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jcpetruzza / barbies

BSD 3-Clause "New" or "Revised" License
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Pathological compilation behavior with GHC 9.8.1 #51

Open m4dc4p opened 9 months ago

m4dc4p commented 9 months ago

I've found that a module using barbies takes exponentially longer to compile under GHC 9.8 as I add fields to a data type.

The repo at https://github.com/m4dc4p/ghc98-bug demonstrates the problem. If you have GHC and cabal installed, you should be able to build with just cabal build:

$ cabal build
Warning: The package list for 'hackage.haskell.org' is 19 days old.
Run 'cabal update' to get the latest list of available packages.
Resolving dependencies...
Build profile: -w ghc-9.8.1 -O1
In order, the following will be built (use -v for more details):
 - ghc98-bug-0.0.0 (lib) (first run)
Configuring library for ghc98-bug-0.0.0..
Preprocessing library for ghc98-bug-0.0.0..
Building library for ghc98-bug-0.0.0..

Uncommenting fields on the HDKType data constructor cause compilation to take longer and longer (and use more memory):

Its worth noting the removing the deriving instance for Eq causes the problem to go away (but, of course, that instance is something we want). Also worth noting that with -O0, the problem goes away. I have not narrowed down which optimization is the problem.

jcpetruzza commented 9 months ago

Thanks for flagging! Just to confirm, this is a regression, correct? Is there a ghc issue tracking this?

m4dc4p commented 9 months ago

Yeh, under 9.6 this does not happen. I reported to GHC just after this as well - https://gitlab.haskell.org/ghc/ghc/-/issues/24462.