[Haskell-cafe] Optimizations and constant unboxed values

[Max Bolingbroke]

Hi,

On 28 May 2010 01:22, Pierre-Etienne Meunier
<pierreetienne.meunier at gmail.com> wrote:
> Then, examining the core (with of course -O3 on)

FYI, -O3 is the same as -O2.

> revealed things like :
>
>  (GHC.Prim.*##
>                  (GHC.Prim.-## 1.0 (GHC.Prim.**## 2.0 -53.0))
>                  (GHC.Prim.**## 2.0 1024.0))
>
> or
>
>  case GHC.Prim.<## x_aB9 (GHC.Prim.**## 2.0 -1021.0) of _  {...

GHC does constant-fold many operations on floats/doubles at the Core
level (see PrelRules.lhs) but there is no RULE for **. Perhaps there
should be.

> Then I wondered if this was really the last stage of GHC's optimizations,

I eyeballed the C-- code generator and it looks like that doesn't
constant-fold ** either.

> Any clue about how to do it ?

I think that rewriting your code is the only thing you can do. Please
also submit a bug to the tracker and I'm sure that the right RULEs
will get added soon.

Cheers,
Max