Stack profiling defeating stream fusion

[John Ky]

Hello everyone,

I have this problem where enabling profiling using stack undoes stream fusion and I’d like to do my profiling with stream fusion intact.

The following reproduces my problem:

$ git clone g...@github.com:haskell-works/hw-tutorial-performance.git
$ cd hw-tutorial-performance/
$ stack init
$ stack build --executable-profiling
$ time $(find .stack-work/dist/ -type f -name hw-tutorial-performance-rwhe) 1e7
5000000.5

real    0m4.432s
user    0m3.484s
sys    0m0.936s

$ rm -rf .stack-work/
$ stack build
$ time $(find .stack-work/dist/ -type f -name hw-tutorial-performance-rwhe) 1e7
5000000.5

real    0m0.034s
user    0m0.018s
sys    0m0.013s

With profiling, my program taks over 4 seconds to run. Without profiling, it only takes 0.034 seconds.

I believe the problem to be that stream fusion has been disabled by stack.

Does anyone know how to prevent this from happening?

Cheers,

-John

​

[Michael Snoyman]

This isn't in any way a complete answer, but from within your project directory, you can use `stack exec -- ghc` to compile individual main modules, and play around with different profiling settings. My guess here is that -prof-auto it inserting cost centers in such a way that GHC isn't able to inline things, but that's very much a guess.

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