[project @ 2001-02-28 11:48:34 by simonpj]
Add most of the code for constructor specialisation. The comment
below is reproduced from specialise/SpecConstr.lhs.
It doesn't quite work properly yet, because we need to have
rules in scope in a recursive function's own RHS, and that
entails a bit of fiddling I havn't yet completed. But SpecConstr
itself is a nice neat 250 lines of code.
-----------------------------------------------------
Game plan
-----------------------------------------------------
Consider
drop n [] = []
drop 0 xs = []
drop n (x:xs) = drop (n-1) xs
After the first time round, we could pass n unboxed. This happens in
numerical code too. Here's what it looks like in Core:
drop n xs = case xs of
[] -> []
(y:ys) -> case n of
I# n# -> case n# of
0 -> []
_ -> drop (I# (n# -# 1#)) xs
Notice that the recursive call has an explicit constructor as argument.
Noticing this, we can make a specialised version of drop
RULE: drop (I# n#) xs ==> drop' n# xs
drop' n# xs = let n = I# n# in ...orig RHS...
Now the simplifier will apply the specialisation in the rhs of drop', giving
drop' n# xs = case xs of
[] -> []
(y:ys) -> case n# of
0 -> []
_ -> drop (n# -# 1#) xs
Much better!
We'd also like to catch cases where a parameter is carried along unchanged,
but evaluated each time round the loop:
f i n = if i>0 || i>n then i else f (i*2) n
Here f isn't strict in n, but we'd like to avoid evaluating it each iteration.
In Core, by the time we've w/wd (f is strict in i) we get
f i# n = case i# ># 0 of
False -> I# i#
True -> case n of n' { I# n# ->
case i# ># n# of
False -> I# i#
True -> f (i# *# 2#) n'
At the call to f, we see that the argument, n is know to be (I# n#),
and n is evaluated elsewhere in the body of f, so we can play the same
trick as above. However we don't want to do that if the boxed version
of n is needed (else we'd avoid the eval but pay more for re-boxing n).
So in this case we want that the *only* uses of n are in case statements.
So we look for
* A self-recursive function. Ignore mutual recursion for now,
because it's less common, and the code is simpler for self-recursion.
* EITHER
a) At a recursive call, one or more parameters is an explicit
constructor application
AND
That same parameter is scrutinised by a case somewhere in
the RHS of the function
OR
b) At a recursive call, one or more parameters has an unfolding
that is an explicit constructor application
AND
That same parameter is scrutinised by a case somewhere in
the RHS of the function
AND
Those are the only uses of the parameter