stream fusion, concatMap, exisential seed unboxing

[Simon Peyton Jones]

I think it'd help you to open a Trac ticket, give a fully-reproducible test case, including instructions for how to reproduce, and say what isn't happening that should happen. 

What's odd is that loop_s29q looks strict in its Int arg, yet isn't unboxed.  There is a way to get the strictness analysis to run twice -flate-dmd-anal.  You could try that.

Simon

|  -----Original Message-----
|  From: Glasgow-haskell-users [mailto:glasgow-haskell-users-
|  bounces at haskell.org] On Behalf Of Christian Höner zu Siederdissen
|  Sent: 01 February 2015 12:18
|  To: Glasgow-Haskell-Users
|  Subject: stream fusion, concatMap, exisential seed unboxing
|  
|  Hi everybody,
|  
|  I'm playing around with concatMap in stream fusion (the vector package
|  to be exact).
|  
|  concatMapM :: Monad m => (a->m (Stream m b)) -> Stream m a -> Stream m
|  b concatMapM f (Stream ...) = ...
|  
|  I can get my concatMap to behave nicely and erase all Stream and Step
|  constructors but due to the existential nature of the Stream seeds,
|  they are re-boxed for the inner stream (which is kind-of annoying
|  given that the seed is immediately unboxed again ;-). seq doesn't help
|  here.
|  
|  Otherwise, fusion happens for streams and vectors, so that is ok. But
|  boxing kills performance, criterion says.
|  
|  Do we have s.th. in place that could help here? Currently I could use
|  the vector-concatMap which creates intermediate arrays, my version
|  which has boxed seeds, or hermit but that is too inconvenient for non-
|  ghc savy users.
|  
|  Viele Gruesse,
|  Christian
|  
|  
|  
|  Fusing concatMapM:
|  
|  concatMapM f (SM.Stream ostep t _) = SM.Stream step (Left t) Unknown
|    where step (Left t) = do r <- ostep t
|                             case r of
|                              SM.Done       -> return $ SM.Done
|                              SM.Skip    t' -> return $ SM.Skip (Left
|  t')
|                              SM.Yield a t' -> do s <- f a
|                                                  return $ SM.Skip
|  (Right (s,t'))
|          step (Right (SM.Stream istep s _,t)) = do r <- istep s
|                                                    case r of
|                                                      SM.Done       ->
|  return $ SM.Skip    (Left t)
|                                                      SM.Skip    s' ->
|  return $ SM.Skip    (Right (SM.Stream istep s' Unknown,t))
|                                                      SM.Yield x s' ->
|  return $ SM.Yield x (Right (SM.Stream istep s' Unknown,t))
|          {-# INLINE [0] step #-}
|  {-# INLINE [1] concatMapM #-}
|  
|  testConcatMapM :: Int -> Int
|  testConcatMapM k = seq k $ U.unId
|                   . SM.foldl' (+) 0
|                   . concatMap (\i -> SM.enumFromTo 5 k)
|                   $ SM.enumFromTo 3 k
|  {-# NOINLINE testConcatMapM #-}
|  
|  CORE:
|  
|  testConcatMapM
|  testConcatMapM =
|    \ k_aCA ->
|      let! { I# ipv_s1xv ~ _ <- k_aCA } in ### inner loop
|      letrec {
|        $s$wfoldlM'_loop_s29q
|        $s$wfoldlM'_loop_s29q =
|          \ sc_s29i sc1_s29j sc2_s29k ->
|  ### unboxing
|            let! { I# x_a1LA ~ _ <- sc1_s29j } in
|            case tagToEnum# (<=# x_a1LA ipv_s1xv) of _ {
|              False -> $s$wfoldlM'_loop1_s29c sc_s29i sc2_s29k;
|              True ->
|                $s$wfoldlM'_loop_s29q
|  ### reboxing
|                  (+# sc_s29i x_a1LA) (I# (+# x_a1LA 1)) sc2_s29k
|            };
|  ### outer loop
|        $s$wfoldlM'_loop1_s29c
|        $s$wfoldlM'_loop1_s29c =
|          \ sc_s29a sc1_s29b ->
|            case tagToEnum# (<=# sc1_s29b ipv_s1xv) of _ {
|              False -> sc_s29a;
|              True ->
|                case tagToEnum# (<=# 5 ipv_s1xv) of _ {
|                  False -> $s$wfoldlM'_loop1_s29c sc_s29a (+# sc1_s29b
|  1); ### boxed seed (I# 6)
|                  True -> $s$wfoldlM'_loop_s29q (+# sc_s29a 5) (I# 6)
|  (+# sc1_s29b 1)
|                }
|            }; } in
|      let! { __DEFAULT ~ ww_s20G <- $s$wfoldlM'_loop1_s29c 0 3 } in
|      I# ww_s20G
|  
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