#7367: Optimiser / Linker Problem on amd64 --------------------------------------------+------------------------------ Reporter: wurmli | Owner: Type: bug | Status: new Priority: normal | Milestone: 7.8.1 Component: Build System | Version: 7.6.1 Resolution: | Keywords: Operating System: Linux | Architecture: x86_64 Type of failure: Runtime performance bug | (amd64) Test Case: | Difficulty: Unknown Blocking: | Blocked By: | Related Tickets: --------------------------------------------+------------------------------ Comment (by rwbarton): Actually, `flopS` uses its "continuation" `f` linearly, like the CPS translation of an ordinary imperative function. Teaching GHC to recognize this sounds like quite an undertaking, though. This seems like a typical instance of how it is hard to decide whether to float out a let. The trouble with let-floating from the user's perspective is that when GHC wrongly decides to float out a let, it is virtually impossible to stop it from doing so. One way out is `-fno-full-laziness`, but GHC probably was correctly floating out a lot of other things, and it's difficult and tedious to locate and transform all those parts of the code. One suggestion is to identify situations in which let-floating can never be harmful, and allow `-fno-full-laziness` (or perhaps a new flag, `-fsome-full-laziness`?) to perform those floatings. For example, I don't think it can ever cost to float a binding whose RHS is a lambda to the top level. That rule would float out `flopp` and `swap` from `flop` here, and compiling with `-fno-full-laziness` would then eliminate all allocations. It would also be very useful to be able to control let-floating locally with some kind of pragma or magic built-in function. I'm sure this idea has been explored before and I also have some ideas of how to go about it, but I want to get a little more familiar with the structure of GHC before proposing anything. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/7367#comment:7 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler