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RE: Notes from the GROW'10 workshop panel (GCC research opportunities workshop)

Hi all,

Dorit and I just got an anonymous ;) feedback about GCC vs LLVM following
our email about GROW'10 panel questions so we are just forwarding it here

The reasons I have seen for using llvm/clang are basically two-fold: gcc is too slow and too
complicated. (This is true even for a company with significant in-house gcc expertise.) The C
language parser for llvm (clang) is far faster than the gcc equivalent, easier to modify, and easier
to build into a separate library. Speed is a major decision for choosing clang/llvm, as OpenCL needs
to be complied on the fly. The llvm infrastructure is also far easier to manipulate and modify than
gcc. This is particularly important as implementing OpenCL means building complier backends to
generate Nvidia's PTX or AMD's IL, adding specific vector extensions, and supporting various
language intrinsics. I don't know how much of an issue the licensing issues may be. These issues,
plus significant corporate backing, appear to be really driving llvm in the OpenCL area. My
impression is that for gcc to be competitive it will have to offer both comparable compilation speed
and dramatically better code optimizations. Even then, I'm not sure if the difficulty of working
with it will be considered a good tradeoff for most companies.


-----Original Message-----
From: [] On Behalf Of Dorit Nuzman
Sent: Sunday, April 11, 2010 2:54 PM
Subject: Notes from the GROW'10 workshop panel (GCC research opportunities workshop)

Dear all,

We would like to share notes from the lively panel discussion at
GROW'10 (GCC Research Opportunities Workshop) that took place at the
end of January in Pisa, Italy (alongside the HiPEAC conference).
The main topic of the discussion was:

      How to make GCC more attractive to researchers,
      and how GCC can benefit from researchers?

Here is the list of major points and wishes raised by participants:

* Need to encourage cleanup/infrastructure work on GCC and provide
stable/flexible/extensible APIs (the question is how to encourage such
infrastructure work...?)

* Encourage people to work on infrastructure and full implementation
that actually works: Lobby for high quality conferences to reserve a quota
of the accepted papers to papers that focus on *implementation* work (and
start with HiPEAC!)

* Follow up to the above: Encourage research that actually works:
Lobby for conferences to favor research work that is tested on a realistic
environment (e.g. pass all of SPECcpu...), and that is reproducible.
Then GCC and the community could immediately benefit from the results and
not wait for many years before someone decides to reproduce/redo research
ideas in GCC.

* Get statistics on percentage of papers/projects that use compilers other
than GCC, and ask them why... (By the way, why was OpenCL implemented only
on LLVM and not on GCC?)

* Open up GCC to be used as a library by other tools, so that other tools
could use its analysis facilities. For example, having alias analysis as an
API rather than a pass that needs to be applied and then collect
information. Allow developers/tools access those functions outside GCC
(should be a high-level API).

* Follow up to the above: Need to come up with a common API /
standardization / common levels of abstractions. Decide on how to
coordinate efforts and find commonalities.

* Need a simple pass manager that can list all available passes
and allow any scheduling (providing dependency information).

* Make more visible/accessible guides for building/extending GCC.

* Better advertize Google Summer of Code, and provide more mentoring.

* Send feedback on which plugin events are desired to add to next releases
of GCC.

* GCC tries to auto-parallelize the programs it compiles. Will GCC itself
be made more multi-threaded and run faster in a multi-core environment...?

We've collected these and other comments/suggestions before/at/after the
workshop, on the GROW'10 wiki page:

We hope these notes would help improve GCC and its appeal/usability for
researches (or at least raise more discussion!)

Grigori Fursin and Dorit Nuzman
(GROW'10 organizers)

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