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/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)
+			}
+		}
+	}
+}