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Check if variable is register type before taken address

It is forbidden to take the address of a variable defined as
register.

--- a/expr.c

+++ b/expr.c

@@ -369,7 +369,7 @@

 {

 	register Node *np;

 	Type *tp;

-	char paren, op;

+	char paren, op, *err;

 	uint8_t t;

 	switch (yytoken) {

@@ -410,8 +410,9 @@

 	switch (op) {

 	case OADDR:

 		if (!np->b.lvalue)

-			goto bad_operand;

-		/* TODO: check if variable is register */

+			goto no_lvalue;

+		if (np->code == emitsym && np->u.sym->s.isregister)

+			goto reg_address;

 		tp = mktype(tp, PTR, NULL, 0);

 		break;

 	case OEXC:

@@ -438,8 +439,16 @@

 	return unarycode(op, tp, np);

+no_lvalue:

+	err = "lvalue required in unary expression";

+	goto error;

+reg_address:

+	err = "address of register variable '%s' requested";

+	goto error;

 bad_operand:

-	error("bad operand in unary expression");

+	err = "bad operand in unary expression";

+error:

+	error(err, yytext);

 }

 static Node *