Are yacc parsers portable ?

[Naim Abdullah]

[I am a sporadic reader of this newsgroup, so please forgive me if this
 question has been beaten to death here].
 
Is the C parser produced by yacc, portable ? I was looking at it and it had
stuff like:

yyparse() {

	short yys[YYMAXDEPTH];

	register short yystate, *yyps, yyn;
        ...
	yyps= &yys[-1];
	...
}

It seems to me that negative indices on arrays are probably not portable.
Is it really necessary for yacc to do these tricks, or can it produce
portable code that is just as efficient ? Is bison any better in this respect ?

Thank you.

		      Naim Abdullah
		      Dept. of EECS,
		      Northwestern University

		      Internet: naim at eecs.nwu.edu
		      Uucp: {oddjob, chinet, att}!nucsrl!naim