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LET'S BUILD A COMPILER!
By
Jack W. Crenshaw, Ph.D.
24 July 1988
Part I: INTRODUCTION
*****************************************************************
* *
* COPYRIGHT NOTICE *
* *
* Copyright (C) 1988 Jack W. Crenshaw. All rights reserved. *
* *
*****************************************************************
INTRODUCTION
This series of articles is a tutorial on the theory and practice
of developing language parsers and compilers. Before we are
finished, we will have covered every aspect of compiler
construction, designed a new programming language, and built a
working compiler.
Though I am not a computer scientist by education (my Ph.D. is in
a different field, Physics), I have been interested in compilers
for many years. I have bought and tried to digest the contents
of virtually every book on the subject ever written. I don't
mind telling you that it was slow going. Compiler texts are
written for Computer Science majors, and are tough sledding for
the rest of us. But over the years a bit of it began to seep in.
What really caused it to jell was when I began to branch off on
my own and begin to try things on my own computer. Now I plan to
share with you what I have learned. At the end of this series
you will by no means be a computer scientist, nor will you know
all the esoterics of compiler theory. I intend to completely
ignore the more theoretical aspects of the subject. What you
_WILL_ know is all the practical aspects that one needs to know
to build a working system.
This is a "learn-by-doing" series. In the course of the series I
will be performing experiments on a computer. You will be
expected to follow along, repeating the experiments that I do,
and performing some on your own. I will be using Turbo Pascal
4.0 on a PC clone. I will periodically insert examples written
in TP. These will be executable code, which you will be expected
to copy into your own computer and run. If you don't have a copy
of Turbo, you will be severely limited in how well you will be
able to follow what's going on. If you don't have a copy, I urge
you to get one. After all, it's an excellent product, good for
many other uses!
Some articles on compilers show you examples, or show you (as in
the case of Small-C) a finished product, which you can then copy
and use without a whole lot of understanding of how it works. I
hope to do much more than that. I hope to teach you HOW the
things get done, so that you can go off on your own and not only
reproduce what I have done, but improve on it.
This is admittedly an ambitious undertaking, and it won't be done
in one page. I expect to do it in the course of a number of
articles. Each article will cover a single aspect of compiler
theory, and will pretty much stand alone. If all you're
interested in at a given time is one aspect, then you need to
look only at that one article. Each article will be uploaded as
it is complete, so you will have to wait for the last one before
you can consider yourself finished. Please be patient.
The average text on compiler theory covers a lot of ground that
we won't be covering here. The typical sequence is:
o An introductory chapter describing what a compiler is.
o A chapter or two on syntax equations, using Backus-Naur Form
(BNF).
o A chapter or two on lexical scanning, with emphasis on
deterministic and non-deterministic finite automata.
o Several chapters on parsing theory, beginning with top-down
recursive descent, and ending with LALR parsers.
o A chapter on intermediate languages, with emphasis on P-code
and similar reverse polish representations.
o Many chapters on alternative ways to handle subroutines and
parameter passing, type declarations, and such.
o A chapter toward the end on code generation, usually for some
imaginary CPU with a simple instruction set. Most readers
(and in fact, most college classes) never make it this far.
o A final chapter or two on optimization. This chapter often
goes unread, too.
I'll be taking a much different approach in this series. To
begin with, I won't dwell long on options. I'll be giving you
_A_ way that works. If you want to explore options, well and
good ... I encourage you to do so ... but I'll be sticking to
what I know. I also will skip over most of the theory that puts
people to sleep. Don't get me wrong: I don't belittle the
theory, and it's vitally important when it comes to dealing with
the more tricky parts of a given language. But I believe in
putting first things first. Here we'll be dealing with the 95%
of compiler techniques that don't need a lot of theory to handle.
I also will discuss only one approach to parsing: top-down,
recursive descent parsing, which is the _ONLY_ technique that's
at all amenable to hand-crafting a compiler. The other
approaches are only useful if you have a tool like YACC, and also
don't care how much memory space the final product uses.
I also take a page from the work of Ron Cain, the author of the
original Small C. Whereas almost all other compiler authors have
historically used an intermediate language like P-code and
divided the compiler into two parts (a front end that produces
P-code, and a back end that processes P-code to produce
executable object code), Ron showed us that it is a
straightforward matter to make a compiler directly produce
executable object code, in the form of assembler language
statements. The code will _NOT_ be the world's tightest code ...
producing optimized code is a much more difficult job. But it
will work, and work reasonably well. Just so that I don't leave
you with the impression that our end product will be worthless, I
_DO_ intend to show you how to "soup up" the compiler with some
optimization.
Finally, I'll be using some tricks that I've found to be most
helpful in letting me understand what's going on without wading
through a lot of boiler plate. Chief among these is the use of
single-character tokens, with no embedded spaces, for the early
design work. I figure that if I can get a parser to recognize
and deal with I-T-L, I can get it to do the same with IF-THEN-
ELSE. And I can. In the second "lesson," I'll show you just
how easy it is to extend a simple parser to handle tokens of
arbitrary length. As another trick, I completely ignore file
I/O, figuring that if I can read source from the keyboard and
output object to the screen, I can also do it from/to disk files.
Experience has proven that once a translator is working
correctly, it's a straightforward matter to redirect the I/O to
files. The last trick is that I make no attempt to do error
correction/recovery. The programs we'll be building will
RECOGNIZE errors, and will not CRASH, but they will simply stop
on the first error ... just like good ol' Turbo does. There will
be other tricks that you'll see as you go. Most of them can't be
found in any compiler textbook, but they work.
A word about style and efficiency. As you will see, I tend to
write programs in _VERY_ small, easily understood pieces. None
of the procedures we'll be working with will be more than about
15-20 lines long. I'm a fervent devotee of the KISS (Keep It
Simple, Sidney) school of software development. I try to never
do something tricky or complex, when something simple will do.
Inefficient? Perhaps, but you'll like the results. As Brian
Kernighan has said, FIRST make it run, THEN make it run fast.
If, later on, you want to go back and tighten up the code in one
of our products, you'll be able to do so, since the code will be
quite understandable. If you do so, however, I urge you to wait
until the program is doing everything you want it to.
I also have a tendency to delay building a module until I
discover that I need it. Trying to anticipate every possible
future contingency can drive you crazy, and you'll generally
guess wrong anyway. In this modern day of screen editors and
fast compilers, I don't hesitate to change a module when I feel I
need a more powerful one. Until then, I'll write only what I
need.
One final caveat: One of the principles we'll be sticking to here
is that we don't fool around with P-code or imaginary CPUs, but
that we will start out on day one producing working, executable
object code, at least in the form of assembler language source.
However, you may not like my choice of assembler language ...
it's 68000 code, which is what works on my system (under SK*DOS).
I think you'll find, though, that the translation to any other
CPU such as the 80x86 will be quite obvious, though, so I don't
see a problem here. In fact, I hope someone out there who knows
the '86 language better than I do will offer us the equivalent
object code fragments as we need them.
THE CRADLE
Every program needs some boiler plate ... I/O routines, error
message routines, etc. The programs we develop here will be no
exceptions. I've tried to hold this stuff to an absolute
minimum, however, so that we can concentrate on the important
stuff without losing it among the trees. The code given below
represents about the minimum that we need to get anything done.
It consists of some I/O routines, an error-handling routine and a
skeleton, null main program. I call it our cradle. As we
develop other routines, we'll add them to the cradle, and add the
calls to them as we need to. Make a copy of the cradle and save
it, because we'll be using it more than once.
There are many different ways to organize the scanning activities
of a parser. In Unix systems, authors tend to use getc and
ungetc. I've had very good luck with the approach shown here,
which is to use a single, global, lookahead character. Part of
the initialization procedure (the only part, so far!) serves to
"prime the pump" by reading the first character from the input
stream. No other special techniques are required with Turbo 4.0
... each successive call to GetChar will read the next character
in the stream.
{--------------------------------------------------------------}
program Cradle;
{--------------------------------------------------------------}
{ Constant Declarations }
const TAB = ^I;
{--------------------------------------------------------------}
{ Variable Declarations }
var Look: char; { Lookahead Character }
{--------------------------------------------------------------}
{ Read New Character From Input Stream }
procedure GetChar;
begin
Read(Look);
end;
{--------------------------------------------------------------}
{ Report an Error }
procedure Error(s: string);
begin
WriteLn;
WriteLn(^G, 'Error: ', s, '.');
end;
{--------------------------------------------------------------}
{ Report Error and Halt }
procedure Abort(s: string);
begin
Error(s);
Halt;
end;
{--------------------------------------------------------------}
{ Report What Was Expected }
procedure Expected(s: string);
begin
Abort(s + ' Expected');
end;
{--------------------------------------------------------------}
{ Match a Specific Input Character }
procedure Match(x: char);
begin
if Look = x then GetChar
else Expected('''' + x + '''');
end;
{--------------------------------------------------------------}
{ Recognize an Alpha Character }
function IsAlpha(c: char): boolean;
begin
IsAlpha := upcase(c) in ['A'..'Z'];
end;
{--------------------------------------------------------------}
{ Recognize a Decimal Digit }
function IsDigit(c: char): boolean;
begin
IsDigit := c in ['0'..'9'];
end;
{--------------------------------------------------------------}
{ Get an Identifier }
function GetName: char;
begin
if not IsAlpha(Look) then Expected('Name');
GetName := UpCase(Look);
GetChar;
end;
{--------------------------------------------------------------}
{ Get a Number }
function GetNum: char;
begin
if not IsDigit(Look) then Expected('Integer');
GetNum := Look;
GetChar;
end;
{--------------------------------------------------------------}
{ Output a String with Tab }
procedure Emit(s: string);
begin
Write(TAB, s);
end;
{--------------------------------------------------------------}
{ Output a String with Tab and CRLF }
procedure EmitLn(s: string);
begin
Emit(s);
WriteLn;
end;
{--------------------------------------------------------------}
{ Initialize }
procedure Init;
begin
GetChar;
end;
{--------------------------------------------------------------}
{ Main Program }
begin
Init;
end.
{--------------------------------------------------------------}
That's it for this introduction. Copy the code above into TP and
compile it. Make sure that it compiles and runs correctly. Then
proceed to the first lesson, which is on expression parsing.
*****************************************************************
* *
* COPYRIGHT NOTICE *
* *
* Copyright (C) 1988 Jack W. Crenshaw. All rights reserved. *
* *
*****************************************************************