[Haskell-cafe] GHC optimisations

[Simon Peyton-Jones]

| 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