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Re: GSOC Question about the parallelization project


On Tue, Mar 20, 2018 at 3:49 PM, David Malcolm <dmalcolm@redhat.com> wrote:
> 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.

Yes.  There's also global tree modification like chaining new
pointer types into TYPE_POINTER_TO and friends so some
helpers in tree.c need to be guarded as well.

> 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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