#8793: Improve GHC.Event.IntTable performance -------------------------------------+------------------------------------- Reporter: cdk | Owner: Type: task | Status: patch Priority: normal | Milestone: 8.0.1 Component: Core Libraries | Version: 7.6.3 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime | Unknown/Multiple performance bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by jscholl):
Interesting, though I don't yet understand the details. Could you boil out a standalone example that demonstrates just this single issue? I.e. two versions of a function, one of which repeats the stack check and one of which doesn't, and show the code side by side?
but it seems hard or impossible to correctly identity such unused arguments (I mean, it is used, but only in a function which does not use it...).
Well GHC's strictness analyser should find exactly this case. I'm
If I understand cmm correctly, we jump to the start of $wa in line 34, so the IO version repeats the stack check. The corresponding instruction in the pure version is in line 33, here we jump to cCT, so behind our stack check. This should also be possible in the IO version as the function only uses constant stack space and if it was available once, it should stay available until we deallocate it, right? puzzled why it does not. Again, could you spare a moment to make a standalone reproducer for just this issue? Or at least a smallish function I can compile in isolation to see this argument not disappearing.
No, I did not mean the optimizer in this case. The unused argument is optimized out, I was just thinking whether GHC could warn if an argument is never used (as it does with unused variables), as this sometimes indicated unfinished code which should either be removed (or documented as such) or finished. But I think this comes with too many cases where you want an unused argument, either to pass in a type via proxy or to satisfy another functions expectations. And if a function is recursive, unused arguments have to be passed on. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8793#comment:15 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler