cmd/gc: fix escape analysis bug

The code assumed that the only choices were EscNone, EscScope, and EscHeap,
so that it makes sense to set EscScope only if the current setting is EscNone.
Now that we have the many variants of EscReturn, this logic is false, and it was
causing important EscScopes to be ignored in favor of EscReturn.

Fixes #4360.

R=ken2
CC=golang-dev, lvd
https://golang.org/cl/6816103

src/cmd/gc/esc.c

@@ -1005,8 +1005,8 @@ escwalk(EscState *e, int level, Node *dst, Node *src)
 				if((src->esc&EscMask) != EscReturn)
 					src->esc = EscReturn;
 				src->esc |= 1<<((dst->vargen-1) + EscBits);
+				goto recurse;
 			}
-			goto recurse;
 		}
 	}
 
@@ -1014,7 +1014,7 @@ escwalk(EscState *e, int level, Node *dst, Node *src)
 
 	switch(src->op) {
 	case ONAME:
-		if(src->class == PPARAM && leaks && src->esc == EscNone) {
+		if(src->class == PPARAM && leaks && src->esc != EscHeap) {
 			src->esc = EscScope;
 			if(debug['m'])
 				warnl(src->lineno, "leaking param: %hN", src);

src/cmd/gc/subr.c

@@ -219,6 +219,8 @@ warnl(int line, char *fmt, ...)
 	va_start(arg, fmt);
 	adderr(line, fmt, arg);
 	va_end(arg);
+	if(debug['m'])
+		flusherrors();
 }
 
 void

test/escape5.go

@@ -117,3 +117,28 @@ func leakrecursive2(p, q *int) (*int, *int) { // ERROR "leaking param: p" "leaki
 	return p, q
 }
 
+
+var global interface{}
+
+type T1 struct {
+	X *int
+}
+
+type T2 struct {
+	Y *T1
+}
+
+func f8(p *T1) (k T2) { // ERROR "leaking param: p to result k" "leaking param: p"
+	if p == nil {
+		k = T2{}
+		return
+	}
+
+	global = p // should make p leak always
+	return T2{p}
+}
+
+func f9() {
+	var j T1 // ERROR "moved to heap: j"
+	f8(&j) // ERROR "&j escapes to heap"
+}