[Haskell-cafe] Two GHC-related GSoC Proposals

Patrick Palka patrick at parcs.ath.cx
Mon Apr 22 04:34:41 CEST 2013

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
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

Thanks for the feedback.

On Sun, Apr 21, 2013 at 5:55 PM, Carter Schonwald <
carter.schonwald at 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 <patrick at 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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