cmd/gc: fix escape analysis bug

Was not handling &x.y[0] and &x.y.z correctly where
y is an array or struct-valued field (not a pointer).

R=ken2
CC=golang-dev
https://golang.org/cl/6551059

src/cmd/gc/esc.c

@@ -926,9 +926,15 @@ escwalk(EscState *e, int level, Node *dst, Node *src)
 		}
 		break;
 
+	case ODOT:
+		escwalk(e, level, dst, src->left);
+		break;
+
 	case OINDEX:
-		if(isfixedarray(src->type))
+		if(isfixedarray(src->left->type)) {
+			escwalk(e, level, dst, src->left);
 			break;
+		}
 		// fall through
 	case OSLICE:
 	case ODOTPTR:

test/escape2.go

@@ -1211,3 +1211,21 @@ func foo137() {
 		}()
 	}()
 }
+
+func foo138() *byte {
+	type T struct {
+		x [1]byte
+	}
+	t := new(T) // ERROR "new.T. escapes to heap"
+	return &t.x[0] // ERROR "&t.x.0. escapes to heap"
+}
+
+func foo139() *byte {
+	type T struct {
+		x struct {
+			y byte
+		}
+	}
+	t := new(T) // ERROR "new.T. escapes to heap"
+	return &t.x.y // ERROR "&t.x.y escapes to heap"
+}

test/escape4.go

@@ -37,3 +37,21 @@ func f2() {} // ERROR "can inline f2"
 // No inline for panic, recover.
 func f3() { panic(1) }
 func f4() { recover() }
+
+func f5() *byte {
+	type T struct {
+		x [1]byte
+	}
+	t := new(T) // ERROR "new.T. escapes to heap"
+	return &t.x[0] // ERROR "&t.x.0. escapes to heap"
+}
+
+func f6() *byte {
+	type T struct {
+		x struct {
+			y byte
+		}
+	}
+	t := new(T) // ERROR "new.T. escapes to heap"
+	return &t.x.y // ERROR "&t.x.y escapes to heap"
+}