TinyCompiler: A compiler in a week-end
116 comments
·February 20, 2025glouwbug
fooker
I’d argue that you should start with doing an interpreter and push it as long as you can.
But once you’re ready to generate code, use llvm or something like that because you are going to hit very problematic roadblocks early in your effort that will kill your interest in compilers otherwise.
Some examples are: ensuring you can compare expressions for equality, ensuring that changed expressions don’t violate dominance, SSA conversion, avoiding infinite loops when analyzing cfgs. If you don’t start with knowledge of several things like this, you are looking at rewriting your compiler a dozen times. That is a great way to learn about compilers, but perhaps not the first compiler you make.
glouwbug
That's right. Crenshaw's LBAC can make you an interpreter too. LBAC's magic lies in single token read ahead, single character identifiers, and single character digits. It parses its BNF grammar only with recursive function calls (no stacks, no tables for operator precedence, etc). The final language you build is almost useless, but it is done without a standard library (eg. resizable strings, resizable arrays, stacks, queues, associative containers), and serves as an introduction to get one hooked on more advanced compiler theory like SSA. I love OP's work. I consider OP's implementation (and similarities) to be a modern day LBAC. It's just that LBAC is one of those things I'd take to the grave. I have never found such a concise introductory text that produces a working final example in such delightful piecewise prose.
muth02446
Shameless plug for another relatively small compiler written in Python with no dependencies: http://cwerg.org
mingodad
But if otherwise anyone want to try with Yacc/Lex I have an online interpreter/editor with more than 250 examples to try/study at https://mingodad.github.io/parsertl-playground/playground/ and just included a grammar for tinycompiler there (select "Tinycompiler parser" from "Examples" then click "Parse" to see a parse tree for the content in "Input source").
bwfan123
lovely from-scratch implementation, without the magic and complexity of lex and yacc. brings back memories from 30+ years ago when I implemented a subset of C in C using lex and yacc in a compiler course.
in your blog, i would have liked to see the BNF grammar (without having to peek into the code).
now, heres your next assignment :) - do the same for functional programming (lambda calc) or logic programming (prolog). That would round out the education on languages and compilers for computation.
nurettin
Not to take away from the author or you, but we had things like this 10 years ago https://github.com/ymyzk/tinyc/tree/master/tinyc
glouwbug
Unless I am mistaken, this uses LLVM?
ymyzk
I'm the author of ymyzk/tinyc and was surprised to see my project mentioned here after more than 10 years. Since the README is written in Japanese, let me answer in English here. The project has two backends: one for NASM (x86) and another for LLVM IR.
nurettin
It can. It can also output x86 nasm, here's the relevant part.
https://github.com/ymyzk/tinyc/blob/master/tinyc/generator/n...
sitkack
Only a happy customer, but if you want an amazing experience building a compiler I highly recommend David Beazley's Compiler Course (or anything else you can sign up for) https://www.dabeaz.com/
cellularmitosis
While I loved this week-long course, I was a bit disappointed that we offloaded all of the code generation to LLVM. I think it would be fantastic to stretch this into a two-week course and actually emit assembly.
sitkack
When I took the course, each person did their own thing when it came to code generation. I don't recall anyone using LLVM. I think emitting assembly is easier than using LLVM.
I would love a a longer course.
dailykoder
The fire.wend code[1] made me smile! I love how oneself tries to push boundaries with a language that barely can do anything, just do prove it to yourself that is works.
Code is 10/10. Go for merge! :-)
- [1] https://github.com/ssloy/tinycompiler/blob/main/test-program...
keyle
Funnily enough, wend looks like what fun programming means to me. C like (prefixed types) syntax, strongly typed but without heartaches of pointers, and simple types.
Every features on top of that has either leaky abstractions and/or nightmare scenarios.
That said I'm not claiming the world should run on this type of code. Or should it.
glouwbug
I think OP is just trying to demystify the complexity of compliers for the sake of an educational resource. It's high quality work, compressed to a very small line count
atan2
I'm aware people here frown upon paid content but I am currently taking this course and it's excellent:
https://pikuma.com/courses/create-a-programming-language-com...
gustavopezzi
Hey there. Thanks for the mention and for the kind words.
jcuenod
Thanks! I appreciate your discussion of semantic analysis for wend. I've literally just embarked on creating a (yet another) "compiles to JSX" language[0][1]. I'm using ANTLR for my lexer/parser, but I've just hit a point where I can start doing semantic analysis :)
pjmlp
Great project, this is how language exploration should start, and then carry it from there.
fogleman
I've been a big fan of the author's tiny renderer. Nice to see a tiny compiler too!
kragen
That sounds interesting! He seems to have four tiny renderers pinned on his GitHub page; is https://github.com/ssloy/tinyrenderer the one you're recommending? What do you like about it?
mukunda_johnson
Pretty neat. I would have expected it to take a bit longer, but I suppose the optimizing step is the real magic in a compiler.
zvmaz
I don't think one can understand compilers in a "week-end".
haqreu
The "week-end" part refers to the fact that I wrote it in one week-end with little to no experience in compiler design. You can check the commit history, 12 to 14 Jan, 2024 :)
not_a_bot_4sho
I think the "week-end" reference up above was a little tongue in cheek, since the modern spelling is a compound word: https://linguix.com/english/common-mistake/week_end_hyphen
Not to take away from this awesome sharing.
sieve
Compilers are some of the simplest "complicated" programs out there if you start by throwing yacc/bison/antlr/parser generators in the garbage.
Production compilers are complicated because of the feature set of languages and performance requirements.
You can write a lexer + parser + treewalk interpreter for a simple language in a day if you know what you are doing.
whartung
The hardest part (IMHO) about getting a simple language running, particularly for beginners, is the “expression”. Mostly for your typical algebraic style infix expressions with precedence.
Just getting the grammar straight on the naturally recursive structures can be a trick. They also touch a large portion of the code generation and run time.
Get:
(a + c/2) * sqrt(b)
sieve
> The hardest part (IMHO) about getting a simple language running, particularly for beginners, is the “expression”
This is why teaching material should be tailored towards teaching rather than implementing demented stuff from the real world just because it is "there." You can easily implement a parser that ignores precedence and tell the reader that we are using brackets to force precedence instead.
In a real-word parser, you might use something like this[1] or some other home-grown algorithm if you didn't know about it. Doesn't matter as long as the parser works.
[1] Top-Down operator precedence (Pratt) parsing https://eli.thegreenplace.net/2010/01/02/top-down-operator-p...
andai
Is that really the hard part? I figured that part out before I knew much programming: made a simple Excel type formula parser. (A horrible, horrible solution based around replacing strings and manually counting parentheses... but it worked!)
I never got much farther than that though, I've looked into making a compiler many times and got overwhelmed every time.
rurban
(a + c/2) * sqrt(b)
(* (+ a (/ c 2)) (sqrt b))
Rohansi
Pratt parsers are a really nice way to handle mathematical expressions like this vs. the usual recursive descent.
https://journal.stuffwithstuff.com/2011/03/19/pratt-parsers-...
n42
well, in my personal experience, parsing is the easiest part. it's what you do with the AST once you have one that is the hard part. but for some reason an overwhelming amount of writing is about frontends when you could teach the concepts to a layman with some boxes and arrows.
milesrout
I think the hardest part is function calls. Implementing the standard calling convention is really fiddly and annoying when you have to save and restore registers around function calls if you want to do it efficiently (keeping track of which registers are dirty, doing parallel loads of registers into the right argument registers, spilling when appropriate, etc. It is fiddly.
The alternative is going to an IR and doing full register allocation but then you need to implement Lengauer-Tarjan to get into SSA, all the same parallel load stuff for phi functions/block arguments, out-of-SSA, reconstruct in optimisation passes all the information you discard by going to a linear IR, etc.
RossBencina
> lexer + parser + treewalk interpreter
If all you do is construct an AST and interpret it it's not really a compiler is it. It doesn't compile from source to target. At most you're describing a compiler front end (arguably), or an interpreter.
I would expect:
lexer + parser + AST definition and construction + semantic analysis/type checking + X + codegen to ASM/WASM/C
where X includes
- definition of an intermediate representation (IR)
- lowering AST to IR
- static analysis
- improvers/optimisation passes (at least some simple stuff)
- code gen including register allocation
EDIT: I notice that the author of the original article has also started work on an optimizing compiler: https://ssloy.github.io/tinyoptimizer/
sieve
> If all you do is construct an AST and interpret it it's not really a compiler is it.
What if I dumped the AST to disk, called it "bytecode" and updated the "interpreter" to run this "bytecode"? Would you consider that to be a compiler? :-)
> where X includes
I don't consider any of these things to be essential. There are entire languages out there which run on interpreters of some kind. There are others which compile down to C/JS. Many compilers use LLVM as their backend. By your definition, none of them use (or are) compilers.
jcuenod
Honestly, antlr made this pretty straightforward to me. I didn't want to work in java, but they have a ton of targets. You can definitely write it all yourself, and that's a great learning exercise. But I wanted to get a parser for a language idea I had in mind and it took a couple of days with antlr (https://github.com/chicory-lang/chicory)
sieve
Problem with these tools is, you have to spend time wrangling them instead of learning to write a lexer/parser yourself.
A recursive-descent parser is a beautiful thing and can be implemented very quickly.
sitkack
5 days if you don't, but have the right tutor.
sieve
If you know basic programming (js/python), a day is more than enough to get the concept across using a tiny language. Could probably be done in an hour.
The problem, always, is people are given terrible reading recommendations which makes the subject more complicated than it is.
duped
All a compiler does is take a well defined data structure and transform it into another, sometimes by encoding or sometimes by decoding. Really it's bread and butter stuff that every engineer does everyday but with the benefit that you can say something is undefined/out of scope way easier than talking to APIs you don't control.
The fun part are the magic algorithms you sometimes get to use
jcranmer
> All a compiler does is take a well defined data structure and transform it into another
Sorry, having worked on language semantics topics, "well-defined" is not how I would describe either a programming language or the behavior of machine code.
storoj
There should be a well-defined list of undefined behaviors.
null
mattgreenrocks
One weekend is enough to survey the general concepts. That’s extremely useful on its own, as now you have a better idea of how to orient yourself.
ramon156
You set your own bar. you could understand "how a compiler" in a day, but how much do you want to know?
sylware
On the same way than cproc+QBE I guess, 70% of gcc speed in my benchmarks.
The more real-life alternatives to those abominations of gcc and clang, the merrier.
I wish the linux kernel devs did care to keep the door _reasonably_ open for such compilers (with assembler source files as alternative to inline assembly, some extensions avoidance and niche expensive compiler features).
This is another case of why super complex syntax computer languages should be avoided like hell (c++, rust, etc).
sitkack
QBE is ... problematic. The source has no comments, single letter variable names. It is not written to be extended by anyone else other than the author.
I would recommend libfirm over QBE by at least 10km.
https://libfirm.github.io/ https://github.com/libfirm/libfirm
kragen
It won't surprise you to learn that I'm always interested in hearing about alternatives to QBE, but from the documentation, libfirm sounds more like a heavyweight alternative to LLVM, not a lightweight alternative to QBE. How big is it?
sitkack
9x larger in LoC, 50x larger in comments.
I had a wall of supporting tokei but decided against it, but the short of it is, LLVM is phenomenally massive, and libfirm at 135kloc and 35kloc of comments is but a raindrop that is in the 15Mloc+ that is LLVM.
Perhaps instead of looking towards libfirm, we should look at cranelift. It is of comparable size and actually includes 12kloc of documentation, not just comments.
https://github.com/bytecodealliance/wasmtime/tree/main/crane...
contificate
On the topic of QBE, I've always felt that someone aiming to do the same scope of project ought to make their IR a syntactic subset of LLVM IR. If you do that, your test suite can involve invoking LLVM for a comparison.
As for QBE itself, many of the core transformations are fairly standard, which makes it somewhat more approachable for me (e.g. Cooper et al's dominance algorithm, Rideau et al's parallel moves algorithm, etc.). Of course, this doesn't negate the fact that it's not as "hackable" as they probably intended.
sylware
Missing RISC-V code generation.
But it is written in plain and simple C99, so it is at least much less toxic than LLVM.
I wonder how cparser (did not check if it was plain and simple C)+libfirm compare in performance to cproc+QBE on my benchmarks. May have to take some time to check that.
Whatever the results, it is always good to have, again, a real life alternative for optimizing C toolchains.
The main issues are the heavy usage of gcc extensions by linux (and glibc, etc).
sitkack
Haven't used it, but this looks rather complete https://github.com/libfirm/libfirm/tree/master/ir/be/riscv
pjmlp
Circle frontend, that includes all C++17 (when it was current) and the Rust related extensions, was implemented by a single guy.
sitkack
Relativity was discovered by a single guy.
pjmlp
Which confirms complex matters aren't an issue, being able to handle them might be.
ninalanyon
Not really. Relativity goes back to Galileo at least, and it's development by Einstein relied on Lorentz and Riemann among others. Science really is a process of /standing on the shoulders of giants/ as Newton is reputed to have said.
That doesn't diminish Einstein's achievement of course.
delta_p_delta_x
Interesting off-topic—I noticed this paragraph:
> Personally, I get a twisted pleasure wearing a T-shirt with this print in the 2020s, even though I hate being a walking billboard. But the number of angry comments from confused people makes up for any moral discomfort of advertising on my belly!
It's possible the 'angry comments' are from people who may have mistaken it for an antisemitic, white supremacist symbol[1].
haqreu
Hum. Did not anticipate that, thank you for pointing out.
kragen
Where is that paragraph?
null
I appreciate how wonderfully simple and dependency free this is. More often than not people just want to write a compiler of sorts without bison or yacc, or LLVM, and just convert expressions into assembler or vm-like instructions that _just run_. This is a great starting point for that, and I wish I had something like it 10 years ago.
(Crenshaw's Let's Build a Compiler is an excellent source too if you want to go one step lower and go standard library free, focusing only on function call stacks: https://compilers.iecc.com/crenshaw/)