Re: [GHC] #8598: IO hack in demand analyzer gets in the way of CPR

13 Feb 2017

      #8598: IO hack in demand analyzer gets in the way of CPR
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        Reporter:  nomeata           |                Owner:  (none)
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.6.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |            Test Case:  T8598
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by akio):

 I have come across an example where adding a new item to the lattice is
 actually useful:

 {{{#!hs
 {-# LANGUAGE MagicHash #-}
 {-# LANGUAGE UnboxedTuples #-}
 module Float(fun) where

 import GHC.Exts
 import GHC.Integer (decodeDoubleInteger, encodeDoubleInteger)

 fun :: Double -> Double
 fun x@(D# x#)
   | x == 0 = 0
   | otherwise = case isDoubleFinite x of
       I# y# -> case decodeDoubleInteger x# of
         (# i, j #) -> fun (D# (encodeDoubleInteger i (y# +# j)))

 foreign import ccall unsafe "isDoubleFinite" isDoubleFinite :: Double ->
 Int
 }}}

 Here `fun` is a tail-recursive function, and I think it can be CPRed just
 fine. However the current GHC fails to give `fun` a CPR property because
 it removes too much information in `deferAfterIO`. Specifically:

 1. In the first iteration of the fixpoint calculation, the recursive to
 call to `fun` gets the CPR result of `Diverges`.
 2. However, the case expression that scrutinizes `isDoubleFinite x` causes
 a call to `deferAfterIO`, which turns the CPR result to `Dunno NoCPR`.
 3. This in turn means the whole function doesn't get any useful CPR
 property.

 If we had `ExitCPR` (or `Exits` to match the current naming convention) in
 the lattice, then the step (2) would have returned `Exits` instead of
 `Dunno NoCPR`, and allowed `fun` to get a useful CPR property.

 I don't know how often this comes up in practice. With nested CPR, this
 will become a much bigger problem because it essentially means no tail-
 recursive IO function can get a useful CPR property.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8598#comment:13
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