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Oops, I Wrote a C++ Compiler

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Re: Oops, I Wrote a C++ Compiler

#11

Post by lilyball »

peterkelly wrote: post #9
amelius wrote: post #3

Please don't call it C++ if it doesn't even support constructors and destructors. People will be trying to port their existing code, and will be unpleasantly surprised. > But the truth is, I’m just going to keep listening to users and improving the parts I can. I am not trying to recreate GCC or LLVM. Why not use GCC or LLVM as part of your project? That seems like a better investment of your time.

> Why not use GCC or LLVM as part of your project? That's not how the world gets compiler people. Someone had to write GCC and LLVM, and someone will have to write whatever comes next. Languages and tooling only advance because there are those who choose to take the hard road instead of "just using that thing that someone else made".

The successor to LLVM isn't going to be written by someone just hacking on an app and decides they could use a basic compiler. It's going to be written by someone who has the explicit goal of replacing LLVM, fully understands LLVM's design and tradeoffs, and has ideas about an architecture for a successor that improves on the design.

Encouraging people to use LLVM instead of home-growing a toy compiler can still get you compiler people. LLVM is just the backend, not the frontend, so you still need to write the compiler frontend yourself, and once you've learned LLVM, if you're so inclined you may start hacking on LLVM and therefore become a backend compiler person too.

Re: Oops, I Wrote a C++ Compiler

#12

Post by comex »

jchw wrote: post #6
amelius wrote: post #3

Please don't call it C++ if it doesn't even support constructors and destructors. People will be trying to port their existing code, and will be unpleasantly surprised. > But the truth is, I’m just going to keep listening to users and improving the parts I can. I am not trying to recreate GCC or LLVM. Why not use GCC or LLVM as part of your project? That seems like a better investment of your time.

It is very well explained? GCC and Clang would be nightmares to integrate into an app that runs on many platforms including iOS, and even once you did, you'd still need to implement an interpreter, because you're not allowed to JIT on the app store. Compiling a toolchain for a desktop platform is easy. Cross compiling a toolchain is moderately easy. But embedding a toolchain into an iOS app, where you have to get it…

For Clang/LLVM, it wouldn’t be that hard. It’s one source tree to build (Clang is in a separate repo from LLVM core, but you check it out into a specific subdirectory and they build together), all C++, no external dependencies, and the result is a set of static libraries you can easily link into an iOS app.

As for interpreting — there are a few options. The simplest approach would be to use LLVM‘s built-in IR interpreter (lli), which did exist in 2010.

Alternately, if you want more control: these days, the targets supported by upstream LLVM include:

- eBPF

- WebAssembly

- AVR

The first two are bytecode formats, both designed for (relative) simplicity and portability. There are open-source interpreters available for both, or alternately they’re both simple enough that you could relatively easily write your own. AVR, on the other hand, is the actual instruction set used by the Arduino; I’m not sure what the status is of open-source AVR emulators (I see a few projects on GitHub but don’t know how good or complete they are), but there’s probably something out there that would work.

But to be fair, all three of those targets were merged into LLVM upstream only recently, much later than 2010. Also, 2010 was the year that Clang got C++ support working well enough to become self-hosting, according to Wikipedia [1]; it was still relatively experimental, not nearly as much of a “safe bet” as it is today. (Still, even then, it was pretty much guaranteed to work better than writing your own C++ compiler from scratch! :p)

GCC is older but much harder to embed, even today. For one thing, it’s under the GPL. iCircuit is closed source, and even if the author were willing to open it, there are supposedly legal issues with publishing GPL code to the App Store. And there are technical obstacles as well: last I checked, GCC still expects to be run as the main executable as part of a traditional Unix toolchain, not embedded as a library. If that has changed, it was only recently.

[1] https://en.m.wikipedia.org/wiki/Clang#Status_history

Re: Oops, I Wrote a C++ Compiler

#13

Post by fmihaila »

chubot wrote: post #2

The process of creating the parser went very smoothly thanks to this. The real work involves creating C# syntax classes that mirror the grammar. These classes form the Abstract Syntax Tree (AST) of the compiler. To address this problem, I use Zephyr ASDL to describe the abstract syntax of a language (which CPython also uses!) It's a little DSL that lets you write an ML-style algebraic data type, and generates a bunch…

For those writing language processors in Java, I highly recommend having a look at JastAdd [1]. It's a great framework for implementing semantic analysis using attribute grammars [2]. The tool automatically generates Java classes to represent ASTs, based on a high-level description (a CFG grammar) of the desired AST shape. You can use any Java-based parser as long as the AST it produces is constructed from those classes. That's only the starting point; the real power is in the excellent support for expressing attribute computation. It uses a high-level, lazy equational formalism extended by the programmer with custom logic, which is then turned into automatically-generated Java code. Compilers/interpreters written in JastAdd tend to be very well structured and easy to maintain. It's one of my all-time favourite libraries, actively maintained, and very well documented.

[1] http://jastadd.org/web/documentation/concept-overview.php

[2] https://en.wikipedia.org/wiki/Attribute_grammar

Re: Oops, I Wrote a C++ Compiler

#15

Post by AgentME »

jchw wrote: post #6
amelius wrote: post #3

Please don't call it C++ if it doesn't even support constructors and destructors. People will be trying to port their existing code, and will be unpleasantly surprised. > But the truth is, I’m just going to keep listening to users and improving the parts I can. I am not trying to recreate GCC or LLVM. Why not use GCC or LLVM as part of your project? That seems like a better investment of your time.

It is very well explained? GCC and Clang would be nightmares to integrate into an app that runs on many platforms including iOS, and even once you did, you'd still need to implement an interpreter, because you're not allowed to JIT on the app store. Compiling a toolchain for a desktop platform is easy. Cross compiling a toolchain is moderately easy. But embedding a toolchain into an iOS app, where you have to get it…

If they integrated Clang, then they could use Clang to compile code to WebAssembly, which can be executed by iOS's Javascript engine and pre-processed by wasm-metering[0] to set execution op limits so infinite loops can be recovered from.

[0] https://github.com/ewasm/wasm-metering

Re: Oops, I Wrote a C++ Compiler

#17

Post by archgoon »

maxxxxx wrote: post #14

Nice! I think doing this is a great learning experience. I think I understand how to write a basic compiler but I am completely stumped by writing an optimizer. How do you model that?

The llvm compiler has it's optimization routines here:

https://github.com/llvm-mirror/llvm/tree/master/lib/Transfor...

You can play around with how it takes in llvm code to optimized llvm code by using the opt tool.

Here's a basic example.

  $ cat test.c
  #include 
  int main() {
    for (int i = 0; i
The command `opt -S -print-before-all -O2` can be manually modified to specify exactly which compiler passes you want to see. You can run `opt --help` to see a list of all compiler passes. (you can figure out what code performs by looking at the * * * IR Dump Before `MODULE NAME` * * * and finding the associated logic in the Lib directory)

In the case of llvm, the idea for them is they break the code into things called 'basic blocks' which, along with using Single Static Assignment, are more easily analyzed, transformed, and simplified.

Re: Oops, I Wrote a C++ Compiler

#18

Post by lopatin »

fmihaila wrote: post #13
chubot wrote: post #2

The process of creating the parser went very smoothly thanks to this. The real work involves creating C# syntax classes that mirror the grammar. These classes form the Abstract Syntax Tree (AST) of the compiler. To address this problem, I use Zephyr ASDL to describe the abstract syntax of a language (which CPython also uses!) It's a little DSL that lets you write an ML-style algebraic data type, and generates a bunch…

For those writing language processors in Java, I highly recommend having a look at JastAdd [1]. It's a great framework for implementing semantic analysis using attribute grammars [2]. The tool automatically generates Java classes to represent ASTs, based on a high-level description (a CFG grammar) of the desired AST shape. You can use any Java-based parser as long as the AST it produces is constructed from those clas…

And for Scala, there's Kiama https://bitbucket.org/inkytonik/kiama

Re: Oops, I Wrote a C++ Compiler

#20

Post by School-Cotton »

craftyguy wrote: post #5
amelius wrote: post #3

Please don't call it C++ if it doesn't even support constructors and destructors. People will be trying to port their existing code, and will be unpleasantly surprised. > But the truth is, I’m just going to keep listening to users and improving the parts I can. I am not trying to recreate GCC or LLVM. Why not use GCC or LLVM as part of your project? That seems like a better investment of your time.

> Please don't call it C++ if it doesn't even support constructors and destructors. At least they recognize this is missing, and call it out. > Why not use GCC or LLVM as part of your project? That seems like a better investment of your time. Learning opportunity? Maybe they're trying to solve a problem? Or do you think the LLVM folks should have just used GCC instead of wasting their time on LLVM?

Aren't constructors and destructors basically the main idea of c++?
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