cmd/gc: optimize existence-only map lookups

The compiler converts 'val, ok = m[key]' to

        tmp, ok = <runtime call>
        val = *tmp

For lookups of the form '_, ok = m[key]',
the second statement is unnecessary.
By not generating it we save a nil check.

Change-Id: I21346cc195cb3c62e041af8b18770c0940358695
Reviewed-on: https://go-review.googlesource.com/1975Reviewed-by: Russ Cox <rsc@golang.org>

src/cmd/gc/walk.c

@@ -794,8 +794,6 @@ walkexpr(Node **np, NodeList **init)
 		//   var,b = mapaccess2*(t, m, i)
 		//   a = *var
 		a = n->list->n;
-		var = temp(ptrto(t->type));
-		var->typecheck = 1;
 		fn = mapfn(p, t);
 		r = mkcall1(fn, getoutargx(fn->type), init, typename(t), r->left, key);
 
@@ -806,10 +804,17 @@ walkexpr(Node **np, NodeList **init)
 			r->type->type->down->type = n->list->next->n->type;
 		n->rlist = list1(r);
 		n->op = OAS2FUNC;
+
+		// don't generate a = *var if a is _
+		if(!isblank(a)) {
+			var = temp(ptrto(t->type));
+			var->typecheck = 1;
 			n->list->n = var;
 			walkexpr(&n, init);
 			*init = list(*init, n);
 			n = nod(OAS, a, nod(OIND, var, N));
+		}
+
 		typecheck(&n, Etop);
 		walkexpr(&n, init);
 		// mapaccess needs a zero value to be at least this big.

test/nilcheck.go

@@ -182,3 +182,8 @@ func f4(x *[10]int) {
 	_ = &x[9] // ERROR "nil check"
 }
 
+func f5(m map[string]struct{}) bool {
+	// Existence-only map lookups should not generate a nil check
+	_, ok := m[""]
+	return ok
+}