cmd/compile: fix stringtoslicebytetmp optimization

Fixes #14973.

Change-Id: Iea68c9deca9429bde465c9ae05639209fe0ccf72
Reviewed-on: https://go-review.googlesource.com/21175Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/cmd/compile/internal/gc/order.go

@@ -705,28 +705,31 @@ func orderstmt(n *Node, order *Order) {
 		order.out = append(order.out, n)
 		cleantemp(t, order)
 
-		// n->right is the expression being ranged over.
+	case ORANGE:
+		// n.Right is the expression being ranged over.
 		// order it, and then make a copy if we need one.
 		// We almost always do, to ensure that we don't
 		// see any value changes made during the loop.
-	// Usually the copy is cheap (e.g., array pointer, chan, slice, string are all tiny).
-	// The exception is ranging over an array value (not a slice, not a pointer to array),
+		// Usually the copy is cheap (e.g., array pointer,
+		// chan, slice, string are all tiny).
+		// The exception is ranging over an array value
+		// (not a slice, not a pointer to array),
 		// which must make a copy to avoid seeing updates made during
 		// the range body. Ranging over an array value is uncommon though.
-	case ORANGE:
-		t := marktemp(order)
 
+		// Mark []byte(str) range expression to reuse string backing storage.
+		// It is safe because the storage cannot be mutated.
+		if n.Right.Op == OSTRARRAYBYTE {
+			n.Right.Op = OSTRARRAYBYTETMP
+		}
+
+		t := marktemp(order)
 		n.Right = orderexpr(n.Right, order, nil)
 		switch n.Type.Etype {
 		default:
 			Fatalf("orderstmt range %v", n.Type)
 
-			// Mark []byte(str) range expression to reuse string backing storage.
-		// It is safe because the storage cannot be mutated.
 		case TARRAY:
-			if n.Right.Op == OSTRARRAYBYTE {
-				n.Right.Op = OSTRARRAYBYTETMP
-			}
 			if n.List.Len() < 2 || isblank(n.List.Second()) {
 				// for i := range x will only use x once, to compute len(x).
 				// No need to copy it.
@@ -734,10 +737,9 @@ func orderstmt(n *Node, order *Order) {
 			}
 			fallthrough
 
+		case TCHAN, TSTRING:
 			// chan, string, slice, array ranges use value multiple times.
 			// make copy.
-		// fall through
-		case TCHAN, TSTRING:
 			r := n.Right
 
 			if r.Type.Etype == TSTRING && r.Type != Types[TSTRING] {
@@ -748,12 +750,11 @@ func orderstmt(n *Node, order *Order) {
 
 			n.Right = ordercopyexpr(r, r.Type, order, 0)
 
+		case TMAP:
 			// copy the map value in case it is a map literal.
 			// TODO(rsc): Make tmp = literal expressions reuse tmp.
 			// For maps tmp is just one word so it hardly matters.
-		case TMAP:
 			r := n.Right
-
 			n.Right = ordercopyexpr(r, r.Type, order, 0)
 
 			// n->alloc is the temp for the iterator.

src/runtime/string_test.go

@@ -10,6 +10,10 @@ import (
 	"testing"
 )
 
+// Strings and slices that don't escape and fit into tmpBuf are stack allocated,
+// which defeats using AllocsPerRun to test other optimizations.
+const sizeNoStack = 100
+
 func BenchmarkCompareStringEqual(b *testing.B) {
 	bytes := []byte("Hello Gophers!")
 	s1, s2 := string(bytes), string(bytes)
@@ -158,7 +162,7 @@ func TestGostringnocopy(t *testing.T) {
 }
 
 func TestCompareTempString(t *testing.T) {
-	s := "foo"
+	s := strings.Repeat("x", sizeNoStack)
 	b := []byte(s)
 	n := testing.AllocsPerRun(1000, func() {
 		if string(b) != s {
@@ -221,7 +225,7 @@ func TestIntStringAllocs(t *testing.T) {
 }
 
 func TestRangeStringCast(t *testing.T) {
-	s := "abc"
+	s := strings.Repeat("x", sizeNoStack)
 	n := testing.AllocsPerRun(1000, func() {
 		for i, c := range []byte(s) {
 			if c != s[i] {