[Haskell] Expecting more inlining for bit shifting

roconnor at theorem.ca roconnor at theorem.ca
Wed Oct 11 06:06:41 EDT 2006

On Wed, 11 Oct 2006, Simon Peyton-Jones wrote:

> Correct.  GHC generates
> 	case (x# >=# 0#) of { True -> ...; False -> ... }
> But the argument discount only applies when we have
> 	case y of { ... }
> So you really want a discount for the args of a primop.

Do you think it should be that general?  I was thinking the discount 
should only apply in the situtation where a case expression contains an 
expression with one free varaible that is a function argument, and all 
operations are primitive.

So I was thinking the right place to patch is in sizeExpr:

     size_up (Case (Var v) _ _ alts)
         | v `elem` top_args
         = ...

And make this branch activate is a wider range of circumstances.  SamB 
is/was working on such a patch.

But making sure that all operations are primitive is not quite right, for 
instance in

f :: Int -> ...
f x | gcd x 21 > 1 = ...

we cannot give x an argument discount because (gcd (5::Int) 21) is not 
rewritten into 1 (for some strange reason).

So, is there a way of deciding if a primitive op will be rewritten if all 
its arguements are given?

Russell O'Connor                                      <http://r6.ca/>
``All talk about `theft,''' the general counsel of the American Graphophone
Company wrote, ``is the merest claptrap, for there exists no property in
ideas musical, literary or artistic, except as defined by statute.''

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