Code reading: LuaJIT about blog

[lukego]

Please leave a comment if you know anything about LuaJIT internals :).

[lukego]

Could be that what I really want is better perf integration. For example, for perf top to be able to zoom in and show me machine code annotated with IR code. That would be something :-).

[darius]

I'm sure I know less than you, but I read a little of the source after Mike's talk and could stand to read more.

[lukego]

@darius maybe we can thrash our way through it together a bit.

I'm looking now in lj_asm_trace and it seems like the machine code is created by iterating backwards from the last IR instruction. Each instruction is assembled in order and emits some machine code (also in backwards order?).

Thinking aloud...

Then it would seem like you could output a really neat interleaved IR/mcode dump because the order of instructions would be identical in both and each mcode instruction would correspond to one definite IR instruction. Sound reasonable?

Then I wonder if such an interleaved trace could be exported to perf top so that it could be annotated in realtime with profiler information (as you get with C code). I wonder if that would require some hacking on perftools too? I haven't yet dug into the details of the data file that LuaJIT optionally exports for use by perf -- I know the current usage is quite limited (only matching instructions to trace numbers but not showing the IR/mcode).

Could also be that perf is the wrong tool and it would be better to add this to LuaJIT's own profiler.

[darius]

Yes, looks like it's emitting back-to-front. Makes me feel a little vindicated since my own x86 machine-code emitter worked that way and it got me a few funny looks.

The dump sounds reasonable too, though you'd want to make it a separate function to keep any overhead out of the normal asm function -- that's important, I assume?

Will email you tonight to catch up.

[nico-abram]

The dropbox link in the reddit thread seems dead (For the thesis you mention). Do you have any other uploads of it?

[darius]

Not on my laptop anymore, I'm afraid.