gc: avoid false positives when using scalar struct fields.

The escape analysis code does not make a distinction between
scalar and pointers fields in structs. Non-pointer fields
that escape should not make the whole struct escape.

R=lvd, rsc
CC=golang-dev, remy
https://golang.org/cl/5489128

src/cmd/gc/esc.c

@@ -469,10 +469,14 @@ escassign(Node *dst, Node *src)
 		escflows(dst, src);
 		break;
 
+	case ODOT:
+		// A non-pointer escaping from a struct does not concern us.
+		if(src->type && !haspointers(src->type))
+			break;
+		// fallthrough
 	case OCONV:
 	case OCONVIFACE:
 	case OCONVNOP:
-	case ODOT:
 	case ODOTMETH:	// treat recv.meth as a value with recv in it, only happens in ODEFER and OPROC
 			// iface.method already leaks iface in esccall, no need to put in extra ODOTINTER edge here
 	case ODOTTYPE:

test/escape2.go

@@ -126,10 +126,36 @@ func (b *Bar) NoLeak() int { // ERROR "b does not escape"
 	return *(b.ii)
 }
 
+func (b *Bar) Leak() *int { // ERROR "leaking param: b"
+	return &b.i // ERROR "&b.i escapes to heap"
+}
+
 func (b *Bar) AlsoNoLeak() *int { // ERROR "b does not escape"
 	return b.ii
 }
 
+func (b Bar) AlsoLeak() *int { // ERROR "leaking param: b"
+	return b.ii
+}
+
+func (b Bar) LeaksToo() *int { // ERROR "leaking param: b"
+	v := 0	// ERROR "moved to heap: v"
+	b.ii = &v // ERROR "&v escapes"
+	return b.ii
+}
+
+func (b *Bar) LeaksABit() *int { // ERROR "b does not escape"
+	v := 0	// ERROR "moved to heap: v"
+	b.ii = &v // ERROR "&v escapes"
+	return b.ii
+}
+
+func (b Bar) StillNoLeak() int { // ERROR "b does not escape"
+	v := 0
+	b.ii = &v // ERROR "&v does not escape"
+	return b.i
+}
+
 func goLeak(b *Bar) { // ERROR "leaking param: b"
 	go b.NoLeak()
 }
@@ -155,6 +181,10 @@ func (b *Bar2) AlsoNoLeak() []int { // ERROR "b does not escape"
 	return b.ii[0:1]
 }
 
+func (b Bar2) AgainNoLeak() [12]int { // ERROR "b does not escape"
+	return b.i
+}
+
 func (b *Bar2) LeakSelf() { // ERROR "leaking param: b"
 	b.ii = b.i[0:4] // ERROR "b.i escapes to heap"
 }