runtime: implement eqstring in assembly.

BenchmarkCompareStringEqual               10.4          7.33          -29.52%
BenchmarkCompareStringIdentical           3.99          3.67          -8.02%
BenchmarkCompareStringSameLength          9.80          6.84          -30.20%
BenchmarkCompareStringDifferentLength     1.09          0.95          -12.84%
BenchmarkCompareStringBigUnaligned        75220         76071         +1.13%
BenchmarkCompareStringBig                 69843         74746         +7.02%

LGTM=bradfitz, josharian
R=golang-codereviews, bradfitz, josharian, dave, khr
CC=golang-codereviews
https://golang.org/cl/105280044

src/pkg/runtime/asm_386.s

@@ -1096,6 +1096,28 @@ TEXT runtime·memeq(SB),NOSPLIT,$0-12
 	MOVL	count+8(FP), BX
 	JMP	runtime·memeqbody(SB)
 
+// eqstring tests whether two strings are equal.
+// See runtime_test.go:eqstring_generic for
+// equivlaent Go code.
+TEXT runtime·eqstring(SB),NOSPLIT,$0-17
+	MOVL	s1len+4(FP), AX
+	MOVL	s2len+12(FP), BX
+	CMPL	AX, BX
+	JNE	different
+	MOVL	s1str+0(FP), SI
+	MOVL	s2str+8(FP), DI
+	CMPL	SI, DI
+	JEQ	same
+	CALL	runtime·memeqbody(SB)
+	MOVB	AX, v+16(FP)
+	RET
+same:
+	MOVB	$1, v+16(FP)
+	RET
+different:
+	MOVB	$0, v+16(FP)
+	RET
+
 TEXT bytes·Equal(SB),NOSPLIT,$0-25
 	MOVL	a_len+4(FP), BX
 	MOVL	b_len+16(FP), CX

src/pkg/runtime/asm_amd64.s

@@ -1058,6 +1058,28 @@ TEXT runtime·memeq(SB),NOSPLIT,$0-24
 	MOVQ	count+16(FP), BX
 	JMP	runtime·memeqbody(SB)
 
+// eqstring tests whether two strings are equal.
+// See runtime_test.go:eqstring_generic for
+// equivlaent Go code.
+TEXT runtime·eqstring(SB),NOSPLIT,$0-33
+	MOVQ	s1len+8(FP), AX
+	MOVQ	s2len+24(FP), BX
+	CMPQ	AX, BX
+	JNE	different
+	MOVQ	s1str+0(FP), SI
+	MOVQ	s2str+16(FP), DI
+	CMPQ	SI, DI
+	JEQ	same
+	CALL	runtime·memeqbody(SB)
+	MOVB	AX, v+32(FP)
+	RET
+same:
+	MOVB	$1, v+32(FP)
+	RET
+different:
+	MOVB	$0, v+32(FP)
+	RET
+
 // a in SI
 // b in DI
 // count in BX

src/pkg/runtime/asm_amd64p32.s

@@ -715,6 +715,28 @@ TEXT runtime·memeq(SB),NOSPLIT,$0-12
 	MOVL	count+8(FP), BX
 	JMP	runtime·memeqbody(SB)
 
+// eqstring tests whether two strings are equal.
+// See runtime_test.go:eqstring_generic for
+// equivlaent Go code.
+TEXT runtime·eqstring(SB),NOSPLIT,$0-17
+	MOVL	s1len+4(FP), AX
+	MOVL	s2len+12(FP), BX
+	CMPL	AX, BX
+	JNE	different
+	MOVL	s1str+0(FP), SI
+	MOVL	s2str+8(FP), DI
+	CMPL	SI, DI
+	JEQ	same
+	CALL	runtime·memeqbody(SB)
+	MOVB	AX, v+16(FP)
+	RET
+same:
+	MOVB	$1, v+16(FP)
+	RET
+different:
+	MOVB	$0, v+16(FP)
+	RET
+
 // a in SI
 // b in DI
 // count in BX

src/pkg/runtime/asm_arm.s

@@ -646,6 +646,33 @@ _next:
 	MOVW	$0, R0
 	RET
 
+// eqstring tests whether two strings are equal.
+// See runtime_test.go:eqstring_generic for
+// equivlaent Go code.
+TEXT runtime·eqstring(SB),NOSPLIT,$-4-17
+	MOVW	s1len+4(FP), R0
+	MOVW	s2len+12(FP), R1
+	MOVW	$0, R7
+	CMP	R0, R1
+	MOVB.NE R7, v+16(FP)
+	RET.NE
+	MOVW	s1str+0(FP), R2
+	MOVW	s2str+8(FP), R3
+	MOVW	$1, R8
+	MOVB	R8, v+16(FP)
+	CMP	R2, R3
+	RET.EQ
+	ADD	R2, R0, R6
+_eqnext:
+	CMP	R2, R6
+	RET.EQ
+	MOVBU.P	1(R2), R4
+	MOVBU.P	1(R3), R5
+	CMP	R4, R5
+	BEQ	_eqnext
+	MOVB	R7, v+16(FP)
+	RET
+
 // We have to resort to TLS variable to save g(R10) and
 // m(R9). One reason is that external code might trigger
 // SIGSEGV, and our runtime.sigtramp don't even know we

src/pkg/runtime/runtime_test.go

@@ -202,3 +202,41 @@ func testSetPanicOnFault(t *testing.T, addr uintptr) {
 	println(*p)
 	t.Fatalf("still here - should have faulted on address %#x", addr)
 }
+
+func eqstring_generic(s1, s2 string) bool {
+	if len(s1) != len(s2) {
+		return false
+	}
+	// optimization in assembly versions:
+	// if s1.str == s2.str { return true }
+	for i := 0; i < len(s1); i++ {
+		if s1[i] != s2[i] {
+			return false
+		}
+	}
+	return true
+}
+
+func TestEqString(t *testing.T) {
+	// This isn't really an exhaustive test of eqstring, it's
+	// just a convenient way of documenting (via eqstring_generic)
+	// what eqstring does.
+	s := []string{
+		"",
+		"a",
+		"c",
+		"aaa",
+		"ccc",
+		"cccc"[:3], // same contents, different string
+		"1234567890",
+	}
+	for _, s1 := range s {
+		for _, s2 := range s {
+			x := s1 == s2
+			y := eqstring_generic(s1, s2)
+			if x != y {
+				t.Errorf(`eqstring("%s","%s") = %t, want %t`, s1, s2, x, y)
+			}
+		}
+	}
+}

src/pkg/runtime/string.goc

@@ -206,18 +206,6 @@ func concatstrings(s Slice) (res String) {
 	res = concatstring(s.len, (String*)s.array);
 }
 
-func eqstring(s1 String, s2 String) (v bool) {
-	if(s1.len != s2.len) {
-		v = false;
-		return;
-	}
-	if(s1.str == s2.str) {
-		v = true;
-		return;
-	}
-	v = runtime·memeq(s1.str, s2.str, s1.len);
-}
-
 int32
 runtime·strcmp(byte *s1, byte *s2)
 {