail
+_16_or_more:
+	CMPQ BX, $16
+	JA   _17_to_31
+	MOVOU (AX), X1
+	LEAQ -15(DI)(CX*1), CX
+loop16:
+	MOVOU (DI), X2
+	PCMPEQB X1, X2
+	PMOVMSKB X2, SI
+	CMPQ  SI, $0xffff
+	JE   success
+	ADDQ $1,DI
+	CMPQ DI,CX
+	JB loop16
+	JMP fail
+_17_to_31:
+	LEAQ 1(DI)(CX*1), CX
+	SUBQ BX, CX
+	MOVOU -16(AX)(BX*1), X0
+	MOVOU (AX), X1
+loop17to31:
+	MOVOU (DI), X2
+	PCMPEQB X1,X2
+	PMOVMSKB X2, SI
+	CMPQ  SI, $0xffff
+	JE   partial_success17to31
+	ADDQ $1,DI
+	CMPQ DI,CX
+	JB loop17to31
+	JMP fail
+partial_success17to31:
+	MOVOU -16(BX)(DI*1), X3
+	PCMPEQB X0, X3
+	PMOVMSKB X3, SI
+	CMPQ  SI, $0xffff
+	JE success
+	ADDQ $1,DI
+	CMPQ DI,CX
+	JB loop17to31
+fail:
+	MOVQ $-1, ret+32(FP)
+	RET
+success:
+	SUBQ s+0(FP), DI
+	MOVQ DI, ret+32(FP)
+	RET
+
+
 TEXT bytes·IndexByte(SB),NOSPLIT,$0-40
 	MOVQ s+0(FP), SI
 	MOVQ s_len+8(FP), BX

src/strings/strings.go

@@ -143,43 +143,6 @@ func ContainsRune(s string, r rune) bool {
 	return IndexRune(s, r) >= 0
 }
 
-// Index returns the index of the first instance of sep in s, or -1 if sep is not present in s.
-func Index(s, sep string) int {
-	n := len(sep)
-	switch {
-	case n == 0:
-		return 0
-	case n == 1:
-		return IndexByte(s, sep[0])
-	case n == len(s):
-		if sep == s {
-			return 0
-		}
-		return -1
-	case n > len(s):
-		return -1
-	}
-	// Rabin-Karp search
-	hashsep, pow := hashStr(sep)
-	var h uint32
-	for i := 0; i < n; i++ {
-		h = h*primeRK + uint32(s[i])
-	}
-	if h == hashsep && s[:n] == sep {
-		return 0
-	}
-	for i := n; i < len(s); {
-		h *= primeRK
-		h += uint32(s[i])
-		h -= pow * uint32(s[i-n])
-		i++
-		if h == hashsep && s[i-n:i] == sep {
-			return i - n
-		}
-	}
-	return -1
-}
-
 // LastIndex returns the index of the last instance of sep in s, or -1 if sep is not present in s.
 func LastIndex(s, sep string) int {
 	n := len(sep)

src/strings/strings_amd64.go

@@ -0,0 +1,49 @@
+// Copyright 2015 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package strings
+
+// indexShortStr returns the index of the first instance of c in s, or -1 if c is not present in s.
+// indexShortStr requires 2 <= len(c) <= shortStringLen
+func indexShortStr(s, c string) int // ../runtime/asm_$GOARCH.s
+const shortStringLen = 31
+
+// Index returns the index of the first instance of sep in s, or -1 if sep is not present in s.
+func Index(s, sep string) int {
+	n := len(sep)
+	switch {
+	case n == 0:
+		return 0
+	case n == 1:
+		return IndexByte(s, sep[0])
+	case n <= shortStringLen:
+		return indexShortStr(s, sep)
+	case n == len(s):
+		if sep == s {
+			return 0
+		}
+		return -1
+	case n > len(s):
+		return -1
+	}
+	// Rabin-Karp search
+	hashsep, pow := hashStr(sep)
+	var h uint32
+	for i := 0; i < n; i++ {
+		h = h*primeRK + uint32(s[i])
+	}
+	if h == hashsep && s[:n] == sep {
+		return 0
+	}
+	for i := n; i < len(s); {
+		h *= primeRK
+		h += uint32(s[i])
+		h -= pow * uint32(s[i-n])
+		i++
+		if h == hashsep && s[i-n:i] == sep {
+			return i - n
+		}
+	}
+	return -1
+}

src/strings/strings_generic.go

@@ -0,0 +1,47 @@
+// Copyright 2015 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build !amd64
+
+package strings
+
+// TODO: implements short string optimization on non amd64 platforms
+// and get rid of strings_amd64.go
+
+// Index returns the index of the first instance of sep in s, or -1 if sep is not present in s.
+func Index(s, sep string) int {
+	n := len(sep)
+	switch {
+	case n == 0:
+		return 0
+	case n == 1:
+		return IndexByte(s, sep[0])
+	case n == len(s):
+		if sep == s {
+			return 0
+		}
+		return -1
+	case n > len(s):
+		return -1
+	}
+	// Rabin-Karp search
+	hashsep, pow := hashStr(sep)
+	var h uint32
+	for i := 0; i < n; i++ {
+		h = h*primeRK + uint32(s[i])
+	}
+	if h == hashsep && s[:n] == sep {
+		return 0
+	}
+	for i := n; i < len(s); {
+		h *= primeRK
+		h += uint32(s[i])
+		h -= pow * uint32(s[i-n])
+		i++
+		if h == hashsep && s[i-n:i] == sep {
+			return i - n
+		}
+	}
+	return -1
+}

src/strings/strings_test.go

@@ -59,6 +59,59 @@ var indexTests = []IndexTest{
 	{"abc", "b", 1},
 	{"abc", "c", 2},
 	{"abc", "x", -1},
+	// test special cases in Index() for short strings
+	{"", "ab", -1},
+	{"bc", "ab", -1},
+	{"ab", "ab", 0},
+	{"xab", "ab", 1},
+	{"xab"[:2], "ab", -1},
+	{"", "abc", -1},
+	{"xbc", "abc", -1},
+	{"abc", "abc", 0},
+	{"xabc", "abc", 1},
+	{"xabc"[:3], "abc", -1},
+	{"xabxc", "abc", -1},
+	{"", "abcd", -1},
+	{"xbcd", "abcd", -1},
+	{"abcd", "abcd", 0},
+	{"xabcd", "abcd", 1},
+	{"xyabcd"[:5], "abcd", -1},
+	{"xbcqq", "abcqq", -1},
+	{"abcqq", "abcqq", 0},
+	{"xabcqq", "abcqq", 1},
+	{"xyabcqq"[:6], "abcqq", -1},
+	{"xabxcqq", "abcqq", -1},
+	{"xabcqxq", "abcqq", -1},
+	{"", "01234567", -1},
+	{"32145678", "01234567", -1},
+	{"01234567", "01234567", 0},
+	{"x01234567", "01234567", 1},
+	{"xx01234567"[:9], "01234567", -1},
+	{"", "0123456789", -1},
+	{"3214567844", "0123456789", -1},
+	{"0123456789", "0123456789", 0},
+	{"x0123456789", "0123456789", 1},
+	{"xyz0123456789"[:12], "0123456789", -1},
+	{"x01234567x89", "0123456789", -1},
+	{"", "0123456789012345", -1},
+	{"3214567889012345", "0123456789012345", -1},
+	{"0123456789012345", "0123456789012345", 0},
+	{"x0123456789012345", "0123456789012345", 1},
+	{"", "01234567890123456789", -1},
+	{"32145678890123456789", "01234567890123456789", -1},
+	{"01234567890123456789", "01234567890123456789", 0},
+	{"x01234567890123456789", "01234567890123456789", 1},
+	{"xyz01234567890123456789"[:22], "01234567890123456789", -1},
+	{"", "0123456789012345678901234567890", -1},
+	{"321456788901234567890123456789012345678911", "0123456789012345678901234567890", -1},
+	{"0123456789012345678901234567890", "0123456789012345678901234567890", 0},
+	{"x0123456789012345678901234567890", "0123456789012345678901234567890", 1},
+	{"xyz0123456789012345678901234567890"[:33], "0123456789012345678901234567890", -1},
+	{"", "01234567890123456789012345678901", -1},
+	{"32145678890123456789012345678901234567890211", "01234567890123456789012345678901", -1},
+	{"01234567890123456789012345678901", "01234567890123456789012345678901", 0},
+	{"x01234567890123456789012345678901", "01234567890123456789012345678901", 1},
+	{"xyz01234567890123456789012345678901"[:34], "01234567890123456789012345678901", -1},
 }
 
 var lastIndexTests = []IndexTest{