Regression, simplifier explosion with Accelerate, cannot compile, increasing tick factor is not a workaround
In Accelerate we are encountering problems with the simplifier exploding the number of terms, which consequently cause huge compile times. This has only happened since the release of 7.10.1 and it appears to still be happening in the ghc-7.10 branch.
In the attached Slice.hs you can see what is about the worst case example. Trying to compile this with
[1 of 1] Compiling Data.Array.Accelerate.CUDA.Array.Slice ( Slice.hs, Slice.o ) ghc: panic! (the 'impossible' happened) (GHC version 220.127.116.1150602 for x86_64-apple-darwin): Simplifier ticks exhausted When trying UnfoldingDone $j_s1FRp To increase the limit, use -fsimpl-tick-factor=N (default 100) If you need to do this, let GHC HQ know, and what factor you needed To see detailed counts use -ddump-simpl-stats Total ticks: 1668604
Bumping the tick factor to 150 stops this from happening, but at that point the simplifier blows up. See output.txt for the output of
ghc -v. After half an hour we end up with ~4 million terms. At this point ghc takes up about 9GB of memory and I have to kill it.
In order to reproduce this, you will need the
accelerate package and all its various dependencies. It doesn't have to be the latest from github. The version on hackage also works with the example I have given. I will try to come up with a test case that is not so heavy on dependencies, but I wanted to ensure this was raised before 7.10.2 is released.
Steps to reproduce:
cabal install accelerate ghc -O2 -fsimpl-tick-factor=150 Slice.hs