| 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.
Does that help?
If one of the folk listening to this thread wanted to add a page to the GHC
Commentary distilling this thread into Wiki material, I'd be happy to check its
accuracy.
http://hackage.haskell.org/trac/ghc/wiki/Commentary
Simon
_______________________________________________
Haskell-Cafe mailing list
Haskell-Cafe@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-cafe
