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[cc1] Add warning about statements without side effects

This is an useful warning because it indicates to the user
that it is possible that maybe he doesn't want such statement
since it is not going to do anything.

--- a/cc1/code.c

+++ b/cc1/code.c

@@ -466,6 +466,10 @@

 	np->left = lp;

 	np->right = rp;

+	if (lp)

+		np->flags |= lp->flags & NEFFECT;

+	if (rp)

+		np->flags |= rp->flags & NEFFECT;

 	return np;

 }

--- a/cc1/expr.c

+++ b/cc1/expr.c

@@ -530,6 +530,7 @@

 	Node *inc;

 	chklvalue(np);

+	np->flags |= NEFFECT;

 	if (!tp->defined) {

 		errorp("invalid use of undefined type");

@@ -739,6 +740,7 @@

 			break;

 		case '(':

 			lp = arguments(lp);

+			lp->flags |= NEFFECT;

 			break;

 		default:

 			return lp;

@@ -1047,6 +1049,7 @@

 		default: return np;

 		}

 		chklvalue(np);

+		np->flags |= NEFFECT;

 		next();

 		np = (fun)(op, np, assign());

 	}

--- a/cc1/stmt.c

+++ b/cc1/stmt.c

@@ -38,6 +38,8 @@

 static void

 stmtexp(Symbol *lbreak, Symbol *lcont, Switch *lswitch)

 {

+	Node *np;

+

 	if (accept(';'))

 		return;

 	if (yytoken == IDEN && ahead() == ':') {

@@ -45,7 +47,10 @@

 		stmt(lbreak, lcont, lswitch);

 		return;

 	}

-	emit(OEXPR, expr());

+	np = expr();

+	if ((np->flags & NEFFECT) == 0)

+		warn("expression without side effects");

+	emit(OEXPR, np);

 	expect(';');

 }