[llvm-dev] ORC JIT Weekly #7 -- JITEventListener support and Swift Immediate Mode Migration

[Andres Freund via llvm-dev]

Hi,

On 2020-03-09 21:20:44 +0100, Frank Tetzel via llvm-dev wrote:
> I think, debugging and profiling support is very important for a JIT
> engine. I could never get it to work with older LLVM versions. Is there
> example code somewhere available?

Since I added the perf listener I probably can help you with that. What
exactly was the problem you were hitting?

I don't have isolated example code, but it really shouldn't take much
code to set up the event listener. All that PG (which is what I added
the perf listener for) is doing is:
https://github.com/postgres/postgres/blob/master/src/backend/jit/llvm/llvmjit.c#L676

When taking a profile of a program you need to specify -k 1 to perf, to
have compatible clocks. E.g. you need to take it like
$ perf record -k 1 --call-graph dwarf -o /tmp/perf.data -p 22950
and then
you need to inject the JIT information into the profile:
$ perf inject -v --jit -i /tmp/perf.data -o /tmp/perf.jit.data
after that perf.jit.data is a perf profile enriched with the necessary
information, and can normally be inspected by perf report (using -i
/tmp/perf.jit.data).

If you want debug information within the JITed code (e.g. to see
associated source lines or such) that's a separate task - you need to
emit debug information, which is unrelated to the listener itself.

Greetings,

Andres Freund