Memory leak with simple vector code about eta-hackage

[rahulmutt]

Thanks for the report!

I did a brief investigation by compiling this program with -ddump-stg and decompiling the generated class files as well. It turns out the optimizations are happening and the intermediate vectors have been fused away and there's a nice tight loop that calculates the sum via an accumulator variable.

What's interesting is that when I checked out VisualVM, the old generation for the GC was through the roof - 1GB! This means that there's a memory leak going on somewhere. The code is nice and small so it should be easy to investigate. Thank you for constructing a minimal example! I'll take a look at this.

[rahulmutt]

Some notes on the investigation for this:

1. I did memory profiling in VisualVM and discovered that the entire list from 1 to 100,000,000 is retained along with all the intermediate thunks used to construct it. This list was created as a result of fusing the vector operations and lifting out the resulting list to the top-level, creating a CAF (constant applicative form) or a globally shared thunk.

   In Eta, globally shared thunks are referenced through a static field of the class which corresponds to the module for which that thunk was declared (either directly or indirectly). Thus, the head of the list is retained throughout the duration of the program, preventing the GC from collecting the front part of the list. The way to get around this is to make the static reference to the CAF weak so that it can be GC'd when it's unused and re-created again as necessary. See typelead/eta#554.

   This problem doesn't affect GHC since its GC has special handling for top-level closures and thunks which clears out unused heap objects.

2. I turned on -fno-full-laziness to see if I could prevent the optimizer from lifting the list out as a CAF. The optimizer did not lift the CAF this time, but unfortunately, even in this case, it turns out there's head retention going on where the head of the list is retained by a local variable in the function that computes the initial [1..100,000,000] list. This problem can be fixed by typelead/eta#79.

Two bugs with one program, great work @rpeszek! :)