cmd/gc: properly race-instrument for loops

Instrumentation of ntest expression should go to ntest->init.
Same for nincr.
Fixes #5340.

R=golang-dev, daniel.morsing
CC=golang-dev
https://golang.org/cl/10026046

src/cmd/gc/racewalk.c

@@ -122,8 +122,20 @@ racewalknode(Node **np, NodeList **init, int wr, int skip)
 	if(debug['w'] > 1)
 		dump("racewalk-before", n);
 	setlineno(n);
-	if(init == nil || init == &n->ninit)
+	if(init == nil)
 		fatal("racewalk: bad init list");
+	if(init == &n->ninit) {
+		// If init == &n->ninit and n->ninit is non-nil,
+		// racewalknode might append it to itself.
+		// nil it out and handle it separately before putting it back.
+		l = n->ninit;
+		n->ninit = nil;
+		racewalklist(l, nil);
+		racewalknode(&n, &l, wr, skip);  // recurse with nil n->ninit
+		appendinit(&n, l);
+		*np = n;
+		return;
+	}
 
 	racewalklist(n->ninit, nil);
 
@@ -255,13 +267,7 @@ racewalknode(Node **np, NodeList **init, int wr, int skip)
 		// side effects are safe.
 		// n->right may not be executed,
 		// so instrumentation goes to n->right->ninit, not init.
-		// If right->ninit is non-nil, racewalknode might append it to itself.
-		// nil it out and handle it separately before putting it back.
-		l = n->right->ninit;
-		n->right->ninit = nil;
-		racewalklist(l, nil);
-		racewalknode(&n->right, &l, wr, 0);
-		appendinit(&n->right, l);
+		racewalknode(&n->right, &n->right->ninit, wr, 0);
 		goto ret;
 
 	case ONAME:
@@ -400,12 +406,8 @@ racewalknode(Node **np, NodeList **init, int wr, int skip)
 ret:
 	if(n->op != OBLOCK)  // OBLOCK is handled above in a special way.
 		racewalklist(n->list, init);
-	l = nil;
-	racewalknode(&n->ntest, &l, 0, 0);
-	n->ninit = concat(n->ninit, l);
-	l = nil;
-	racewalknode(&n->nincr, &l, 0, 0);
-	n->ninit = concat(n->ninit, l);
+	racewalknode(&n->ntest, &n->ntest->ninit, 0, 0);
+	racewalknode(&n->nincr, &n->nincr->ninit, 0, 0);
 	racewalklist(n->nbody, nil);
 	racewalklist(n->nelse, nil);
 	racewalklist(n->rlist, nil);

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

@@ -306,6 +306,94 @@ func TestRaceRange(t *testing.T) {
 	}
 }
 
+func TestRaceForInit(t *testing.T) {
+	c := make(chan int)
+	x := 0
+	go func() {
+		c <- x
+	}()
+	for x = 42; false; {
+	}
+	<-c
+}
+
+func TestNoRaceForInit(t *testing.T) {
+	done := make(chan bool)
+	c := make(chan bool)
+	x := 0
+	go func() {
+		for {
+			_, ok := <-c
+			if !ok {
+				done <- true
+				return
+			}
+			x++
+		}
+	}()
+	i := 0
+	for x = 42; i < 10; i++ {
+		c <- true
+	}
+	close(c)
+	<-done
+}
+
+func TestRaceForTest(t *testing.T) {
+	done := make(chan bool)
+	c := make(chan bool)
+	stop := false
+	go func() {
+		for {
+			_, ok := <-c
+			if !ok {
+				done <- true
+				return
+			}
+			stop = true
+		}
+	}()
+	for !stop {
+		c <- true
+	}
+	close(c)
+	<-done
+}
+
+func TestRaceForIncr(t *testing.T) {
+	done := make(chan bool)
+	c := make(chan bool)
+	x := 0
+	go func() {
+		for {
+			_, ok := <-c
+			if !ok {
+				done <- true
+				return
+			}
+			x++
+		}
+	}()
+	for i := 0; i < 10; x++ {
+		i++
+		c <- true
+	}
+	close(c)
+	<-done
+}
+
+func TestNoRaceForIncr(t *testing.T) {
+	done := make(chan bool)
+	x := 0
+	go func() {
+		x++
+		done <- true
+	}()
+	for i := 0; i < 0; x++ {
+	}
+	<-done
+}
+
 func TestRacePlus(t *testing.T) {
 	var x, y, z int
 	ch := make(chan int, 2)

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

@@ -175,3 +175,20 @@ type inltype struct {
 func inlinetest(p **inltype) *inltype {
 	return *p
 }
+
+type iface interface {
+	Foo() *struct{ b bool }
+}
+
+type Int int
+
+func (i Int) Foo() *struct{ b bool } {
+	return &struct{ b bool }{false}
+}
+
+func TestNoRaceForInfiniteLoop(t *testing.T) {
+	var x Int
+	// interface conversion causes nodes to be put on init list
+	for iface(x).Foo().b {
+	}
+}