#8763: forM_ [1..N] does not get fused (allocates 50% more) -------------------------------------+------------------------------------- Reporter: nh2 | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.8.1 Component: Compiler | 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: #7206 | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by simonpj): Thanks for the explanation. Try this {{{ forM_2 :: (Foldable t, Monad m) => t a -> (a -> m b) -> m () forM_2 xs f = let c x k = f x >> k {-# INLINE c #-} in foldr c (return ()) xs }}} and use `forM_2` instead of `forM_` in the outer calls in `f` and `g`. I then get good results for both. How does this work? Well by marking `c` as INLINE, I prevent `f` from inlining into it -- remember, the promise of INLINE things is that what you write gets inlined. And this is what we want: `c` is small, just `f x >> k`, and inlining it is very very good. Without the INLINE pragmas on `c` we have something like {{{ let f = BIG in let c x k = f x >> k in BODY }}} Since `f` occurs just once, we inline `f` to give {{{ let c x k = BIG x >> k in BODY }}} and now `c` becomes too big to inline. This is a classic inlining dilemma: do we inline `f` into `c` or `c` into `BODY`? The latter is much better in this case. I think we could build this into the libraries just by changing the definition of `mapM_`. Do you agree? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8763#comment:76 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler