Very poor constant folding

[augustss]

GHC seems to lack constant folding for encodeFloat/decodeFloat. For many examples, compile the numeric-limits package on hackage, or just this snippet:

epsilon :: Double
epsilon = r
  where r = 1 - encodeFloat (m-1) e
        (m, e) = decodeFloat (1 `asTypeOf` r)

This has a very simple value, but GHC refuses to compute it at compile time. This in turn inhibits other constant folding that should be going on.

[simonpj]

Would you like to propose the specific constant-folding rules that you believe should be implemented?

[simonmar]

It looks like we should have constant folding for:

encodeFloatInteger :: Integer -> Int# -> Float#
encodeDoubleInteger :: Integer -> Int# -> Double#

decodeDoubleInteger :: Double# -> (# Integer, Int# #)

doubleFromInteger :: Integer -> Double#
floatFromInteger :: Integer -> Float#

which should be easy enough with the new Integer literals. Ian, could you look into that?

[igloo]

This now works in HEAD.