cmd/gc, runtime: avoid unnecessary copy on type assertion.

When the first result of a type assertion is blank, the compiler would still copy out a potentially large non-interface type.

Fixes #1021.

R=golang-dev, bradfitz, rsc
CC=golang-dev
https://golang.org/cl/6812079

src/cmd/gc/builtin.c

@@ -54,6 +54,8 @@ char *runtimeimport =
 	"func @\"\".assertI2I2(@\"\".typ *byte, @\"\".iface any) (@\"\".ret any, @\"\".ok bool)\n"
 	"func @\"\".assertI2T(@\"\".typ *byte, @\"\".iface any) (@\"\".ret any)\n"
 	"func @\"\".assertI2T2(@\"\".typ *byte, @\"\".iface any) (@\"\".ret any, @\"\".ok bool)\n"
+	"func @\"\".assertI2TOK(@\"\".typ *byte, @\"\".iface any) (@\"\".ok bool)\n"
+	"func @\"\".assertE2TOK(@\"\".typ *byte, @\"\".iface any) (@\"\".ok bool)\n"
 	"func @\"\".ifaceeq(@\"\".i1 any, @\"\".i2 any) (@\"\".ret bool)\n"
 	"func @\"\".efaceeq(@\"\".i1 any, @\"\".i2 any) (@\"\".ret bool)\n"
 	"func @\"\".ifacethash(@\"\".i1 any) (@\"\".ret uint32)\n"

src/cmd/gc/runtime.go

@@ -76,6 +76,8 @@ func assertI2I(typ *byte, iface any) (ret any)
 func assertI2I2(typ *byte, iface any) (ret any, ok bool)
 func assertI2T(typ *byte, iface any) (ret any)
 func assertI2T2(typ *byte, iface any) (ret any, ok bool)
+func assertI2TOK(typ *byte, iface any) (ok bool)
+func assertE2TOK(typ *byte, iface any) (ok bool)
 
 func ifaceeq(i1 any, i2 any) (ret bool)
 func efaceeq(i1 any, i2 any) (ret bool)

src/cmd/gc/walk.c

@@ -686,6 +686,31 @@ walkexpr(Node **np, NodeList **init)
 		n->ninit = nil;
 		r = n->rlist->n;
 		walkexprlistsafe(n->list, init);
+		if(isblank(n->list->n) && !isinter(r->type)) {
+			strcpy(buf, "assert");
+			p = buf+strlen(buf);
+			if(isnilinter(r->left->type))
+				*p++ = 'E';
+			else
+				*p++ = 'I';
+			*p++ = '2';
+			*p++ = 'T';
+			*p++ = 'O';
+			*p++ = 'K';
+			*p = '\0';
+			
+			fn = syslook(buf, 1);
+			ll = list1(typename(r->type));
+			ll = list(ll, r->left);
+			argtype(fn, r->left->type);
+			n1 = nod(OCALL, fn, N);
+			n1->list = ll;
+			n = nod(OAS, n->list->next->n, n1);
+			typecheck(&n, Etop);
+			walkexpr(&n, init);
+			goto ret;
+		}
+
 		r->op = ODOTTYPE2;
 		walkexpr(&r, init);
 		ll = ascompatet(n->op, n->list, &r->type, 0, init);

src/pkg/runtime/iface.c

@@ -278,6 +278,13 @@ runtime·assertI2T2(Type *t, Iface i, ...)
 	copyout(t, &i.data, ret);
 }
 
+void
+runtime·assertI2TOK(Type *t, Iface i, bool ok)
+{
+	ok = i.tab!=nil && i.tab->type==t;
+	FLUSH(&ok);
+}
+
 static void assertE2Tret(Type *t, Eface e, byte *ret);
 
 // func ifaceE2T(typ *byte, iface any) (ret any)
@@ -334,6 +341,13 @@ runtime·assertE2T2(Type *t, Eface e, ...)
 	copyout(t, &e.data, ret);
 }
 
+void
+runtime·assertE2TOK(Type *t, Eface e, bool ok)
+{
+	ok = t==e.type;
+	FLUSH(&ok);
+}
+
 // func convI2E(elem any) (ret any)
 void
 runtime·convI2E(Iface i, Eface ret)