strings: remove overengineered Compare implementation

The function is here ONLY for symmetry with package bytes.
This function should be used ONLY if it makes code clearer.
It is not here for performance. Remove any performance benefit.

If performance becomes an issue, the compiler should be fixed to
recognize the three-way compare (for all comparable types)
rather than encourage people to micro-optimize by using this function.

Change-Id: I71f4130bce853f7aef724c6044d15def7987b457
Reviewed-on: https://go-review.googlesource.com/3012
Reviewed-by: Rob Pike <r@golang.org>

src/runtime/asm_386.s

@@ -1431,9 +1431,6 @@ TEXT runtime·cmpstring(SB),NOSPLIT,$0-20
 	MOVL	AX, ret+16(FP)
 	RET
 
-TEXT strings·Compare(SB),NOSPLIT,$0
-        JMP	runtime·cmpstring(SB)
-
 TEXT bytes·Compare(SB),NOSPLIT,$0-28
 	MOVL	s1+0(FP), SI
 	MOVL	s1+4(FP), BX

src/runtime/asm_amd64.s

@@ -1364,9 +1364,6 @@ TEXT runtime·cmpstring(SB),NOSPLIT,$0-40
 	MOVQ	AX, ret+32(FP)
 	RET
 
-TEXT strings·Compare(SB),NOSPLIT,$0
-        JMP	runtime·cmpstring(SB)
-
 TEXT bytes·Compare(SB),NOSPLIT,$0-56
 	MOVQ	s1+0(FP), SI
 	MOVQ	s1+8(FP), BX

src/runtime/asm_amd64p32.s

@@ -832,9 +832,6 @@ TEXT runtime·cmpstring(SB),NOSPLIT,$0-20
 	MOVL	AX, ret+16(FP)
 	RET
 
-TEXT strings·Compare(SB),NOSPLIT,$0
-        JMP	runtime·cmpstring(SB)
-
 TEXT bytes·Compare(SB),NOSPLIT,$0-28
 	MOVL	s1+0(FP), SI
 	MOVL	s1+4(FP), BX

src/runtime/noasm.go

@@ -10,11 +10,6 @@ package runtime
 
 import _ "unsafe" // for go:linkname
 
-//go:linkname strings_Compare strings.Compare
-func strings_Compare(s1, s2 string) int {
-	return cmpstring(s1, s2)
-}
-
 func cmpstring(s1, s2 string) int {
 	l := len(s1)
 	if len(s2) < l {

src/strings/compare.go

@@ -0,0 +1,28 @@
+// 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
+
+// Compare returns an integer comparing two strings lexicographically.
+// The result will be 0 if a==b, -1 if a < b, and +1 if a > b.
+//
+// Compare is included only for symmetry with package bytes.
+// It is usually clearer and always faster to use the built-in
+// string comparison operators ==, <, >, and so on.
+func Compare(a, b string) int {
+	// NOTE(rsc): This function does NOT call the runtime cmpstring function,
+	// because we do not want to provide any performance justification for
+	// using strings.Compare. Basically no one should use strings.Compare.
+	// As the comment above says, it is here only for symmetry with package bytes.
+	// If performance is important, the compiler should be changed to recognize
+	// the pattern so that all code doing three-way comparisons, not just code
+	// using strings.Compare, can benefit.
+	if a == b {
+		return 0
+	}
+	if a < b {
+		return -1
+	}
+	return +1
+}

src/strings/strings_decl.go

@@ -6,10 +6,3 @@ 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
-
-// Compare returns an integer comparing two strings lexicographically.
-// The result will be 0 if a==b, -1 if a < b, and +1 if a > b.
-//
-// In most cases it is simpler to use the built-in comparison operators
-// ==, <, >, and so on.
-func Compare(a, b string) int // ../runtime/noasm.go or ../runtime/asm_*.s