cmd/gc: treat &T{} like new(T): allocate on stack if possible

Fixes #4618.

R=ken2
CC=golang-dev
https://golang.org/cl/7278048

src/cmd/gc/sinit.c

@@ -953,7 +953,7 @@ void
 anylit(int ctxt, Node *n, Node *var, NodeList **init)
 {
 	Type *t;
-	Node *a, *vstat;
+	Node *a, *vstat, *r;
 
 	t = n->type;
 	switch(n->op) {
@@ -964,7 +964,14 @@ anylit(int ctxt, Node *n, Node *var, NodeList **init)
 		if(!isptr[t->etype])
 			fatal("anylit: not ptr");
 
-		a = nod(OAS, var, callnew(t->type));
+		r = nod(ONEW, N, N);
+		r->typecheck = 1;
+		r->type = t;
+		r->esc = n->esc;
+		walkexpr(&r, init);
+
+		a = nod(OAS, var, r);
+
 		typecheck(&a, Etop);
 		*init = list(*init, a);
 

test/fixedbugs/issue4618.go

+// run
+
+// Copyright 2013 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.
+
+package main
+
+import (
+	"fmt"
+	"os"
+	"testing"
+)
+
+type T struct { int }
+
+var globl *T
+
+func F() {
+	t := &T{}
+	globl = t
+}
+
+func G() {
+	t := &T{}
+	_ = t
+}
+
+func main() {
+	nf := testing.AllocsPerRun(100, F)
+	ng := testing.AllocsPerRun(100, G)
+	if int(nf) != 1 {
+		fmt.Printf("AllocsPerRun(100, F) = %v, want 1\n", nf)
+		os.Exit(1)
+	}
+	if int(ng) != 0 {
+		fmt.Printf("AllocsPerRun(100, G) = %v, want 0\n", ng)
+		os.Exit(1)
+	}
+}