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Markdown
139 lines
No EOL
4 KiB
Markdown
sjs<br>
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[sami@samhuri.net](mailto:sami@samhuri.net)<br>
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published : 2009-09-22<br>
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updated : 2010-01-19
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Overview
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========
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A compiler for fun and education. Written (mostly) in Ruby and based
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on the tutorial "Let's Build a Compiler" by Jack Crenshaw[1].
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[1]: http://compilers.iecc.com/crenshaw/
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The semantics are simple and familiar to most programmers. Eager
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evaluation, assignment with the equals sign, arithmetic using + - *
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and /, loops, if/else statement, etc. Integers are the only data
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type.
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While the parser still closely resembles Crenshaw's recursive descent
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parser, back-end generates x86 machine code using a homegrown
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assembler in ~1000 lines of Ruby (just 650 lines of real code).
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NOTE: OS X is the only platform that compiles working binaries right
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now. ELF support for Linux coming ... eventually.
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Pre-requisites
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==============
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OS X
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----
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You need Ruby and gcc. Ruby is standard on Macs but you'll need to
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install Xcode for gcc. You can also compile it yourself or use
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MacPorts, or [homebrew](http://github.com/mxcl/homebrew).
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Linux
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-----
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You need Ruby and ld - which lives in the binutils package.
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% sudo aptitude install ruby binutils
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That's it!
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Compiling
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=========
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The build script should detect your platform. If not append 'elf' or
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'macho' to the command.
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% ./build.rb filename.code [elf | macho]
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The resulting native executable is called 'filename' and you should be
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able it run it directly.
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% ./filename
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Syntax in 2 minutes
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===================
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The parser starts by parsing a block of code. A block consists of one
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or more statements. Whitespace is largely ignored beyond delimiting
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tokens, so statements can be grouped on one line or spread out over
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multiple lines. With no explicit terminator this can look strange so
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we will see how it works out when the syntax evolves into something
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more complicated.
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There are variables and integers. That's honestly about it. There
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are no functions or function calls, no closures, arrays, hashes, or
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anything else.
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Supported statements are:
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* assignment<br> e.g. foo = 4096
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* if/else<br> e.g. if x < 0 a=0 else a=1 end
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* while<br> e.g. while x > 0 x=x-1 end
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* until<br> e.g. until x == 0 x=x-1 end
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* break<br> e.g. break
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* repeat<br> e.g. repeat x=x-1 if x == 0 break end end
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* for<br> e.g. for i=1 to 5 x=x+1 end
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* do<br> e.g. do 5 x=x+1 end
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* print <expr><br> e.g. print 42
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Supported operations are the following, in increasing order of
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precedence:
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* add + and subtract -
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* multiply * and divide /
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* relations: == != < > <= >=
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* boolean not !
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* [unimplemented] or ||
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* [unimplemented] and &&
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* bit or | and bit xor ^
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* bit and &
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* unary plus + and minus -
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Parentheses are used to force a specific order of evaluation.
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As far as booleans go, 0 is false and everything else is true. Right
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now there are only integers so this makes sense.
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Internals
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=========
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It wasn't much fun generating assembly text, so I wrote an x86
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assembler library in Ruby. It implements just the instructions needed
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for this compiler and is by no means complete. It only does 32-bit
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and no prefixes are supported. It's basically just a handful of
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instructions and mod-rm encoding. I use the system's linker and have
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no intention of writing my own, don't worry!
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ELF support is still in C and not published in the repository. The
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class to output Mach-O binaries is found in asm/machofile.rb.
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The asm/ directory holds the assembler but also the Mach-O code, for
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now. This is my first assembler and first time working with the x86
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ISA, so it probably isn't great. It outputs horribly inefficient code
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and there are no optimizations.
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I did not write this compiler with the intention of anyone else
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reading it but there are a reasonable amount of comments.
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What next?
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==========
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Whatever interests me really, I don't know yet.. Right now I need to
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clean up some of the code, now that object files of any size can be
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generated and tests pass again.
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Happy hacking!<br>
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-sjs |