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

Hi All,

Just a couple of updates this week:

First: there is a preview patch up at https://reviews.llvm.org/D75838 to
enable use of JITEventListeners in RTDyldObjectLinkingLayer. I've only done
very preliminary testing on it, but was able to debug simple JIT'd programs
on Linux using the GDB registration listener. If you've been wanting to
move to ORC but held up by lack of debugger / profiler registration, chime
in on the review -- I'd appreciate feedback and help with testing. I will
also post a bug tomorrow for investigating similar (or hopefully better)
debugger registration functionality for JITLink.

The other news is that Swift's immediate mode has moved from MCJIT  to
LLJIT. The first patch for this was posted on GitHub a few weeks back, but
blocked while I tracked down some failures in Swift script test cases on
Linux. Those failures have now been fixed and Swift's immediate mode moved
to LLJIT in
https://github.com/apple/swift/commit/7a0754b127b5fdfeaa2461be44265059c1fb162d
.

-- Lang.
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Hi Lang,
> First: there is a preview patch up at https://reviews.llvm.org/D75838
> to enable use of JITEventListeners in RTDyldObjectLinkingLayer. I've
> only done very preliminary testing on it, but was able to debug
> simple JIT'd programs on Linux using the GDB registration listener.
> If you've been wanting to move to ORC but held up by lack of debugger
> / profiler registration, chime in on the review -- I'd appreciate
> feedback and help with testing. I will also post a bug tomorrow for
> investigating similar (or hopefully better) debugger registration
> functionality for JITLink.
Does this also include support for PerfEventListener for profiling with
perf?

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?

Best regards,
Frank

+1. Once ORC JIT supports debugging and profiling, it will have everything
we need.

Thanks, Lang!


On Mon, Mar 9, 2020 at 9:21 PM Frank Tetzel via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Hi Lang,
>
> > First: there is a preview patch up at https://reviews.llvm.org/D75838
> > to enable use of JITEventListeners in RTDyldObjectLinkingLayer.
I've
> > only done very preliminary testing on it, but was able to debug
> > simple JIT'd programs on Linux using the GDB registration
listener.
> > If you've been wanting to move to ORC but held up by lack of
debugger
> > / profiler registration, chime in on the review -- I'd appreciate
> > feedback and help with testing. I will also post a bug tomorrow for
> > investigating similar (or hopefully better) debugger registration
> > functionality for JITLink.
>
> Does this also include support for PerfEventListener for profiling with
> perf?
>
> 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?
>
> Best regards,
> Frank
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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Hi Frank,

Does this also include support for PerfEventListener for profiling
with
> perf?

This patch just enables use of the JITEventListeners with
RTDyldObjectLinkingLayer. In theory it should bring
RTDyldObjectLinkingLayer users up to parity with MCJIT's support. (Note:
Windows and Linux still use RTDyldObjectLinkingLayer by default, MacOS uses
JITLink but can be manually switched back to RTDyldObjectLinkingLayer if
event listeners are needed).

So if PerfEventListener wasn't working for you before I wouldn't expect
this to fix the issue. On the other hand it makes sense to try it and file
a bug, since it *should* work.

What OS have you been seeing problems with?

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?

Agreed -- We definitely want this. My plan at the moment is to make sure
the JITEventListener API is at parity with MCJIT to make it easy for
clients to transition. The long term plan is to re-implement debugging and
profiling support from the ground up via ObjectLinkingLayer::Plugins, which
are much more flexible than JITEventListeners.

I was hoping to land some example code today, but hit a bug with object
buffer ownership. I hope to sort this out and (re)commit the event listener
support and example code tomorrow.

Cheers,
Lang.

On Mon, Mar 9, 2020 at 1:21 PM Frank Tetzel <s1445051 at
mail.zih.tu-dresden.de>
wrote:
> Hi Lang,
>
> > First: there is a preview patch up at https://reviews.llvm.org/D75838
> > to enable use of JITEventListeners in RTDyldObjectLinkingLayer.
I've
> > only done very preliminary testing on it, but was able to debug
> > simple JIT'd programs on Linux using the GDB registration
listener.
> > If you've been wanting to move to ORC but held up by lack of
debugger
> > / profiler registration, chime in on the review -- I'd appreciate
> > feedback and help with testing. I will also post a bug tomorrow for
> > investigating similar (or hopefully better) debugger registration
> > functionality for JITLink.
>
> Does this also include support for PerfEventListener for profiling with
> perf?
>
> 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?
>
> Best regards,
> Frank
>
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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