Simon Peyton-Jones
| So, my hypothesis is that the inliner doesn't recognise that | ``if (x >= 0) then ...'' is effectively a case analysis on x, and thus the | argument discount is not fired. So we need to figure out how to extend | this criterion for when to apply the argument discount.
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.
The relevant file is coreSyn/CoreUnfold.lhs, and the function is calcUnfoldingGuidance.
Actually it is sizeExpr. (Even so, apparantly I've been figuring this out the hard way...) The brach that currently handles these is the size_up (Case e _ _ alts) = nukeScrutDiscount (size_up e) `addSize` foldr (addSize . size_up_alt) sizeZero alts branch. I've got a patch that seems like it ought to do a bettter job, but it doesn't seem to give the $wrotate functions any discount (the $wshift functions having been tagged by the {-# INLINE shift #-} pragmas I added all over). Unfortunately I left it at home and I'm at school right now :-(. It does get run sometimes, but I'm not sure if it is run for rotate or that its results are kept...
I see some notes there with primops, namely:
PrimOpId op -> primOpSize op (valArgCount args) -- foldr addSize (primOpSize op) (map arg_discount args) -- At one time I tried giving an arg-discount if a primop -- is applied to one of the function's arguments, but it's -- not good. At the moment, any unlifted-type arg gets a -- 'True' for 'yes I'm evald', so we collect the discount even -- if we know nothing about it. And just having it in a primop -- doesn't help at all if we don't know something more.
At the call site, the call f x y gets f's arg-discount for x if x is evaluated. But in the case of primitive types we don't just want "evaluated", we want to know the value. So one could refine that. The relevant function is interestingArg in simplCore/SimplUtils.
I might point out that the current code would throw out those discounts (the nukeSrutDiscounts in that case).
| (This whole idea of argument discounting seems rather ad hoc. Is it not | possible try out an inline, and remove it if in the end it doesn't get | reduced in size sufficently?)
Yes, you could try that too. It might result in a lot of wasted work, but it'd be a reasonable thing to try. The relevant code is in simplCore/Simplify.lhs
Simon