From: David Malcolm <dmalcolm@redhat.com>
To: Richard Biener <richard.guenther@gmail.com>,
Sebastiaan Peters <sebaspe97@hotmail.com>,
Sebastiaan Peters <sebpeters@outlook.com>
Cc: "gcc@gcc.gnu.org" <gcc@gcc.gnu.org>
Subject: Re: GSOC Question about the parallelization project
Date: Tue, 20 Mar 2018 14:49:00 -0000 [thread overview]
Message-ID: <1521557360.5688.30.camel@redhat.com> (raw)
In-Reply-To: <CAFiYyc1CYNEYS=TgEOSbQJSQRMr1Vndw0M7bnZnVLZ=KsGMz6Q@mail.gmail.com>
On Tue, 2018-03-20 at 14:02 +0100, Richard Biener wrote:
> On Mon, Mar 19, 2018 at 9:55 PM, Richard Biener
> <richard.guenther@gmail.com> wrote:
> > On March 19, 2018 8:09:32 PM GMT+01:00, Sebastiaan Peters <sebaspe9
> > 7@hotmail.com> wrote:
> > > > The goal should be to extend TU wise parallelism via make to
> > > > function
> > >
> > > wise parallelism within GCC.
> > >
> > > Could you please elaborate more on this?
> >
> > In the abstract sense you'd view the compilation process separated
> > into N stages, each function being processed by each. You'd assign
> > a thread to each stage and move the work items (the functions)
> > across the set of threads honoring constraints such as an IPA stage
> > needing all functions completed the previous stage. That allows you
> > to easier model the constraints due to shared state (like no pass
> > operating on two functions at the same time) compared to a model
> > where you assign a thread to each function.
> >
> > You'll figure that the easiest point in the pipeline to try this
> > 'pipelining' is after IPA has completed and until RTL is generated.
> >
> > Ideally the pipelining would start as early as the front ends
> > finished parsing a function and ideally we'd have multiple
> > functions in the RTL pipeline.
> >
> > The main obstacles will be the global state in the compiler of
> > which there is the least during the GIMPLE passes (mostly cfun and
> > current_function_decl plus globals in the individual passes which
> > is easiest dealt with by not allowing a single pass to run at the
> > same time in multiple threads). TLS can be used for some of the
> > global state plus of course some global data structures need
> > locking.
>
> Oh, and just to mention - there are a few things that may block
> adoption in the end
> like whether builds are still reproducible (we allocate things like
> DECL_UID from
> global pools and doing that somewhat randomly because of threading
> might - but not
> must - change code generation). Or that some diagnostics will appear
> in
> non-deterministic order, or that dump files are messed up (both
> issues could be
> solved by code dealing with the issue, like buffering and doing a re-
> play in
> program order). I guess reproducability is important when it comes
> down to
> debugging code-generation issues - I'd prefer to debug gcc when it
> doesn't run
> threaded but if that doesn't reproduce an issue that's bad.
>
> So the most important "milestone" of this project is to identify such
> issues and
> document them somewhere.
One issue would be the garbage-collector: there are plenty of places in
GCC that have hidden assumptions that "a collection can't happen here"
(where we have temporaries that reference GC-managed objects, but which
aren't tracked by GC-roots).
I had some patches for that back in 2014 that I think I managed to drop
on the floor (sorry):
https://gcc.gnu.org/ml/gcc-patches/2014-11/msg01300.html
https://gcc.gnu.org/ml/gcc-patches/2014-11/msg01340.html
https://gcc.gnu.org/ml/gcc-patches/2014-11/msg01510.html
The GC's allocator is used almost everywhere, and is probably not
thread-safe yet.
FWIW I gave a talk at Cauldron 2013 about global state in GCC. Beware:
it's five years out-of-date, but maybe is still relevant in places?
https://dmalcolm.fedorapeople.org/gcc/global-state/
https://gcc.gnu.org/ml/gcc/2013-05/msg00015.html
(I tackled this for libgccjit by instead introducing a mutex, a "big
compiler lock", jit_mutex in gcc/jit/jit-playback.c, held by whichever
thread is calling into the rest of the compiler sources).
Hope this is helpful
Dave
[...]
next prev parent reply other threads:[~2018-03-20 14:49 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-03-16 16:25 Sebastiaan Peters
2018-03-19 9:20 ` Richard Biener
2018-03-19 15:28 ` Sebastiaan Peters
2018-03-19 17:37 ` Richard Biener
2018-03-19 19:09 ` Sebastiaan Peters
2018-03-19 20:55 ` Richard Biener
2018-03-20 13:02 ` Richard Biener
2018-03-20 14:49 ` David Malcolm [this message]
2018-03-20 15:07 ` Richard Biener
2018-03-21 8:51 ` Sebastiaan Peters
2018-03-21 10:34 ` Richard Biener
2018-03-21 19:04 ` Sebastiaan Peters
2018-03-22 12:27 ` Richard Biener
2018-03-23 15:05 ` Sebastiaan Peters
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