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https://github.com/boothby/repiet

A compiler for the esoteric language Piet, targeting multiple backends.
https://github.com/boothby/repiet

piet python

Last synced: 8 months ago
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A compiler for the esoteric language Piet, targeting multiple backends.

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README 

          
# Repiet: The Brutalizing Piet Recompiler



A Piet program welcoming you to repiet; a mostly-white field with thin, multicolored lines on the left side and a diagonal arrangement of red tee-shapes on the right side.  Execute it to find out what it does!



Repiet is a compiler for the [Piet](http://www.dangermouse.net/esoteric/piet.html)

graphical language, written in Python.  The name "repiet" is meant to convey that

it can recompile Piet programs into Piet.  Additionally, there are Python and C++

backends (more to come, and please contribute!).



A small, colorful, square Piet program which is used to illustrate various compiler passes.



To compile a Piet program, we lex it, parse it, and optionally perform some

optimizations.  The result of parsing and subsequent passes are parse graphs; our

intermediate representation.  The compiler (`repiet.compiler.compiler`)

collaborates with backends to grok this representation, and emit a program in a

target language.



We describe `repiet` as a "brutalizing recompiler" because it will take your

beautiful, hand-crafted Piet program and reconstruct it in a

[brutalist](https://en.wikipedia.org/wiki/Brutalist_architecture) style.  Who

knows why you'd want that.  The images referenced in this README are a somewhat

[randomly-generated Piet program](https://codegolf.stackexchange.com/a/3226) --

tweaked a little bit so it does *something* and also terminates.  Spoiler: it

takes a number from the user, emits the corresponding ascii character, and 

terminates, but spends most of its time NOPping.



## First Pass: Lexing



A visualization of a lexed Piet program, where pixel regions have been merged into 'lexemes', which contain numbers indicating the number of codels they contain; their corners decorated with arrows indicating dp/cc  directions, and sliding regions are decorated with arrows indicating how sliding through those regions proceeds.



The lexer (`repiet.lexer.Lexer`) computes lexemes, or cardinally-connected sets

of same-colored pixels, and identifies whitespace and blocking features 

(by default, these are all pixels which are neither white nor coding colors; this

may be controlled through `opinions` described in `repiet.util`).  As the lexemes

are computed, we locate eight corners of each; corresponding to the  eight (`dp`,

`cc`) states.  As whitespace is located, we identify the 4 pixels reached by

sliding in each of four `dp` directions.



The role of the lexer is to eliminate pixel-based computations from subsequent

passes.



## Second Pass: Parsing



A visualiation of a parsed Piet program, where lexemes have been split into one or more parse nodes, and unreachable nodes have been discarded.  Parse nodes retain the shapes of their original lexemes, contain text indicating the operation emitted by that node, and have arrows to their destinations -- branching operations have multiple outgoing arrows.  The initial node is indicated with a large triangle, and a termination point is indicated with a heavy box containing an X.



The parser begins in the upper-left corner of the image, and computes a parse

graph whose nodes correspond to (lexeme, `dp`, `cc`) states.  For each node, we

compute the operation (if any) resulting from the (`dp`, `cc`)-ward transition

from the lexeme, as well if the next visited lexeme (if any).  If the operation

 is `switch` or `pointer`, there are two or four possible next-visited lexemes

respectively.



The recompiled version of the example program with optimization level zero (parsing only).



The role of the parser is to eliminate the `dp` and `cc` from subsequent passes.

Thus, interpreters and compilers of the intermediate representations need only

grok parse graph, handle input, output, and other stack-based operations.



### Intermediate Representation



A parse graph is a [rooted digraph](https://en.wikipedia.org/wiki/Rooted_digraph)

containing Nodes (`repiet.util.Node`) which consist of a name, zero or more

operations, and a list of outgoing neighbor's names. To compile this

representation into another language, one need only implement the

[trampoline](https://en.wikipedia.org/wiki/Trampoline_(computing)) pattern.



A node may have zero, one, two, or four children.  These respectively correspond

to halting, jumping, and the `switch` or `pointer` operations.  Hence, a node's

operation list is constrained such that only the final operation may be `switch`

or `pointer`.  To implement these two operations, a compiled program pops a value

from the stack, takes that value modulo 2 or 4, respectively, and uses the result

as the index to the list of children.  If the stack is empty, the value is taken

to be zero.



The backends (`repiet.backends`) are quite rudimentary, and only grok this very

simple IR.  Thus, the only optimizations available to us are those which perform

surgery on parse graphs.



## Optimizing Passes



A visualiation of a traced Piet program, where parse nodes have been merged into trace nodes.  Parse nodes retain the shapes of their original lexemes, and have arrows to their destinations -- branching operations have multiple outgoing arrows.  The initial node is indicated with a large triangle, and a termination point is indicated with a heavy box containing an X.



We implement a Tracer (`repiet.tracer.Tracer`) which collects a parse graph into

sequences of non-branching operations.  The result is a new parse graph, typically

with fewer nodes. Depending on the backend chosen, this may be a slight 

optimization.



The recompiled version of the example program with optimization level one (parsing and tracing).



Additionally, we implement a rudimentary Static Evaluator 

(`repiet.optimizer.StaticEvaluator`), which maintains compile-time stack while

tracing through instructions.  Presently, the static evaluator stops whenever the

program (a) takes input from the user, (b) tries to pop from an empty stack, or 

(c) attempts to roll beyond the depth of the stack. Further optimizations may be

possible: the values coming off of an empty stack could be treated as symbols,

for example.  Currently the backends are incapable of representing symbolic

variables, so such operations would require quite a bit of work.



The static evaluator is a work in progress, and it looks reasonable to interpose

a stack-depth analyzer between the tracer and static evaluator: the language spec

recommends skipping instructions that pop from an empty stack, which is especially

visible in this page's working example.  The existing static evaluator does not

drop those instructions, but they're apparently rare in hand-crafted or assembled

Piet programs.



# Installing and Using `repiet`



Repiet is a `python` package, with a standard `setup.py`.  To get the very latest,

fetch the git repo and install from there.



    git clone https://github.com/boothby/repiet.git

    cd repiet

    python setup.py install

 

Otherwise, just run `pip install repiet` and you're off to the races.  The primary

interface to `repiet` is the module's `__main__`, but you can also `import` it

`repiet` from python and poke around the module structure.  The `__main__` can be

used either as an executable python module,



    python -m repiet ...



or depending on how it's been installed, directly from the command line



    repiet ...



## But is it Faster Than C?






Yes!  Well, let's back up a minute for the folks missing context.  Recently there

have been a spate of blog posts of folks claiming that their pet language is

"faster than C" by implementing a feature-incomplete version of the 

[wc](http://man7.org/linux/man-pages/man1/wc.1.html) utility and running a single

benchmark to demonstrate superiority.



After cloning the `repiet` repo, point your command line at that directory.  We've

placed a `wc` utility into the `assets` directory, and we're going to time it

versus the `wc` supplied by my operating system (you don't need to know any

details about my system, of course).



    $ cd assets

    $ repiet wc.png -o wc.c --backend c --codel_size 10 -O 2

    $ gcc wc.c -o wc -O3

  

Okay, now our wc utility is built; let's grab some data to test.



    $ wget https://github.com/dwyl/english-words/raw/master/words_dictionary.json



Now, we're off to the races!  Don't worry, I didn't cherrypick these benchmarks

at all!



    $ time cat words_dictionary.json | wc -w

    740204



    real	0m0.145s

    user	0m0.136s

    sys	0m0.036s



Wow that's fast.  Now let's our hand-optimized Piet hotrod!



    $ time cat words_dictionary.json | ./wc

    740204

    real	0m0.131s

    user	0m0.140s

    sys	0m0.028s



Holy wowzers, that's extremely peppy!  Good thing I only ran those tests once,

and didn't show a distribution of runtimes!



So there you go, Piet is faster than C.  Tell your grandma.



# Notes on Style and Quality



This project was written purely for the amusement of the author.  Hence, the code

in this project is idiomatic and [golfed](https://en.wikipedia.org/wiki/Code_golf)

to a minor extent.  Where this could cause difficulty for the reader, extensive

comments have been written to explain what's going on.  Other folks have written

tools for Piet which use short-codes for various operations.  The 3-letter opcodes

used by `repiet` are probably not used elsewhere.



Also it might be worth pointing out that the author has never taken a course on

compilers, read a book about compilers, or generally done much more than read

basic definitions about compilers.  If somebody's got some well-reasoned opinions

about how this project sucks, please be nice, but do share.  Several parts are

significantly works in progress, and there are "tests" but they are not integrated

and do not provide a reasonable degree of coverage.



# Credits and License



The current version of this code is almost entirely the work of Kelly Boothby, and

distributed under the MIT license. If you look far enough back in the git history,

you'll find an interpreter written by Ross Tucker which is distributed on DMM's

website without any mention of licesnse.  That interpreter was hijacked, modified

into a rudimentary compiler, and deleted once obsolete.  Two lines of Ross's code

survive untouched, where the virtual machine performs a `roll` operation.  It is

the author's opinion that these lines fall under fair use.



Redistribution of Ross's code (which appears in early commits) is a bit of a grey

area, and we only claim copyright for the diffs to that code and for the other 

files in the repo.  If anybody knows Ross, and Ross takes issue with that code

living in the history, please submit an issue and we can revise history to make

good.