[Openmcl-devel] Porting the OpenMCL Compiler

[James Bielman]

Hi,

I've been spending a fair bit of time studying the OpenMCL internals
and I now think I'm dangerous enough to consider a port to ARM
(probably Linux first, then hopefully Windows CE).

Obviously this is a huge task and I certainly don't expect to get very
far anytime soon, but hopefully I can learn a lot from the process
either way.

I'm planning to start out implementing kernel subprimitives and the
LAP assembler, but I'd like to get some advice on register usage,
since the ARM has far fewer registers than PowerPC.

Basically, there are 16 GPRs, r0-r15, except r15 is the PC, r14 is the
link register, and r13 is typically the control stack pointer.  So,
apart from any tricks to be done with reusing lr for other purposes
(although I'd think that, being a GPR, it could fulfill the same
purpose as the LOC-PC register on PPC?), we are left with 13 GPRs.

I don't know if there are any guidelines about how many registers are
necessary for it to be worth paritioning into boxed and unboxed.  If
this isn't enough then obviously life gets more complicated...

I wrote a little Lisp program to loop over all the fbound symbols in
the OpenMCL image and disassemble them to a file, then grepped the
output (hopefully correctly) to count register usage.  Here are the
results:

ARG_Z           132643
IMM0            50129
VSP             48012
ARG_Y           34665
FN              33836
SAVE0           32224
TEMP3           19587
SAVE1           18077
ALLOCPTR        11647
SAVE2           11001
TEMP4           9398
LOC-PC          9239
ARG_X           8769
NARGS           8744
SAVE3           7772
TEMP0           7249
TSP             5528
SAVE4           5155
TEMP2           4285
SAVE5           3717
SAVE7           3431
SP              2963
SAVE6           2747
IMM1            2436
ALLOCBASE       2045
IMM2            839
IMM3            517
TEMP1           446
RCONTEXT        293
IMM4            237
IMM5            13

Based on this (and some possibly incorrect common sense), here's what
I've got so far:

r0      imm0            unboxed temp reg
r1      imm1            unboxed temp reg
r2      temp0           boxed temp reg
r3      temp1           boxed temp reg
r4      save0           boxed caller-save reg
r5      save1           boxed caller-save reg
r6      arg_y           second to last argument
r7      arg_z           last argument
r8      nargs           number of function arguments
r9      allocptr        heap free pointer
r10     fn              current function object
r11     rcontext        thread context register
r12     vsp             value stack pointer
r13     sp              control stack pointer
r14     lr              link register
r15     pc              program counter

This is assuming the temp stack pointer could be put in memory
somewhere, perhaps in the tcr?  Also, this doesn't seem like very many
immediate registers, but according to the register counts for PowerPC,
maybe this isn't so bad?

I'm not sure what to use for NFN or FNAME either, hmm.

Also, I'm curious why OpenMCL uses registers for the last arguments
instead of the first, is there a sneaky reason why this is so?

James