strings: add Compare(x, y string) int, for symmetry with bytes.Compare

The implementation is the same assembly (or Go) routine.

Change-Id: Ib937c461c24ad2d5be9b692b4eed40d9eb031412
Reviewed-on: https://go-review.googlesource.com/2828Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/runtime/asm_386.s

@@ -1431,6 +1431,9 @@ 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

@@ -1364,6 +1364,9 @@ 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

@@ -832,6 +832,9 @@ 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

@@ -10,6 +10,11 @@ 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_test.go

+// 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_test
+
+// Derived from bytes/compare_test.go.
+// Benchmarks omitted since the underlying implementation is identical.
+
+import (
+	. "strings"
+	"testing"
+)
+
+var compareTests = []struct {
+	a, b string
+	i    int
+}{
+	{"", "", 0},
+	{"a", "", 1},
+	{"", "a", -1},
+	{"abc", "abc", 0},
+	{"ab", "abc", -1},
+	{"abc", "ab", 1},
+	{"x", "ab", 1},
+	{"ab", "x", -1},
+	{"x", "a", 1},
+	{"b", "x", -1},
+	// test runtime·memeq's chunked implementation
+	{"abcdefgh", "abcdefgh", 0},
+	{"abcdefghi", "abcdefghi", 0},
+	{"abcdefghi", "abcdefghj", -1},
+}
+
+func TestCompare(t *testing.T) {
+	for _, tt := range compareTests {
+		cmp := Compare(tt.a, tt.b)
+		if cmp != tt.i {
+			t.Errorf(`Compare(%q, %q) = %v`, tt.a, tt.b, cmp)
+		}
+	}
+}
+
+func TestCompareIdenticalString(t *testing.T) {
+	var s = "Hello Gophers!"
+	if Compare(s, s) != 0 {
+		t.Error("s != s")
+	}
+	if Compare(s, s[:1]) != 1 {
+		t.Error("s > s[:1] failed")
+	}
+}
+
+func TestCompareStrings(t *testing.T) {
+	n := 128
+	a := make([]byte, n+1)
+	b := make([]byte, n+1)
+	for len := 0; len < 128; len++ {
+		// randomish but deterministic data.  No 0 or 255.
+		for i := 0; i < len; i++ {
+			a[i] = byte(1 + 31*i%254)
+			b[i] = byte(1 + 31*i%254)
+		}
+		// data past the end is different
+		for i := len; i <= n; i++ {
+			a[i] = 8
+			b[i] = 9
+		}
+
+		cmp := Compare(string(a[:len]), string(b[:len]))
+		if cmp != 0 {
+			t.Errorf(`CompareIdentical(%d) = %d`, len, cmp)
+		}
+		if len > 0 {
+			cmp = Compare(string(a[:len-1]), string(b[:len]))
+			if cmp != -1 {
+				t.Errorf(`CompareAshorter(%d) = %d`, len, cmp)
+			}
+			cmp = Compare(string(a[:len]), string(b[:len-1]))
+			if cmp != 1 {
+				t.Errorf(`CompareBshorter(%d) = %d`, len, cmp)
+			}
+		}
+		for k := 0; k < len; k++ {
+			b[k] = a[k] - 1
+			cmp = Compare(string(a[:len]), string(b[:len]))
+			if cmp != 1 {
+				t.Errorf(`CompareAbigger(%d,%d) = %d`, len, k, cmp)
+			}
+			b[k] = a[k] + 1
+			cmp = Compare(string(a[:len]), string(b[:len]))
+			if cmp != -1 {
+				t.Errorf(`CompareBbigger(%d,%d) = %d`, len, k, cmp)
+			}
+			b[k] = a[k]
+		}
+	}
+}

src/strings/strings_decl.go

@@ -6,3 +6,10 @@ 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