wiki:NestedCPR

Version 7 (modified by nomeata, 6 years ago) (diff)

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This is nomeata’s notepad about the nested CPR information:

Related tickets

  • #1600 Main tickets where I mention progress.

Tickets with stuff that would make nested CPR better:

  • #8598 CPR after IO (partly done)

Related testcases

TODOs

  • Does Nick Frisby’s late λ-lifting alleviate problems when CPR’ing join-points?
  • Paper-Writeup of CPR
  • Shouldn’t nested CPR help a lot with Complex-heavy code? Is there something in nofib?
  • Try passing CPR information from the scrunitee to the pattern variables. For that: Reverse flow of analysis for complex scrunitees (for simple, we want the demand coming from the body, for complex, this is not so important.)
  • Why is cacheprof not deterministic? (→ #8611)
  • Use ticky-profiling to learn more about the effects of nested CPR.
  • Look at DmdAnal-related [SLPJ-Tickets] and see which ones are affected by nested-cpr.
  • Idea about join points: ​http://www.haskell.org/pipermail/ghc-devs/2013-December/003481.html

better-ho-cardinality

It would be nice to merge the code structure improvements and notes into master, to keep my branch short. But it is based on better-ho-cardinality, and that is not suitable for merging because of unexpected regressions even in nofib and rtak. So I am investigating.

In these tests, it is related to reading and showing data. Small example: 

main = (read "10" :: Int) `seq` return ()

Baseline: 49832, better-ho-cardinality: 49968. Unfortunately, the changes to, for example, GHC.Read are not small, and probably mostly benign...

Trying to minimize and isolate the problem. After some agressive code deleting, this is where I ended up: 

{-# LANGUAGE RankNTypes #-}

data P a
  = Get (Char -> P a)
  | Result a

Get f1     `mplus` Get f2     = undefined
Result x   `mplus` _          = Result x

newtype ReadP a = R (forall b . (a -> P b) -> P b)

instance Monad ReadP where
  return x  = R (\k -> k x)
  R m >>= f = R (\k -> m (\a -> let R m' = f a in m' k))

readP_to_S (R f) = f Result `seq` ()

ppp :: ReadP a -> ReadP a -> ReadP a
ppp (R f1) (R f2) = R (\k -> f1 k `mplus` f2 k)

paren :: ReadP () -> ReadP ()
paren p = p >> return ()

parens :: ReadP () -> ReadP ()
parens p = optional
 where
  optional  = ppp p mandatory
  mandatory = paren optional

foo = paren ( return () )

foo2 = ppp (parens ( return () )) (parens (return ()))

main = readP_to_S foo `seq` readP_to_S foo2 `seq` return ()

it is important that both paren and parens are used more than once; if there is only one use-site, the problem disappears (which made it hard to find). Also, most other changes prevent the increase in allocations: Removing the Monad instance and turning its methods into regular functions; adding NOINLINE annotations to paren or parens; changing foo2 to ppp foo foo; even removing the dead code that is the first line of the mplus function.

I think I’ll leave it at this point, this is hopefully enough information for someone (likely SPJ) to know whats going on.

Side tracks