Could you show the core difference (for us who don't really understand
#9660: unnecessary indirect jump when returning a case scrutinee -------------------------------------+------------------------------------- Reporter: rwbarton | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.8.3 (CodeGen) | Keywords: Resolution: | Architecture: Unknown/Multiple Operating System: | Difficulty: Unknown Unknown/Multiple | Blocked By: Type of failure: Runtime | Related Tickets: performance bug | Test Case: | Blocking: | Differential Revisions: | -------------------------------------+------------------------------------- Comment (by rwbarton): Replying to [comment:6 schyler]: the original ticket that well)? There is no difference at the Core level. In either case the core is {{{ Test.f = \ (x_arU :: GHC.Types.Int) -> case x_arU of wild_a10j { GHC.Types.I# x1_a10l -> case GHC.Prim.tagToEnum# @ GHC.Types.Bool (GHC.Prim.<# x1_a10l 0) of _ [Occ=Dead] { GHC.Types.False -> GHC.Types.I# (GHC.Prim.+# x1_a10l 1); GHC.Types.True -> wild_a10j } } }}} The difference is that in the generated Cmm, we don't re-enter `wild_a10j` in the True case, since we know it has already been reduced to WHNF by the first `case`.
Also, if you dump and attach the ASM I'd be happy to sift through and find the problem.
I don't have a copy of the instances I was looking at any more, but in one case the change seemed to affect the unique names chosen, which apparently led to laying out functions in a different order... annoying for trying to diff. My main question here is whether it's actually a good idea to use an error/panic in the LambdaFormInfo, or is it better to add a new constructor. I sort of like the error in that if something goes wrong, it's more likely to do so at compile time than at run time; but it's also a bit ugly. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9660#comment:7 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler