cmd/gc: elide self-assignment during return

More efficient, less racy code.

Fixes #4014.

R=ken2, ken
CC=golang-dev
https://golang.org/cl/7275047

src/cmd/gc/walk.c

@@ -1326,8 +1326,12 @@ ascompatee(int op, NodeList *nl, NodeList *nr, NodeList **init)
 		lr->n = safeexpr(lr->n, init);
 
 	nn = nil;
-	for(ll=nl, lr=nr; ll && lr; ll=ll->next, lr=lr->next)
+	for(ll=nl, lr=nr; ll && lr; ll=ll->next, lr=lr->next) {
+		// Do not generate 'x = x' during return. See issue 4014.
+		if(op == ORETURN && ll->n == lr->n)
+			continue;
 		nn = list(nn, ascompatee1(op, ll->n, lr->n, init));
+	}
 
 	// cannot happen: caller checked that lists had same length
 	if(ll || lr)

src/pkg/runtime/race/testdata/regression_test.go

@@ -127,3 +127,21 @@ func divInSlice() {
 	i := 1
 	_ = v[(i*4)/3]
 }
+
+func TestNoRaceReturn(t *testing.T) {
+	c := make(chan int)
+	noRaceReturn(c)
+	<-c
+}
+
+// Return used to do an implicit a = a, causing a read/write race
+// with the goroutine. Compiler has an optimization to avoid that now.
+// See issue 4014.
+func noRaceReturn(c chan int) (a, b int) {
+	a = 42
+	go func() {
+		_ = a
+		c <- 1
+	}()
+	return a, 10
+}