#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 simonpj): Ah, got it now, thanks. Indeed, there is a real issue here. `genPermutations` looks like this: {{{ genPermutations !n !l !r !perm !count = allocaArray n $ \ destF -> do let upd j !f run = do p0 <- peekElemOff perm 0 if p0 == 0 then increment perm count >> run f else do copyArray destF perm n increment perm count flopS destF $ \ flops -> run (f `mappend` F (checksum j flops) flops) let go j !f = if j >= r then return f else upd j f (go (j+1)) go l mempty where checksum i f = if i .&. 1 == 0 then f else -f }}} Now, just inline `checksum` and that call to `flopS` looks like {{{ flopS destF $ \ flops -> run (f `mappend` F (if then flops else -flops) flops) }}} Now `flopS` really does call its argument many times, so work really is saved by lifting out the constant expression to get: {{{ let lvl::Bool = j .&. 1 == 0 in flopS destF $ \ flops -> run (f `mappend` F (if lvl then flops else -flops) flops) }}} As it happens, in this case allocating the thunk may be more expensive than repeating the computation, but that's a bit hard for GHC to work out. Ideas welcome. Simon -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/7367#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler