strings: use runtime assembly for IndexByte

Fixes #3751

R=golang-dev, khr
CC=golang-dev
https://golang.org/cl/12483043

src/pkg/runtime/asm_386.s

@@ -1220,6 +1220,20 @@ TEXT bytes·IndexByte(SB),7,$0
 	MOVL	DI, ret+16(FP)
 	RET
 
+TEXT strings·IndexByte(SB),7,$0
+	MOVL	s+0(FP), SI
+	MOVL	s_len+4(FP), CX
+	MOVB	c+8(FP), AL
+	MOVL	SI, DI
+	CLD; REPN; SCASB
+	JZ 3(PC)
+	MOVL	$-1, ret+12(FP)
+	RET
+	SUBL	SI, DI
+	SUBL	$1, DI
+	MOVL	DI, ret+12(FP)
+	RET
+
 // input:
 //   SI = a
 //   DI = b

src/pkg/runtime/asm_amd64.s

@@ -1224,6 +1224,25 @@ TEXT bytes·IndexByte(SB),7,$0
 	MOVQ s+0(FP), SI
 	MOVQ s_len+8(FP), BX
 	MOVB c+24(FP), AL
+	CALL runtime·indexbytebody(SB)
+	MOVQ AX, ret+32(FP)
+	RET
+
+TEXT strings·IndexByte(SB),7,$0
+	MOVQ s+0(FP), SI
+	MOVQ s_len+8(FP), BX
+	MOVB c+16(FP), AL
+	CALL runtime·indexbytebody(SB)
+	MOVQ AX, ret+24(FP)
+	RET
+
+// input:
+//   SI: data
+//   BX: data len
+//   AL: byte sought
+// output:
+//   AX
+TEXT runtime·indexbytebody(SB),7,$0
 	MOVQ SI, DI
 
 	CMPQ BX, $16
@@ -1281,7 +1300,7 @@ condition:
 	JZ success
 
 failure:
-	MOVQ $-1, ret+32(FP)
+	MOVQ $-1, AX
 	RET
 
 // handle for lengths < 16
@@ -1289,7 +1308,7 @@ indexbyte_small:
 	MOVQ BX, CX
 	REPN; SCASB
 	JZ success
-	MOVQ $-1, ret+32(FP)
+	MOVQ $-1, AX
 	RET
 
 // we've found the chunk containing the byte
@@ -1299,13 +1318,13 @@ ssesuccess:
 	BSFW DX, DX
 	SUBQ SI, DI
 	ADDQ DI, DX
-	MOVQ DX, ret+32(FP)
+	MOVQ DX, AX
 	RET
 
 success:
 	SUBQ SI, DI
 	SUBL $1, DI
-	MOVQ DI, ret+32(FP)
+	MOVQ DI, AX
 	RET
 
 TEXT bytes·Equal(SB),7,$0-49

src/pkg/runtime/asm_arm.s

@@ -667,3 +667,27 @@ _notfound:
 	MOVW	$-1, R0
 	MOVW	R0, ret+16(FP)
 	RET
+
+TEXT strings·IndexByte(SB),7,$0
+	MOVW	s+0(FP), R0
+	MOVW	s_len+4(FP), R1
+	MOVBU	c+8(FP), R2	// byte to find
+	MOVW	R0, R4		// store base for later
+	ADD	R0, R1		// end 
+
+_sib_loop:
+	CMP	R0, R1
+	B.EQ	_sib_notfound
+	MOVBU.P	1(R0), R3
+	CMP	R2, R3
+	B.NE	_sib_loop
+
+	SUB	$1, R0		// R0 will be one beyond the position we want
+	SUB	R4, R0		// remove base
+	MOVW	R0, ret+12(FP) 
+	RET
+
+_sib_notfound:
+	MOVW	$-1, R0
+	MOVW	R0, ret+12(FP)
+	RET

src/pkg/strings/strings.go

@@ -160,16 +160,6 @@ func Index(s, sep string) int {
 	return -1
 }
 
-// IndexByte returns the index of the first instance of c in s, or -1 if c is not present in s.
-func IndexByte(s string, c byte) int {
-	for i := 0; i < len(s); i++ {
-		if s[i] == c {
-			return i
-		}
-	}
-	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/pkg/strings/strings.s

@@ -0,0 +1,5 @@
+// Copyright 2013 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.
+
+// This file is here just to make the go tool happy.

src/pkg/strings/strings_decl.go

@@ -0,0 +1,8 @@
+// Copyright 2013 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
+
+// IndexByte returns the index of the first instance of c in s, or -1 if c is not present in s.
+func IndexByte(s string, c byte) int // ../runtime/asm_$GOARCH.s