Example where ghc 6.10.1 fails to optimize recursive instance function calls

[Syzygies]

Using ghc 6.10.1, I get over a 3x performance boost on the attached toy example, by moving instance function definitions out of the instance declaration, so that recursive calls avoid the class dictionary.

According to SPJ, ghc 6.10.1 is supposed to perform this optimization automatically. I am reporting it as a bug at his request, following a discussion on the glasgow-haskell-users@… list.

[Syzygies]

Toy project exhibiting bug

[simonpj]

Ha! Excellent example. What is happening is that I'm generating stuff like

union d = let u2 = union d in \x. ... u2 ...

If only we had Max's arity analysis, this would generate good code (by inlining u2) but now it doesn't. Max, this is an excellent test case.

Simon

[michalt]

With current HEAD I'm getting exactly the same run-times in both cases. Maybe this was fixed by the simple arity analyser that was recently implemented?

[simonmar]

Yes, looks like another case that the arity analyser catches. Simon, do you agree?

[simonpj]

Yes, this looks like the same win. Ian, could you add a perf test and close? Thanks

Simon

[igloo]

Test added.