cmd/gc: relax address-of escape analysis

Make the loop nesting depth of &x depend on where x is declared,
not on where the &x appears. The latter is only a conservative
estimate of the former. Being more careful can avoid some
variables escaping, and it is easier to reason about.

It would have avoided issue 7313, although that was still a bug
worth fixing.

Not much effect in the tree: one variable in the whole tree
is saved from a heap allocation (something in x509 parsing).

LGTM=daniel.morsing
R=daniel.morsing
CC=golang-codereviews
https://golang.org/cl/62380043

src/cmd/gc/esc.c

@@ -328,6 +328,7 @@ escfunc(EscState *e, Node *func)
 			ll->n->escloopdepth = 0;
 			break;
 		case PPARAM:
+			ll->n->escloopdepth = 1; 
 			if(ll->n->type && !haspointers(ll->n->type))
 				break;
 			if(curfn->nbody == nil && !curfn->noescape)
@@ -335,7 +336,6 @@ escfunc(EscState *e, Node *func)
 			else
 				ll->n->esc = EscNone;	// prime for escflood later
 			e->noesc = list(e->noesc, ll->n);
-			ll->n->escloopdepth = 1; 
 			break;
 		}
 	}
@@ -630,7 +630,6 @@ esc(EscState *e, Node *n)
 			escassign(e, n, a);
 		}
 		// fallthrough
-	case OADDR:
 	case OMAKECHAN:
 	case OMAKEMAP:
 	case OMAKESLICE:
@@ -639,6 +638,24 @@ esc(EscState *e, Node *n)
 		n->esc = EscNone;  // until proven otherwise
 		e->noesc = list(e->noesc, n);
 		break;
+
+	case OADDR:
+		n->esc = EscNone;  // until proven otherwise
+		e->noesc = list(e->noesc, n);
+		// current loop depth is an upper bound on actual loop depth
+		// of addressed value.
+		n->escloopdepth = e->loopdepth;
+		// for &x, use loop depth of x.
+		if(n->left->op == ONAME) {
+			switch(n->left->class) {
+			case PAUTO:
+			case PPARAM:
+			case PPARAMOUT:
+				n->escloopdepth = n->left->escloopdepth;
+				break;
+			}
+		}
+		break;
 	}
 
 	lineno = lno;

test/escape2.go

@@ -1389,3 +1389,13 @@ func foo148(l List) { // ERROR " l does not escape"
 	for p := &l; p.Next != nil; p = p.Next { // ERROR "&l does not escape"
 	}
 }
+
+// related: address of variable should have depth of variable, not of loop
+
+func foo149(l List) { // ERROR " l does not escape"
+	var p *List
+	for {
+		for p = &l; p.Next != nil; p = p.Next { // ERROR "&l does not escape"
+		}
+	}
+}