gc: handle iface == nil in back end

R=ken
OCL=33778
CL=33781

src/cmd/5g/cgen.c

@@ -790,6 +790,25 @@ bgen(Node *n, int true, Prog *to)
 			regfree(&n1);
 			break;
 		}
+		
+		if(isinter(nl->type)) {
+			// front end shold only leave cmp to literal nil
+			if((a != OEQ && a != ONE) || nr->op != OLITERAL) {
+				yyerror("illegal interface comparison");
+				break;
+			}
+			a = optoas(a, types[tptr]);
+			regalloc(&n1, types[tptr], N);
+			agen(nl, &n1);
+			n2 = n1;
+			n2.op = OINDREG;
+			n2.xoffset = 0;
+			nodconst(&tmp, types[tptr], 0);
+			gins(optoas(OCMP, types[tptr]), &n2, &tmp);
+			patch(gbranch(a, types[tptr]), to);
+			regfree(&n1);
+			break;
+		}
 
 		a = optoas(a, nr->type);
 

src/cmd/6g/cgen.c

@@ -743,6 +743,7 @@ bgen(Node *n, int true, Prog *to)
 		nl = n->left;
 		if(nl == N || nl->type == T)
 			goto ret;
+		break;
 	}
 
 	switch(n->op) {
@@ -787,6 +788,25 @@ bgen(Node *n, int true, Prog *to)
 			regfree(&n1);
 			break;
 		}
+		
+		if(isinter(nl->type)) {
+			// front end shold only leave cmp to literal nil
+			if((a != OEQ && a != ONE) || nr->op != OLITERAL) {
+				yyerror("illegal interface comparison");
+				break;
+			}
+			a = optoas(a, types[tptr]);
+			regalloc(&n1, types[tptr], N);
+			agen(nl, &n1);
+			n2 = n1;
+			n2.op = OINDREG;
+			n2.xoffset = 0;
+			nodconst(&tmp, types[tptr], 0);
+			gins(optoas(OCMP, types[tptr]), &n2, &tmp);
+			patch(gbranch(a, types[tptr]), to);
+			regfree(&n1);
+			break;
+		}
 
 		a = optoas(a, nr->type);
 

src/cmd/8g/cgen.c

@@ -838,6 +838,25 @@ bgen(Node *n, int true, Prog *to)
 			regfree(&n1);
 			break;
 		}
+		
+		if(isinter(nl->type)) {
+			// front end shold only leave cmp to literal nil
+			if((a != OEQ && a != ONE) || nr->op != OLITERAL) {
+				yyerror("illegal interface comparison");
+				break;
+			}
+			a = optoas(a, types[tptr]);
+			regalloc(&n1, types[tptr], N);
+			agen(nl, &n1);
+			n2 = n1;
+			n2.op = OINDREG;
+			n2.xoffset = 0;
+			nodconst(&tmp, types[tptr], 0);
+			gins(optoas(OCMP, types[tptr]), &n2, &tmp);
+			patch(gbranch(a, types[tptr]), to);
+			regfree(&n1);
+			break;
+		}
 
 		if(isfloat[nr->type->etype]) {
 			nodreg(&tmp, nr->type, D_F0);

src/cmd/gc/typecheck.c

@@ -323,8 +323,16 @@ reswitch:
 				n->op = OADDSTR;
 		}
 		if(et == TINTER) {
-			n->etype = n->op;
-			n->op = OCMPIFACE;
+			if(l->op == OLITERAL && l->val.ctype == CTNIL) {
+				// swap for back end
+				n->left = r;
+				n->right = l;
+			} else if(r->op == OLITERAL && r->val.ctype == CTNIL) {
+				// leave alone for back end
+			} else {
+				n->etype = n->op;
+				n->op = OCMPIFACE;
+			}
 		}
 		n->type = t;
 		goto ret;