Cowgol is both a compiler toolchain, and a programming language. This page shortly describes the toolchain.

Introduction

Cowgol’s a pretty vanilla compiler in the modern style, with the usual compiler passes (plus some extra). There’s a shared state between all the passes known as the things table which is held in RAM, and the actual instructions (known as iops) are streamed through each pass via files. It does a lot of file access, but it means that if you have enough memory to hold the things table, you can compile unlimited sized programs.

Right now the most complex program built with Cowgol is the code generator itself, which needs a things table of about 35kB.

There’s also a strings table, which contains any constant data referred to by the program. These are accessed as needed so they don’t have to be held in memory.

The front end uses a stack machine representation, while the backend uses a memory machine representation.

The passes

These are, in order:

  • the initialiser: creates a new, empty program, ready for adding tokenised and parsed code to. Normally this is only used for precompiling the runtime library. In normal use, you would simply take a copy of the data files for the runtime library and add your own code to that, so as to save time (the runtime library is big and takes about eight minutes to tokenise and parse).

  • the tokeniser: reads the source files and produces a token stream and the string table.

  • the parser: reads the token stream and parses the Cowgol language out of it. It generates an iop stream containing stack-based front end opcodes.

  • the typechecker: reads in the iops stream containing the front end opcodes and does type analysis and reporting of errors; it also looks up symbolic names which the parser was unable to resolve (so allowing forward references). In addition, it concerts the stack-based front-end opcodes into memory-machine back-end opcodes used by the code generator. This last stage doesn’t really belong here and needs to be split out into a separate pass.

  • the blockifier: this will, eventually, do some simple basic block analysis and dead code removal. What it does now is to rewrite label references based on directives produced by the parser.

  • the classifier: constructs the call graph and determines which subroutines and variables are referred to. Once it knows which variables are used, it assigns them to addresses in the data segment (and in zero page on the 6502).

  • the code generator: turns the memory-machine opcodes into real processor opcodes.

  • the placer: now that the actual code has been generated, we know how big each subroutine is. The placer assigns them addresses in the code segment.

  • the emitter: assembles all the segments and writes out the executable image.

There’s platform-specific code in every pass except the tokeniser, the parser and the blockifier. The BBC Micro version is in src/arch/bbc. (This needs a refactor.)

** The backend model

The backend uses a memory machine abstraction, where every instruction reads from memory and writes to memory. The code generator then caches values in memory in registers. This leads to pretty crummy register utilisation, but it’s very very cheap.

Given an abstract instruction like:

a := b + c

…then a, b and c are all memory references, referred to as an effective address (EA). Each one may be an LEA, VALUE or DEREF, expressed as:

  • LEA: address of an object. object+numoffset+varoffset
  • VALUE: value of an object. [object+numoffset+varoffset]
  • DEREF: value pointed to by an object. [[object]+numoffset+varoffset]

numoffset refers to a constant offset; varoffset refers to an offset in a variable. Either or both may be missing.

EAs are typed, and the backend tracks their types. (Although this may be removed as the only thing it really cares about are the widths.) Constant values are held in memory and referred to by address (the code generator may optimise the fetch away if it wants to).

The typechecker generates abstract code, and then there’s a platform-specific simplification pass which chops instructions up into pieces which the architecture can understand. So:

[i] := [[array] + offset]  # generated from i := array[offset]

…becomes:

[tmp] := array             # 16 bit load constant
[tmp] := [tmp] + [offset]  # 16 bit add
[i] := [[tmp]]             # load value via pointer deref

Sadly, this leads to a lot of repeated work and pretty poor generated code (if [[array] + offset] is referred to again, we should be able to avoid recalculating tmp). I’m thinking of ways to avoid this. Factoring the simplification phase into its own pass will be necessary to improve things here.

Porting to a new architecture

Cowgol’s intended to be portable, although it probably isn’t. It certainly won’t work on big-endian architectures — this may be fixable, but I’ve deliberately avoided thinking about this for simplicity.

All the platform-specific code should be in src/arch. The most complicated part is the code generator. Currently the 6502 code generator is a bit of a mess and needs a proper overhaul. It uses a very simple value cache where it tries to remember which 8-bit values are stored in which register, and flushes them to memory lazily. It’s important to remember that you need to flush the cache whenever doing a pointer access to maintain pointer aliasing rules. There’s no attempt to track values between basic blocks (because it’s really hard); the cache is flushed and values reloaded from memory as needed. 16-bit and 32-bit values aren’t cached at all and are worked on directly in memory.

This is a reasonable strategy for the very register-light old architectures, but would be very wasteful (although simple!) on a register-centric architecture like the ARM. You’ll want to rewrite the code generator from scratch. The 6502 one is about 1.7kloc.

You also need a simplifier, which breaks down complex backend instructions. This is much smaller, but pretty subtle, and I spent ages removing bugs from the 6502 one (and probably adding new ones). If you’re on a register architecture, life may be easier as there are fewer address modes to deal with.

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