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Sandbox space for a rewrite of PCT
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Parrot Alternate Compiler Toolkit

This is a redesign and rebuild of the Parrot Compiler Toolkit (PCT), which is installed with Parrot. PCT provides a quick and easy framework to build high-level languages (HLLs) on Parrot, but has its limitations. The main goal of PACT is to provide the same ease of use as PCT, but add easier extensibility, more power, and more features. This project was sparked by benabik++'s comments during his Google Summer of Code 2011 project.

Currently the project is in the planning stages and this repository contains notes and design documents. Assistance, forks, and commentary are welcome!

Project Goals

  • Ease of Hacking
  • Similar to PCT
  • Modular and Flexible
  • Test-Driven Development
  • Bytecode Generation
  • Typed
  • Optimizations
  • Round-Trip Code
  • Use Compiler Best Practices

Ease of Hacking

PCT was written in PIR, which gives it a lot of access to the Parrot VM, but makes it very difficult to work on. To make the source easier to work on, PACT will be implemented in Winxed, which gives most of the power of direct PIR but in a far more expressive form.

Similar to PCT

There is a lot of experience in the Parrot community with PCT, and its value in developing large compilers has been proved by Rakudo. PACT does not strive for 100% compatibility, but the features and interface of PCT should be kept in mind while designing/building PACT.

Modular and Flexible

While PACT will strive to be a one-size-fits-all toolkit, we must also recognize that someone will always have something that doesn't quite match what PACT does. PACT should be designed in pieces that are easy to use individually, replace, and combine in new and interesting ways. There are some uses in particular to keep in mind:

  • Alternate lexing/parsing frameworks
  • Including low-level code in high-level
    (Think POST nodes in a PAST tree. A more structured version of inline nodes.)
  • Compiling portions of a program.
    (Given a PAST expression, return a POST expression not a full program. Particularly useful for REPLs and eval functions.)

Test-Driven Development

When working on as complicated a system as a compiler, it is very difficult to determine ahead of time what will break when you alter a section of code. A thorough testing framework will help ensure that the compilers that depend on PACT will not break due to updates.

Bytecode Generation

PCT was very tied to the structure of PIR. PACT is intended to be designed with the idea of building directly to bytecode from the start. In particular, this means that no portion of the system can rely on simply generating PIR that does the right thing.

On the other hand, PIR is an extremely useful format. PBC is not stable across versions of parrot, so being able to save PIR bootstrap steps is invaluable. Other backends such as Winxed may also be useful.

Typed

The key to building sane bytecode is maintaining knowledge of what type every value in the tree is intended to be. Because of this, every PACT node should maintain an idea of what type it returns (even if this is 'void'). In addition, passing around information encoded into strings makes further processing of it very difficult so all information in the tree should be a PACT node. (For example, registers should be PACT::Register.new('P', 2), not '$P2'.)

Optimizations

While creating fast code can be done by hand, it is usually far more convenient to write what you mean and have the computer make it faster. While optimizing code is not required for the design of PACT, it should be simple to add and customize. Notably, the tree optimization project should be integrated very deeply into the system.

Some basic optimizations like constant folding (for non-PMC values) and dead code elimination can be implemented quickly. More complex optimizations involving SSA and data-flow analysis are not required, but the ability to perform them should be kept in mind.

Round-Trip Code

Generating high quality code is difficult when the output of the compiler is opaque. PASM is a mostly dead format and PIR is often derided for both the amount of hidden work it does and its compiler IMCC. New assembly and intermediate formats are required. These formats should be as easy to generate and process as possible.

The ability to store arbitrary objects in bytecode files means that not every PBC can be regenerated faithfully from disassembly, but handling the simple cases will keep the disassembler honest and make code generation easier to test. Human-readability is key, but ease of writing is not.

Use Compiler Best Practices

Recommended reading: The Dragon Book (Compilers: Principles, Techniques, and Tools by Aho, Lam, Sethi, and Ullman)

Those who do not learn from history are doomed to repeat it. While converting to multiple intermediate steps or worrying about SSA may seem like unnecessary work, using similar phases and styles as other compilers will make it easy to use the lessons learned from them. GCC and LLVM are designed the way they are because it helps them generate fast and correct code, so emulating them is no bad thing.

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