IO hack in demand analyzer gets in the way of CPR

After a lot of staring at code and comparing unexpected nofib results I found the following:

The IO hack in the demand analyzer (see dmdAnalAlt in StrAnal.lhs and #1592 (closed) for a good discussion) prevents CPR in any function that uses a C call. This is a small example, reduced from the scaleFloat method for doubles:

module Float(fun) where

import GHC.Float (Double(..))
import GHC.Integer (decodeDoubleInteger, encodeDoubleInteger)

fun :: Double -> Double
fun x | isFix           = x
      | otherwise       = case x of
          (D# x#) -> case decodeDoubleInteger x# of
            (# i, j #) -> D# (encodeDoubleInteger i j)
  where
  isFix = isDoubleFinite x == 0

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

Here, fun does current not get the CPR property, and the work gets type GHC.Prim.Double# -> GHC.Types.Double. Why? Because in core, there will be a

    case {__pkg_ccall main isDoubleFinite GHC.Prim.Double#
                                 -> GHC.Prim.State# GHC.Prim.RealWorld
                                 -> (# GHC.Prim.State# GHC.Prim.RealWorld, GHC.Prim.Int# #)}_dQD
           ds_dQA GHC.Prim.realWorld#
    of _ [Occ=Dead, Dmd=<L,A>]
    { (# ds_dQC [Dmd=<L,A>, OS=OneShot], ds_dQB [Dmd=<S,1*U>] #) ->
      ...

where the case body has DmdType m {dQz-><L,1*U(U)> dQA-><L,U>}, but dmdAnalAlt detects that this is evaluating a possibly exiting function and throws aways the information using alt_ty lubDmdType topDmdType.

Would it be ok to lub only the demand on the free variables, but keep the CPR information?

In nofib (if I managed to compare the right results) this does nothing for almost all benchmarks, -9.2% of allocations for mandel and +4.9% for reverse-complement (but these numbers are not obtained very cleanly, and all is happening on top of the better-ho-cardinality branch.

Trac metadata

Trac field	Value
Version	7.6.3
Type	Task
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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