Cliff Hacks Things.

Sunday, December 07, 2008

Adapting propasm for the Pilgrim LMM

Last night, I replaced my Java compiler's hacked code generator with one based around propasm.

Unfortunately, propasm had some baked-in assumptions that gave me some trouble, so I had to rewrite it. The rewrite will go into SVN as soon as it reaches feature-parity. It's a lot simpler now and provides an API for extensible image generation -- which is the feature I needed today.

You see, I'm targeting Phil Pilgrim's adaptation of the LMM, which is the only one with 16-cycle constant execution for register-register instructions. (I developed it independently at Hobee's one night, but it looks like Phil did it first, so as far as I'm concerned it's the Pilgrim LMM.)

The Pilgrim LMM has an unusual property that lets it achieve its speed: it lays code out backwards, from high addresses to low, and uses djnz to update the native and LMM PCs simultaneously. This completely broke poor propasm's brain -- it was designed for single-pass assembly, which of course means that instructions get laid out in the order they're input.

To support this, propasm now generates images in two passes. First, it builds a tree of object describing the image's contents. Second, it walks the tree and generates output. This has the side-effect of allowing optimizations on machine code without requiring a separate object model -- I've implemented a couple simple scalar optimizations over propasm's image objects in my compiler.

My Java compiler plugs a class called rlmm.CodeSegment into propasm's image generation code, which lets it generate the code in execution order (by a CFG walk) but lay it out backwards in memory. Thus, for the sample input from my last post, we now get the following output:

' Start of segment rlmm.CodeSegment@1004901
IF_ALWAYS JMPRET prim_return_ret, #prim_return
method$:L10840700
IF_ALWAYS MOV r0, r2

long @method$:L10840700
long @method$:L15980197
IF_ALWAYS JMPRET prim_jmplt_ret, #prim_jmplt
method$:L29181730
IF_ALWAYS CMPS r2, r1

long @method$:L29181730
IF_ALWAYS RDLONG PC, PC
method$:L15980197
IF_ALWAYS ADDS r2, #1

long @method$:L29181730
IF_ALWAYS RDLONG PC, PC
method$:L15006066
method
IF_ALWAYS MOV r2, #0


Remember to read bottom-to-top! (Pardon the excessive IF_ALWAYS and expansion of calls, this output was disassembled with a naïve algorithm.)

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