llvm dev - Mar 2020 - ORC JIT Weekly #7 -- JITEventListener support and Swift Immediate Mode Migration

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

> 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).
I followed all these steps. During recording, dump files are created in
~/.debug/. Unfortunately, perf report never allowed me to "zoom" into
the jitted function and get an assembly view with profile information.
Either inject or report do not work for some reason. The tools work
fine when I'm using a different JIT library (AsmJit). Maybe it's just
some path issue. I do not really like that the dump files are created
in ~/.debug/.

Last time I tried it was some months ago with LLVM 9. I'll try it
shortly with LLVM trunk/master and report back.

> 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.
Profiling on the assembly level would already be very helpful.

Best regards,
Frank

> Last time I tried it was some months ago with LLVM 9. I'll try it
> shortly with LLVM trunk/master and report back.
Finally, I found the time to try it out again. And it works fine in LLVM
master. I haven't tried any other LLVM version.

Profiling with perf on assembly level works fine. And so does gdb. One
can set pending breakpoints on the to be generated function and
execution stops when calling the function which shows that the jit
integration in gdb works. I have not played around with debug
information.

For the perf support, it's a bit of a pain to remember to enable
LLVM_USE_PERF when building LLVM. Why not enable it by default?

Best regards,
Frank