cmd/gc: allocate stack buffer for ORUNESTR

If result of string(i) does not escape,
allocate a [4]byte temp on stack for it.

Change-Id: If31ce9447982929d5b3b963fd0830efae4247c37
Reviewed-on: https://go-review.googlesource.com/3411
Reviewed-by: Russ Cox <rsc@golang.org>

1 files changed, 35 insertions, 0 deletions

diff --git a/src/runtime/string_test.go b/src/runtime/string_test.go
index 249f431e18..27a44ad645 100644
--- a/src/runtime/string_test.go
+++ b/src/runtime/string_test.go
@@ -186,3 +186,38 @@ func TestStringOnStack(t *testing.T) {
 		t.Fatalf("want: '%v', got '%v'", want, s)
 	}
 }
+
+func TestIntString(t *testing.T) {
+	// Non-escaping result of intstring.
+	s := ""
+	for i := 0; i < 4; i++ {
+		s += string(i+'0') + string(i+'0'+1)
+	}
+	if want := "01122334"; s != want {
+		t.Fatalf("want '%v', got '%v'", want, s)
+	}
+
+	// Escaping result of intstring.
+	var a [4]string
+	for i := 0; i < 4; i++ {
+		a[i] = string(i + '0')
+	}
+	s = a[0] + a[1] + a[2] + a[3]
+	if want := "0123"; s != want {
+		t.Fatalf("want '%v', got '%v'", want, s)
+	}
+}
+
+func TestIntStringAllocs(t *testing.T) {
+	unknown := '0'
+	n := testing.AllocsPerRun(1000, func() {
+		s1 := string(unknown)
+		s2 := string(unknown + 1)
+		if s1 == s2 {
+			t.Fatalf("bad")
+		}
+	})
+	if n != 0 {
+		t.Fatalf("want 0 allocs, got %v", n)
+	}
+}