Reduce allocations when building strings.

Builder has a new CreateByteString function that writes a
null-terimnated byte slice to the buffer. This results in zero
allocations for writing strings.

go/builder.go

@@ -247,16 +247,20 @@ func (b *Builder) EndVector(vectorNumElems int) UOffsetT {
 
 // CreateString writes a null-terminated string as a vector.
 func (b *Builder) CreateString(s string) UOffsetT {
+	return b.CreateByteString([]byte(s))
+}
+
+// CreateByteString writes a null-terminated byteslice as a vector.
+func (b *Builder) CreateByteString(s []byte) UOffsetT {
 	b.Prep(int(SizeUOffsetT), (len(s)+1)*SizeByte)
 	b.PlaceByte(0)
 
-	x := []byte(s)
-	l := UOffsetT(len(x))
+	l := UOffsetT(len(s))
 
 	b.head -= l
-	copy(b.Bytes[b.head:b.head+l], x)
+	copy(b.Bytes[b.head:b.head+l], s)
 
-	return b.EndVector(len(x))
+	return b.EndVector(len(s))
 }
 
 // CreateByteVector writes a ubyte vector

tests/GoTest.sh

@@ -43,8 +43,8 @@ GOPATH=${go_path} go test flatbuffers_test \
                      --test.coverpkg=github.com/google/flatbuffers/go \
                      --cpp_data=${test_dir}/monsterdata_test.mon \
                      --out_data=${test_dir}/monsterdata_go_wire.mon \
-                     --test.bench=. \
-                     --test.benchtime=5s \
+                     --test.bench=Build \
+                     --test.benchtime=3s \
                      --fuzz=true \
                      --fuzz_fields=4 \
                      --fuzz_objects=10000

tests/go_test.go

@@ -518,6 +518,15 @@ func CheckByteLayout(fail func(string, ...interface{})) {
 	check([]byte{4, 0, 0, 0, 'm', 'o', 'o', 'p', 0, 0, 0, 0, // 0-terminated, 3-byte pad
 		3, 0, 0, 0, 'f', 'o', 'o', 0})
 
+	// test 6b: CreateByteString
+
+	b = flatbuffers.NewBuilder(0)
+	b.CreateByteString([]byte("foo"))
+	check([]byte{3, 0, 0, 0, 'f', 'o', 'o', 0}) // 0-terminated, no pad
+	b.CreateByteString([]byte("moop"))
+	check([]byte{4, 0, 0, 0, 'm', 'o', 'o', 'p', 0, 0, 0, 0, // 0-terminated, 3-byte pad
+		3, 0, 0, 0, 'f', 'o', 'o', 0})
+
 	// test 7: empty vtable
 	b = flatbuffers.NewBuilder(0)
 	b.StartObject(0)
@@ -1239,19 +1248,19 @@ func BenchmarkBuildGold(b *testing.B) {
 	buf, offset := CheckGeneratedBuild(b.Fatalf)
 	bytes_length := int64(len(buf[offset:]))
 
-	reuse_str := "MyMonster"
-	reuse_test1 := "test1"
-	reuse_test2 := "test2"
-	reuse_fred := "Fred"
+	reuse_str := []byte("MyMonster")
+	reuse_test1 := []byte("test1")
+	reuse_test2 := []byte("test2")
+	reuse_fred := []byte("Fred")
 
 	b.SetBytes(bytes_length)
 	b.ReportAllocs()
 	for i := 0; i < b.N; i++ {
 		bldr := flatbuffers.NewBuilder(0)
-		str := bldr.CreateString(reuse_str)
-		test1 := bldr.CreateString(reuse_test1)
-		test2 := bldr.CreateString(reuse_test2)
-		fred := bldr.CreateString(reuse_fred)
+		str := bldr.CreateByteString(reuse_str)
+		test1 := bldr.CreateByteString(reuse_test1)
+		test2 := bldr.CreateByteString(reuse_test2)
+		fred := bldr.CreateByteString(reuse_fred)
 
 		example.MonsterStartInventoryVector(bldr, 5)
 		bldr.PrependByte(4)