What's the plan for what control / synchronization structures you'll use in
part 2 of the plan to implement a parallel driver?

Is the idea just to use an IO thread for each compile and block them on
MVars when they encounter dependencies?  Or you can use a pool of worker
threads and a work queue, and only add modules to the work queue when all
their dependencies are met (limits memory use)... many options for
executing a task DAG.  Fortunately the granularity is coarse.


On Sun, Apr 21, 2013 at 10:34 PM, Patrick Palka <patr...@parcs.ath.cx>wrote:

> Good points. I did not take into account whether proposal #2 may be worth
> it in light of -fllvm. I suppose that even if the LLVM codegen is able to
> perform similar optimizations, it would still be beneficial to implement
> proposal #2 as a core-to-core pass because the transformations it performs
> would expose new information to subsequent core-to-core passes. Also,
> Haskell has different overflow rules than C does (whose rules I assume
> LLVM's optimizations are modeled from): In Haskell, integer overflow is
> undefined for all integral types, whereas in C it's only undefined for
> signed integral types. This limits the number of optimizations a C-based
> optimizer can perform on unsigned arithmetic.
> I'm not sure how I would break up the parallel compilation proposal into
> multiple self-contained units of work. I can only think of two units:
> making GHC thread safe, and writing the new parallel compilation driver.
> Other incidental units may come up during development (e.g. parallel
> compilation might exacerbate 
> #4012<http://hackage.haskell.org/trac/ghc/ticket/4012>),
> but I still feel that three months of full time work is ample time to
> complete the project, especially with existing familiarity with the code
> base.
> Thanks for the feedback.
> On Sun, Apr 21, 2013 at 5:55 PM, Carter Schonwald <
> carter.schonw...@gmail.com> wrote:
>> Hey Patrick,
>> both are excellent ideas for work that would be really valuable for the
>> community!
>> (independent of whether or not they can be made into GSOC sided chunks )
>> -------
>> I'm actually hoping to invest some time this summer investigating
>> improving the numerics optimization story in ghc. This is in large part
>> because I'm writing LOTs of numeric codes in haskell presently (hopefully
>> on track to make some available to the community ).
>> That said, its not entirely obvious (at least to me) what a tractable
>> focused GSOC sized subset of the numerics optimization improvement would
>> be, and that would have to also be a subset that has real performance
>> impact and doesn't benefit from eg using -fllvm rather than -fasm .
>> ---------
>> For helping pave the way to better parallel builds, what are some self
>> contained units of work on ghc (or related work on cabal) that might be
>> tractable over a summer? If you can put together a clear roadmap of "work
>> chunks" that are tractable over the course of the summer, I'd favor
>> choosing that work, especially if you can give a clear outline of the plan
>> per chunk and how to evaluate the success of each unit of work.
>> basically: while both are high value projects, helping improve the
>> parallel build tooling (whether in performance or correctness or both!) has
>> a more obvious scope of community impact, and if you can layout a clear
>> plan of work that GHC folks agree with and seems doable, i'd favor that
>> project :)
>> hope this feedback helps you sort out project ideas
>> cheers
>> -Carter
>> On Sun, Apr 21, 2013 at 12:20 PM, Patrick Palka <patr...@parcs.ath.cx>wrote:
>>> Hi,
>>> I'm interested in participating in the GSoC by improving GHC with one of
>>> these two features:
>>> 1) Implement native support for compiling modules in parallel (see 
>>> #910<http://hackage.haskell.org/trac/ghc/ticket/910>).
>>> This will involve making the compilation pipeline thread-safe, implementing
>>> the logic for building modules in parallel (with an emphasis on keeping
>>> compiler output deterministic), and lots of testing and benchmarking. Being
>>> able to seamlessly build modules in parallel will shorten the time it takes
>>> to recompile a project and will therefore improve the life of every GHC
>>> user.
>>> 2) Improve existing constant folding, strength reduction and peephole
>>> optimizations on arithmetic and logical expressions, and optionally
>>> implement a core-to-core pass for optimizing nested comparisons (relevant
>>> tickets include #2132 <http://hackage.haskell.org/trac/ghc/ticket/2132>,
>>> #5615 
>>> <http://hackage.haskell.org/trac/ghc/ticket/5615>,#4101<http://hackage.haskell.org/trac/ghc/ticket/4101>).
>>> GHC currently performs some of these simplifications (via its BuiltinRule
>>> framework), but there is a lot of room for improvement. For instance, the
>>> core for this snippet is essentially identical to the Haskell source:
>>> foo :: Int -> Int -> Int -> Int
>>> foo a b c = 10*((b+7+a+12+b+9)+4) + 5*(a+7+b+12+a+9) + 7 + b + c
>>> Yet the RHS is actually equivalent to
>>> 20*a + 26*b + c + 467
>>> And:
>>> bar :: Int -> Int -> Int
>>> bar a b = a + b - a - b -- the RHS is should be optimized away to 0
>>> Other optimizations include: multiplication and division by powers of
>>> two should be converted to shifts; multiple plusAddr calls with constant
>>> operands should be coalesced into a single plusAddr call; floating point
>>> functions should be constant folded, etc..
>>> GHC should be able to perform all these algebraic simplifications. Of
>>> course, emphasis should be placed on the correctness of such
>>> transformations. A flag for performing unsafe optimizations like assuming
>>> floating point arithmetic is associative and distributive should be added.
>>> This proposal will benefit anybody writing or using numerically intensive
>>> code.
>>> I'm wondering what the community thinks of these projects. Which project
>>> is a better fit for GSoC, or are both a good fit? Is a mentor willing to
>>> supervise one of these projects?
>>> Thanks for your time.
>>> Patrick
>>> (A little about myself: I'm a Mathematics student in the US, and I've
>>> been programming in Haskell for about 3.5 years. Having a keen interest in
>>> Haskell and compilers, I began studying the GHC source about 1 year ago and
>>> I've since gotten a good understanding of its internals, contributing a few
>>> patches along the way.)
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