[release-branch.go1.3] cmd/gc: two escape analysis fixes

««« CL 108860043 / f153208c0a0e
cmd/gc: fix escape analysis for &x inside switch x := v.(type)

The analysis for &x was using the loop depth on x set
during x's declaration. A type switch creates a list of
implicit declarations that were not getting initialized
with loop depths.

Fixes #8176.

LGTM=iant
R=iant
CC=golang-codereviews
https://golang.org/cl/108860043
»»»

««« CL 108870044 / 331dbd4a6334
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  issue 8185 .

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

LGTM=r
R=golang-codereviews, r
CC=bradfitz, golang-codereviews, iant
https://golang.org/cl/107930044

src/cmd/gc/esc.c

@@ -442,6 +442,18 @@ esc(EscState *e, Node *n, Node *up)
 	if(n->op == OFOR || n->op == ORANGE)
 		e->loopdepth++;
 
+	// type switch variables have no ODCL.
+	// process type switch as declaration.
+	// must happen before processing of switch body,
+	// so before recursion.
+	if(n->op == OSWITCH && n->ntest && n->ntest->op == OTYPESW) {
+		for(ll=n->list; ll; ll=ll->next) {  // cases
+			// ll->n->nname is the variable per case
+			if(ll->n->nname)
+				ll->n->nname->escloopdepth = e->loopdepth;
+		}
+	}
+
 	esc(e, n->left, n);
 	esc(e, n->right, n);
 	esc(e, n->ntest, n);
@@ -658,13 +670,24 @@ esc(EscState *e, Node *n, Node *up)
 		// 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.
+		// 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) {
 			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

@@ -1468,3 +1468,25 @@ func foo152() {
 	v := NewV(u)
 	println(v)
 }
+
+// issue 8176 - &x in type switch body not marked as escaping
+
+func foo153(v interface{}) *int { // ERROR "leaking param: v"
+	switch x := v.(type) {
+	case int: // ERROR "moved to heap: x"
+		return &x // ERROR "&x escapes to heap"
+	}
+	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
+}