cmd/gc: fix &result escaping into result

There is a hierarchy of location defined by loop depth:

        -1 = the heap
        0 = function results
        1 = local variables (and parameters)
        2 = local variable declared inside a loop
        3 = local variable declared inside a loop inside a loop
        etc

In general if an address from loopdepth n is assigned to
something in loop depth m < n, that indicates an extended
lifetime of some form that requires a heap allocation.

Function results can be local variables too, though, and so
they don't actually fit into the hierarchy very well.
Treat the address of a function result as level 1 so that
if it is written back into a result, the address is treated
as escaping.

Fixes #8185.

LGTM=iant
R=iant
CC=golang-codereviews
https://golang.org/cl/108870044

src/cmd/gc/esc.c

@@ -673,12 +673,21 @@ esc(EscState *e, Node *n, Node *up)
 		// for &x, use loop depth of x if known.
 		// it should always be known, but if not, be conservative
 		// and keep the current loop depth.
-		if(n->left->op == ONAME && (n->left->escloopdepth != 0 || n->left->class == PPARAMOUT)) {
+		if(n->left->op == ONAME) {
 			switch(n->left->class) {
 			case PAUTO:
+				if(n->left->escloopdepth != 0)
+					n->escloopdepth = n->left->escloopdepth;
+				break;
 			case PPARAM:
 			case PPARAMOUT:
-				n->escloopdepth = n->left->escloopdepth;
+				// PPARAM is loop depth 1 always.
+				// PPARAMOUT is loop depth 0 for writes
+				// but considered loop depth 1 for address-of,
+				// so that writing the address of one result
+				// to another (or the same) result makes the
+				// first result move to the heap.
+				n->escloopdepth = 1;
 				break;
 			}
 		}

test/escape2.go

@@ -1478,3 +1478,15 @@ func foo153(v interface{}) *int { // ERROR "leaking param: v"
 	}
 	panic(0)
 }
+
+// issue 8185 - &result escaping into result
+
+func f() (x int, y *int) { // ERROR "moved to heap: x"
+	y = &x // ERROR "&x escapes to heap"
+	return
+}
+
+func g() (x interface{}) { // ERROR "moved to heap: x"
+	x = &x // ERROR "&x escapes to heap"
+	return
+}