[GHC] #9675: Unreasonable memory usage on large data structures

[GHC]

#9675: Unreasonable memory usage on large data structures
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              Reporter:  Polarina    |            Owner:
                  Type:  bug         |           Status:  new
              Priority:  normal      |        Milestone:
             Component:  Compiler    |          Version:  7.8.3
            Resolution:              |         Keywords:
      Operating System:  Linux       |     Architecture:  x86_64 (amd64)
       Type of failure:  Compile-    |       Difficulty:  Unknown
  time performance bug               |       Blocked By:
             Test Case:              |  Related Tickets:
              Blocking:              |
Differential Revisions:              |
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Comment (by nomeata):

 Thanks for your input.

 The way it breaks optimisation is what I meant with “making some code more
 complicated”: With a sparse list of bound variables, it would have to
 check which fields the optimization would like to use and then generated
 them on demand. I see that that might make the code very ugly..

 The compulsory unfolding you mention would still be of the the shape of a
 huge pattern match, right? So the quadratic behaviour wouldn’t be
 eliminated.

 A comparison of the heap profile with and without `-dverbose-core2core`
 (using this as a poor man’s deepseq after each phase) shows that there
 might be a space leak, as you guess. (Uploading both graphs.)

 @Polarina: A work-around for you might be to not use a data constructor,
 but a newtype around a `Vector (Int -> Int)` (inner type mostly
 irrelevant). You store the functions therein and your accessor functions
 would use `unsafeIndex` and `unsafeCoerce` to get them out again. You can
 wrap that highly unsafe code in a module that can be used in a totally
 safe way.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9675#comment:8>
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