So I decided to introduce myself to the parrot internals by looking
at the JIT. (Partly because I had to tweak it a bit to get it to run
on my machine.) After looking at it a bit, I have some questions,
mostly of the form "wouldn't it be better if X were done like Y?",
and I hope that if the answer is "no, because of Z", or "no, it
really doesn't matter", that my questions will not be taken amiss. :)
Otherwise, perhaps I'll try to put together some patches.
- When loading 32-bit constants, the jit backend uses oris+ori,
keeping a zero in r31 to base it off of. From reading compiler
output, I'm more accustomed to seeing lis+ori, or li+addis, etc..,
which seems simpler; it might also not consume an ALU cycle (I'm not
familiar enough with ppc pipelining to know). Is there a reason for
the oris+ori method?
- Likewise, r31 is used as a zero register, but many ppc instructions
automatically read r0 as zero (for exactly this purpose, AIUI: li is
actually addi ?,r0,?). Would it be helpful to free up r31 for general
use? (Or is there not much register pressure in practice?)
- When emitting calls to Parrot_foo() functions, the code genrator
loads the procedure address as a constant and calls it (via bctrl).
For actual JIT use, this is fine, but for exec_save use, this seems
like a pretty big potential lose. Would it be a bad thing to emit
relocation entries for these calls?
Wim.
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