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From: compilers@ima.UUCP
Newsgroups: mod.compilers
Subject: re: Decus C and "grinding out compil
Message-ID: <136300023@ima.UUCP>
Date: Mon, 6-Jan-86 22:13:00 EST
Article-I.D.: ima.136300023
Posted: Mon Jan  6 22:13:00 1986
Date-Received: Wed, 8-Jan-86 02:46:13 EST
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Approved: compilers@ima.uucp
Nf-ID: #N:ima:136300023:000:1164
Nf-From: ima!compilers    Jan  6 22:13:00 1986


[from cca!decvax!minow (Martin Minow)]

Decus C is a public-domain compiler for a subset of C that runs on
PDP-11's.  It was written about ten years ago in PDP-11 Macro (assembly
language).  (The person who wrote Decus C didn't have access to a
C compiler!)  It is fairly close to Unix V6 C, with a few extensions.
It is more-or-less a proper subset of current (K&R) C's.  Because
it is written in assembler, you would have a tough time porting
it anywhere.

While tools to generate parsers and lexical analysers make part
of the compiler-writing process look easy, code generation is
still a very difficult task.  Doing it right requires you to
make use of every assembly-language trick you ever learned.

You should also beware of using your generator tool where it
isn't appropriate: "if your only tool is a hammer, everything
starts looking like a nail."  For example, Decus C uses a
hand-crafted recursive-descent parser for statements and
operator-precedence for expressions.  A parser-generator
might try to create an lr1 or lalr parser even where it
is inappropriate: it might not be able to switch styles
in mid-parse.

Martin Minow
decvax!minow
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