Memory usage exploding for complex pattern matching

[Victor Miraldo (UU)]

Hello all,

Following up on this, I have created a trac ticket
https://ghc.haskell.org/trac/ghc/ticket/14987#no2 as
suggested a couple days ago.

In order to try to hack my way through and get my library to compile,
I tried progressively
adding more and more pattern synonyms to try to avoid exhaustiveness
checking,yet, something
really surprising happened. Big examples have type errors where small
ones don't!

I tried documenting the process in a repository:
https://github.com/VictorCMiraldo/ghc-14987-repro-pipeline

Unfortunately, however, I couldn't get the self contained repros in
the repository above to throw
type errors. Hence, I'm attaching my full code. Compiling the file
src/Generics/MRSOP/Examples/GoAST.hs
gives type errors, even though it is being generated by the same
template haskell code as the other
files inside Examples, all of which compile just fine. To trigger the
error, just run `stack build`.

It really is a shame that the combination of pattern synonyms that
shows the best memory performance
crashes completely on bigger examples.

Any suggestions are appreciated, thank you very much!

Have a great weekend!
Cheers,
Victor



On Fri, Mar 30, 2018 at 5:23 PM, Ben Gamari <ben at smart-cactus.org> wrote:
> "Victor Miraldo (UU)" <v.cacciarimiraldo at uu.nl> writes:
>
>> Hello,
>>
>>>>>> I have just tried compiling my code with 8.4.2 and using
>>>>>> -fmax-pmcheck-iterations=0,
>>>>>> I gave GHC 12GB of ram and it still ran out (through `ulimit
>>>>>> -v$((1024*1024*12))`).
>>>>>>
>>>>> Hmmm, I'm a bit confused. Why are our results so different? How
>>>>> precisely are you invoking GHC?
>>>>
>>>> Here I meant my whole code, not just the repro. I could have been more clear.
>>>> Nevertheless, I'm calling it through stack:
>>>>
>>> I'll admit that I am a bit lost; Minimal.hs compiles for me with a
>>> maximum residency of ~3.5 GBytes with both -O1 and the PM check enabled
>>> using GHC 8.4.1. Is this not the repro you are referring to?
>>
>> I get the same behavior as you for Minimal.hs.
>>
>> The "my code" above referred to the whole library that I'm developping.
>> In fact, the Minimal.hs file contains a distilled version of that library with
>> a template haskell splice that we are trying to use in one of our
>> fully fledged examples.
>>
> Okay, I just wanted to be certain we were indeed seeing the same
> behavior. Indeed 3.5 GB is quite a lot of memory for a 700 LoC program,
> even one with the deep matches seen in Minimal.hs.
>
> Cheers,
>
> - Ben
>
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