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[cc1] Add warnings in builtin_va_start()

There are a lot of strange conditions in va_start()
that should be informed as warnings to the user,
because any of them can generate wrong code in
some implementations/architectures.

--- a/cc1/builtin.c

+++ b/cc1/builtin.c

@@ -18,7 +18,7 @@

 	if (!valid_va_list(ap->type)) {

 		errorp("incorrect parameters for va_arg");

-		return constnode(zero);

+		goto error;

 	}

 	if (tp == booltype ||

 	    tp == chartype || tp == uchartype || tp == schartype ||

@@ -30,6 +30,9 @@

 	np = node(OBUILTIN, tp, ap, NULL);

 	np->sym = sym;

 	return np;

+

+error:

+	return constnode(zero);

 }

 static Node *

@@ -55,16 +58,31 @@

 builtin_va_start(Symbol *sym)

 {

 	Node *np, *ap, *last;

+	Symbol **p, *lastsym;

+	Type *tp;

 	ap = assign();

 	expect(',');

 	if (yytoken != IDEN)

 		goto error;

-	last = varnode(yylval.sym);

+	lastsym = yylval.sym;

+	last = varnode(lastsym);

 	next();

-	if (!valid_va_list(ap->type))

-		goto error;

+	if (!valid_va_list(ap->type) || !(lastsym->flags&SDECLARED))

+		 goto error;

+

+	for (p = curfun->u.pars; p && *p != lastsym; ++p)

+		/* nothing */;

+	if (!p || *p == NULL || p[1] == NULL || p[1]->type != ellipsistype)

+		warn("second parameter of 'va_start' not last named argument");

+

+	tp = last->type;

+	if (tp == booltype ||

+	    tp == chartype || tp == uchartype || tp == schartype ||

+	    tp == shortype || tp == ushortype) {

+		warn("last parameter before '...' must not be bool, char or short");

+	}

 	np = node(OBUILTIN, voidtype, ap, last);

 	np->sym = sym;

--- a/cc1/code.c

+++ b/cc1/code.c

@@ -470,8 +470,6 @@

 	for (sp = sym->u.pars; sp && *sp; ++sp)

 		emit(ODECL, *sp);

 	fputs("\\\n", outfp);

-	free(sym->u.pars);

-	sym->u.pars = NULL;

 }

 static void

--- a/cc1/decl.c

+++ b/cc1/decl.c

@@ -901,6 +901,8 @@

 	compound(NULL, NULL, NULL);

 	popctx();

 	emit(OEFUN, NULL);

+	free(sym->u.pars);

+	sym->u.pars = NULL;

 	flushtypes();

 	curfun = ocurfun;

 }