PL/0c

A compiler for the PL/0 programming language written in OCaml.

The language has been extended to support the following features:

• Comments of the form { ... }.
• Input and output, respectively handled by operators ? and !.

In the future, we hope to support some additional features:

• Floating-point numbers.
• More logical operators.
• Strings.
• else and else if
• Arrays.
• Procedure arguments.

Lessons Learned

• In the IR, it's a good idea to distinguish between variables and temporaries, so that we can more easily avoid a bunch of redundant load-stores.
• I should store a procedure's Temp generators (e.g. for virtual registers) in the symbol table; right now, we lose that information, and so we can't really generate additional virtual registers in later stages.
• The IR and MIR are too low-level right now, and are thus very hard to work with. In particular, it is basically already assembly, except that we allow an unlimited number of virtual registers (in fact, I even insert function prologues and epilogues!). This significantly complicates instruction selection, as well as register allocation, since it's now very difficult to generate spill code. In the future, I'd instead:
  • Add type info to the IR, so that I can figure out what kind of register is needed to store a particular value. Right now, since we only support 32-bit integers in the source language, I'm able to just hard-code it.
  • To handle stack accesses, I can try to do something similar to what (I believe) LLVM does: use alloca instructions to get ptrs to stack locations to be resolved later.
  • The MIR should still be a pseudo-assembly similar to the IR, but containing additional information:
    • map type info to sizes,
    • track liveness information (def, use, clobber <- IMPORTANT!)
    • DO NOT EMIT PROLOGUES AND EPILOGUES YET! ONLY AFTER REGALLOC, WHEN WE'VE DETERMINED WHAT TO SPILL!
  • Emitting actual assembly should be saved for the VERY END.