runtime: implement string ops in Go

Also implement go:nosplit annotation.  Not really needed
for now, but we'll definitely need it for other conversions.

benchmark                 old ns/op     new ns/op     delta
BenchmarkRuneIterate      534           474           -11.24%
BenchmarkRuneIterate2     535           470           -12.15%

LGTM=bradfitz
R=golang-codereviews, dave, bradfitz, minux
CC=golang-codereviews
https://golang.org/cl/93380044

1 files changed, 24 insertions, 0 deletions

diff --git a/src/pkg/runtime/string_test.go b/src/pkg/runtime/string_test.go
index df3ff06a7d..dbccc24a5b 100644
--- a/src/pkg/runtime/string_test.go
+++ b/src/pkg/runtime/string_test.go
@@ -75,3 +75,27 @@ func BenchmarkCompareStringBig(b *testing.B) {
 	}
 	b.SetBytes(int64(len(s1)))
 }
+
+func BenchmarkRuneIterate(b *testing.B) {
+	bytes := make([]byte, 100)
+	for i := range bytes {
+		bytes[i] = byte('A')
+	}
+	s := string(bytes)
+	for i := 0; i < b.N; i++ {
+		for _ = range s {
+		}
+	}
+}
+
+func BenchmarkRuneIterate2(b *testing.B) {
+	bytes := make([]byte, 100)
+	for i := range bytes {
+		bytes[i] = byte('A')
+	}
+	s := string(bytes)
+	for i := 0; i < b.N; i++ {
+		for _, _ = range s {
+		}
+	}
+}