cross module optimization issues

Simon Peyton-Jones simonpj at
Fri Nov 21 05:33:41 EST 2008

| This project is based on Oleg's Iteratee code; I started using his
| IterateeM.hs and Enumerator.hs files and added my own stuff to
| Enumerator.hs (thanks Oleg, great work as always).  When I started
| cleaning up by moving my functions from Enumerator.hs to MyEnum.hs, my
| minimal test case increased from 19s to 43s.
| I've found two factors that contributed.  When I was cleaning up, I
| also removed a bunch of unused functions from IterateeM.hs (some of
| the test functions and functions specific to his running example of
| HTTP encoding).  When I added those functions back in, and added
| INLINE pragmas to the exported functions in MyEnum.hs, I got the
| performance back.
| In general I hadn't added export lists to the modules yet, so all
| functions should have been exported.

I'm totally snowed under with backlog from my recent absence, so I can't look at this myself, but if anyone else wants to I'd be happy to support with advice and suggestions.

In general, having an explicit export list is good for performance. I typed an extra section in the GHC performance resource to explain why.  In general that page is where we should document user advice for performance in GHC.

I can't explain why *adding* unused functions would change performance though!


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