Primitive constant unfolding

[malosh]

Examining the core generated by ghc with -O2 on a numerical code, I saw things like :

case GHC.Prim.<## x_aB9 (GHC.Prim.**## 2.0 -1021.0) of _  {...

This code being executed each time my code performs an addition ! GHC does not seem to unfold the constants right with at least **##. By the way, it should definitely be possible to specify a Double# constant in hexadecimal.

[igloo]

Test case:

module Q where

foo x = x < (2 ** (-1021) :: Double)

[simonmar]

There are quite a few floating-point operations that we don't constant fold currently - all the trig functions, **, log, sqrt, and so on. One trickiness with implementing this is that the internal representation for Float/Double in the compiler is Rational, and that doesn't support these operations. We'd have to cast down to Double to do the operation and back to Rational again, and only do it when not cross-compiling.

[Khudyakov]

Replying to simonmar:

There is one more problem. Floats and Doubles have special values +∞, -∞ and NaN. They could be represented as Rational (1%0, -1%0, 0%0) but Data.Ratio doesn't allow to construct such values.