cmd/gc: don't emit write barriers for *tmp if tmp=&PAUTO

This is another case where we can say that the address refers to stack.
We create such temps for OSTRUCTLIT initialization.

This eliminates a handful of write barriers today.
But this come up a prerequisite for another change (capturing vars by value),
otherwise we emit writebarriers in writebarrier itself when
capture writebarrier arguments by value.

Change-Id: Ibba93acd0f5431c5a4c3d90ef1e622cb9a7ff50e
Reviewed-on: https://go-review.googlesource.com/3285Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/gc/typecheck.c

@@ -1666,6 +1666,9 @@ reswitch:
 	case OAS:
 		ok |= Etop;
 		typecheckas(n);
+		// Code that creates temps does not bother to set defn, so do it here.
+		if(n->left->op == ONAME && strncmp(n->left->sym->name, "autotmp_", 8) == 0)
+			n->left->defn = n;
 		goto ret;
 
 	case OAS2:

src/cmd/gc/walk.c

@@ -1920,9 +1920,19 @@ callnew(Type *t)
 static int
 isstack(Node *n)
 {
+	Node *defn;
+
 	while(n->op == ODOT || n->op == OPAREN || n->op == OCONVNOP || n->op == OINDEX && isfixedarray(n->left->type))
 		n = n->left;
-	
+
+	// If n is *autotmp and autotmp = &foo, replace n with foo.
+	// We introduce such temps when initializing struct literals.
+	if(n->op == OIND && n->left->op == ONAME && strncmp(n->left->sym->name, "autotmp_", 8) == 0) {
+		defn = n->left->defn;
+		if(defn != N && defn->op == OAS && defn->right->op == OADDR)
+			n = defn->right->left;
+	}
+
 	switch(n->op) {
 	case OINDREG:
 		// OINDREG only ends up in walk if it's indirect of SP.