llvm dev - Oct 2020 - LLVM Developers Meeting JIT BoF -- Request for Topics of Interest

Hi Andres,

That would be of interest to me too. One thing around this I have
been
> wondering about is whether it's realistic to merge the optimization and
> code generation phases - right now we spend a lot of time re-doing
> analyses during codegen that we already had done during optimization.

Sounds good to me. I think there are two sub-topics here:
(1) JIT specifics. E.g. What default optimization pipelines should we
provide in the JIT? The standard 0/1/2/3/s options, or would it make sense
to develop something JIT specific?
(2) General compile time improvements. Everyone will benefit from compile
time improvements, but JIT clients are likely to be extra sensitive to it.
Have we identified any problem areas or redundancies that would be of
interest to the broader LLVM community, and that we could solicit help in
fixing.

Possibly also related to LLJIT design - having LLJIT first
generate
> minimally optimized code and then, while that is in use, doing optimization
> and optimized codegen concurrently, would be neat. It feels like that'd
> fit well into LLJIT, given that it already provides things like
> background compile threads.

Absolutely. Supporting this use-case was one of the motivations for the
concurrency support in OrcV2. It's doable at the moment, but it requires a
fair bit of manual work on the client's part. Implementation and API design
in this area seem like good topics.

-- Lang.

On Fri, Oct 2, 2020 at 10:21 AM Andres Freund <andres at anarazel.de>
wrote:
> Hi,
>
> On 2020-09-29 01:52:48 +0530, Praveen Velliengiri wrote:
> > I'm also interested in reducing the compilation time of code in
JIT
> > component independent of static compiler. Is it sounds interesting? :)
>
> That would be of interest to me too. One thing around this I have been
> wondering about is whether it's realistic to merge the optimization and
> code generation phases - right now we spend a lot of time re-doing
> analyses during codegen that we already had done during optimization.
>
> Possibly also related to LLJIT design - having LLJIT first generate
> minimally optimized code and then, while that is in use, doing optimization
> and optimized codegen concurrently, would be neat. It feels like that'd
> fit well into LLJIT, given that it already provides things like
> background compile threads.
>
> - Andres
>
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Hi,

On 2020-10-02 10:44:43 -0700, Lang Hames wrote:
> Sounds good to me. I think there are two sub-topics here:
> (1) JIT specifics. E.g. What default optimization pipelines should we
> provide in the JIT? The standard 0/1/2/3/s options, or would it make sense
> to develop something JIT specific?
Yea. I've some hopes for "new PM" making it easier to have
maintainable
and customizable pipelines. I've not played around with it too much -
largely because there's no C API last time I checked.


> (2) General compile time improvements. Everyone will benefit from compile
> time improvements, but JIT clients are likely to be extra sensitive to it.
> Have we identified any problem areas or redundancies that would be of
> interest to the broader LLVM community, and that we could solicit help in
> fixing.
I'd guess that some of the things that can be done to significantly
improve JIT performance aren't generally applicable to most other uses
of LLVM. E.g. the overhead of redoing the same analyses for code gen is
mostly an issue on higher optimization levels, and I assume that a large
portion of e.g. clang users using -O3 will do LTO. Where a split between
optimization and code gen seems necessary in number of cases.

> > Possibly also related to LLJIT design - having LLJIT first generate
> > minimally optimized code and then, while that is in use, doing
optimization
> > and optimized codegen concurrently, would be neat. It feels like
that'd
> > fit well into LLJIT, given that it already provides things like
> > background compile threads.
> 
> 
> Absolutely. Supporting this use-case was one of the motivations for the
> concurrency support in OrcV2. It's doable at the moment, but it
requires a
> fair bit of manual work on the client's part. Implementation and API
design
> in this area seem like good topics.
Cool.

Greetings,

Andres Freund

Hi All,

I've listed the current topics of interest below, along with some notes on
each. We only have 30 minutes so we'll barely scratch the surface of these
during the BoF itself. My main aims are for you to meet each other,
identify potential areas of collaboration, identify things that I can do to
unblock you, and get the ball rolling on some conversations that we can
continue on the mailing lists. It looks like we'll have an opportunity to
set up impromptu meet-ups too (see
https://www.youtube.com/watch?v=Qpbefagv6Ts) so if you want to do a deeper
dive on a topic area we can set one or more of those up -- just let me know
in this thread.

Topics so far:

A very brief status update on OrcV2 and JITLink.

Future / Potential use-cases for the JIT.
  -- LLDB, the Swift interpreter, REPLs.

Compile time improvements
  -- Experimenting with / performance-testing custom JIT pipelines (any
volunteers?)
  -- Compile-time improvements for the existing pipeline. This is probably
a broad community project, but JIT users might have interesting workloads /
results to contribute.
  -- Hiding compile times with concurrency.

Profiling, Debugging, PGO
  -- Profiling and Debugging support (especially via JITLink)
  -- How do we integrate PGO (any volunteers to experiment with this?)

Reoptimization
  -- What it is.
  -- Any volunteers to start working on API design and experiments?
  -- Is resource management a problem (do we need to free unoptimized code)
and if so how do we make it safe?

I'll add another of my own topics in here:

Documentation
  -- What would make life better for Orc beginners?
  -- What would make life better for Orc experts?
  -- What would make it easier for you to contribute to Orc?
  -- Any volunteers to help with the documentation effort?

Looking forward to seeing you all on Thursday.

-- Lang.

On Fri, Oct 2, 2020 at 11:21 AM Andres Freund <andres at anarazel.de>
wrote:
> Hi,
>
> On 2020-10-02 10:44:43 -0700, Lang Hames wrote:
> > Sounds good to me. I think there are two sub-topics here:
> > (1) JIT specifics. E.g. What default optimization pipelines should we
> > provide in the JIT? The standard 0/1/2/3/s options, or would it make
> sense
> > to develop something JIT specific?
>
> Yea. I've some hopes for "new PM" making it easier to have
maintainable
> and customizable pipelines. I've not played around with it too much -
> largely because there's no C API last time I checked.
>
>
>
> > (2) General compile time improvements. Everyone will benefit from
compile
> > time improvements, but JIT clients are likely to be extra sensitive to
> it.
> > Have we identified any problem areas or redundancies that would be of
> > interest to the broader LLVM community, and that we could solicit help
in
> > fixing.
>
> I'd guess that some of the things that can be done to significantly
> improve JIT performance aren't generally applicable to most other uses
> of LLVM. E.g. the overhead of redoing the same analyses for code gen is
> mostly an issue on higher optimization levels, and I assume that a large
> portion of e.g. clang users using -O3 will do LTO. Where a split between
> optimization and code gen seems necessary in number of cases.
>
>
> > > Possibly also related to LLJIT design - having LLJIT first
generate
> > > minimally optimized code and then, while that is in use, doing
> optimization
> > > and optimized codegen concurrently, would be neat. It feels like
that'd
> > > fit well into LLJIT, given that it already provides things like
> > > background compile threads.
> >
> >
> > Absolutely. Supporting this use-case was one of the motivations for
the
> > concurrency support in OrcV2. It's doable at the moment, but it
requires
> a
> > fair bit of manual work on the client's part. Implementation and
API
> design
> > in this area seem like good topics.
>
> Cool.
>
> Greetings,
>
> Andres Freund
>
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