On Wed, Aug 22, 2007 at 09:04:11AM +0100, Simon Peyton-Jones wrote:
> | First of all, optimizing mod and div can not be done with PrelRules,
> | because they are not primitives, quot and rem are.
>
> Yes, you can do them with PrelRules! Check out PrelRules.builtinRules.
>
> | Multiplication and division can become shifts:
> |
> | > {-# RULES
> | >
> | > -- x * 2^n --> x `shiftL` n
> | > "x# *# 2#" forall x#. x# *# 2# = x# `iShiftL#` 1#
> | > "2# *# x#" forall x#. 2# *# x# = x# `iShiftL#` 1#
> | > -- etc.
>
>
> | A problem with these rules is that you need a whole lot of them. 32 per
> | operation (on a 32 bit platform), * 4 operations, * 2 separate versions
> | for words and ints = 256.
>
> I think you should be able to a lot better. For example, to do
> constant folding for +# you might think you needed a lot of rules
>
> 1# +# 2# = 3#
> 1# +# 3# = 4#
> etc
>
> But not so! See PrelRules for how to write one rule that does all of
> these at once. I think you can do multiply-to-shift in the same way.
>
> The downside of PrelRules is that it's part of the compiler, not in
> Haskell pragmas; that's what makes it more expressive than rules
> written in source code.
Something I've pondered is adding a more-expressive form of RULES which
works using general pattern matching:
{-# XRULES
"*#-to-shift" (*#)
(CoreLit (CoreInt num)) obj
| num .&. (num - 1) == 0 ->
CoreApp (CoreVar "GHC.Prim.iShiftL#")
[obj, CoreLit (CoreInt (lg2 num))]
obj (CoreLit (CoreInt num))
| num .&. (num - 1) == 0 ->
CoreApp (CoreVar "GHC.Prim.iShiftL#")
[obj, CoreLit (CoreInt (lg2 num))]
#-}
This would require reusing the TH infrastructure, and (depending on how
much we can abstract) might leak too many details of Core to be
useful; on the other hand it would allow some very interesting domain
optimizations to be done.
Views might be nice here.
Opinions on whether something like this is a good idea?
Stefan
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