| 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.
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.
| (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
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