cmd/gc: fix two select temporary bugs

The introduction of temporaries in order.c was not
quite right for two corner cases:

1) The rewrite that pushed new variables on the lhs of
a receive into the body of the case was dropping the
declaration of the variables. If the variables escape,
the declaration is what allocates them.
Caught by escape analysis sanity check.
In fact the declarations should move into the body
always, so that we only allocate if the corresponding
case is selected. Do that. (This is an optimization that
was already present in Go 1.2. The new order code just
made it stop working.)

Fixes #7997.

2) The optimization to turn a single-recv select into
an ordinary receive assumed it could take the address
of the destination; not so if the destination is _.

Fixes #7998.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/100480043

src/cmd/gc/order.c

@@ -781,8 +781,30 @@ orderstmt(Node *n, Order *order)
 				fatal("order select ninit");
 			if(r != nil) {
 				switch(r->op) {
+				default:
+					yyerror("unknown op in select %O", r->op);
+					dump("select case", r);
+					break;
+
 				case OSELRECV:
 				case OSELRECV2:
+					// If this is case x := <-ch or case x, y := <-ch, the case has
+					// the ODCL nodes to declare x and y. We want to delay that
+					// declaration (and possible allocation) until inside the case body.
+					// Delete the ODCL nodes here and recreate them inside the body below.
+					if(r->colas) {
+						t = r->ninit;
+						if(t != nil && t->n->op == ODCL && t->n->left == r->left)
+							t = t->next;
+						if(t != nil && t->n->op == ODCL && t->n->left == r->ntest)
+							t = t->next;
+						if(t == nil)
+							r->ninit = nil;
+					}
+					if(r->ninit != nil) {
+						yyerror("ninit on select recv");
+						dumplist("ninit", r->ninit);
+					}
 					// case x = <-c
 					// case x, ok = <-c
 					// r->left is x, r->ntest is ok, r->right is ORECV, r->right->left is c.
@@ -803,6 +825,11 @@ orderstmt(Node *n, Order *order)
 						// such as in case interfacevalue = <-intchan.
 						// the conversion happens in the OAS instead.
 						tmp1 = r->left;
+						if(r->colas) {
+							tmp2 = nod(ODCL, tmp1, N);
+							typecheck(&tmp2, Etop);
+							l->n->ninit = list(l->n->ninit, tmp2);
+						}
 						r->left = ordertemp(r->right->left->type->type, order, haspointers(r->right->left->type->type));
 						tmp2 = nod(OAS, tmp1, r->left);
 						typecheck(&tmp2, Etop);
@@ -812,6 +839,11 @@ orderstmt(Node *n, Order *order)
 						r->ntest = N;
 					if(r->ntest != N) {
 						tmp1 = r->ntest;
+						if(r->colas) {
+							tmp2 = nod(ODCL, tmp1, N);
+							typecheck(&tmp2, Etop);
+							l->n->ninit = list(l->n->ninit, tmp2);
+						}
 						r->ntest = ordertemp(tmp1->type, order, 0);
 						tmp2 = nod(OAS, tmp1, r->ntest);
 						typecheck(&tmp2, Etop);
@@ -821,6 +853,10 @@ orderstmt(Node *n, Order *order)
 					break;
 
 				case OSEND:
+					if(r->ninit != nil) {
+						yyerror("ninit on select send");
+						dumplist("ninit", r->ninit);
+					}
 					// case c <- x
 					// r->left is c, r->right is x, both are always evaluated.
 					orderexpr(&r->left, order);

src/cmd/gc/walk.c

@@ -666,6 +666,9 @@ walkexpr(Node **np, NodeList **init)
 		r = n->rlist->n;
 		walkexprlistsafe(n->list, init);
 		walkexpr(&r->left, init);
+		if(isblank(n->list->n))
+			n1 = nodnil();
+		else
 			n1 = nod(OADDR, n->list->n, N);
 		n1->etype = 1; // addr does not escape
 		fn = chanfn("chanrecv2", 2, r->left->type);

test/fixedbugs/issue7997.go

+// compile
+
+// Copyright 2014 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// /tmp/x.go:3: internal error: f &p (type *int) recorded as live on entry
+
+package p
+
+func f(ch chan int) *int {
+	select {
+	case p1x := <-ch:
+		return &p1x
+	default:
+		// ok
+	}
+	select {
+	case p1 := <-ch:
+		return &p1
+	default:
+		// ok
+	}
+	select {
+	case p2 := <-ch:
+		return &p2
+	case p3 := <-ch:
+		return &p3
+	default:
+		// ok
+	}
+	select {
+	case p4, ok := <-ch:
+		if ok {
+			return &p4
+		}
+	default:
+		// ok
+	}
+	select {
+	case p5, ok := <-ch:
+		if ok {
+			return &p5
+		}
+	case p6, ok := <-ch:
+		if !ok {
+			return &p6
+		}
+	default:
+		// ok
+	}
+	return nil
+}

test/fixedbugs/issue7998.go

+// compile
+
+// Copyright 2014 The Go Authors.  All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// /tmp/x.go:5: cannot use _ as value
+
+package p
+
+func f(ch chan int) bool {
+	select {
+	case _, ok := <-ch:
+		return ok
+	}
+	_, ok := <-ch
+	_ = ok
+	select {
+	case _, _ = <-ch:
+		return true
+	}
+	return false
+}