re: Decus C and "grinding out compil

[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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