Re: Hand-written rec-descent parser of GCC-4.1 is WRONG!!!.

[Joe Buck <Joe dot Buck at synopsys dot COM>]

On Wed, Mar 16, 2005 at 02:41:12AM +0100, jc-nospam@jr-pizarro.jazztel.es wrote:
> > http://gcc.gnu.org/gcc-4.1/changes.html
> > 
> > New Languages and Language specific improvements
> > C and Objective-C
> > 
> >     * The old Bison-based C and Objective-C parser has been replaced
> >  by a new, faster hand-written recursive-descent parser.
> 
> Hahahahaha, WRONG!!!!!!
> It's one historical error of many years of stupid engineers, developers,
> programmers and boss!!!!!

OK, we are all stupid, and you think that you are smart.

> "Hand-written recursive-descent parser" == LL(k)
> however, people does it as LL(1).
> 
> The grammar of Pascal language can be LL(1), but the grammar of
> C, C++ and Objective-C together won't can be LL(1).

But yet gcc 3.4 has a hand-written recursive descent parser for C++.  How
can this be?  The answer, of course, is that it is not LL(1).

> However, they together can be LALR(1) (or impossible in worst-case) from
> Yacc/Bison.

Make that "impossible".  C++ is not LALR(1), and in fact it requires
unbounded lookahead.  Back when we had a Bison parser for C++, it was
made to work by putting another pass between the lexer and the parser to
look ahead far enough to disambiguate some of the nasty cases.  However,
there were many corners of the language that it could not parse correctly.
That's why we replaced it.

> Writing Hand-written recursive-descent parser miss-cleans the source code
> and goes hardfully to maintain it!!!

Not if you know how to write one correctly.

> The best option is a clean grammar in Yacc/Bison!.

GCC had such parsers for over a decade, and yet they are being replaced.

Bison remains a good solution in many cases, especially for languages
specifically designed to be easy to parse with an LALR parser (that is,
languages that don't look like C).