Re: GSOC Question about the parallelization project

[David Malcolm <dmalcolm at redhat dot 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

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