tagToEnum# leads to some silly closures
I don't know how important this is in practice, but it looks unfortunate.
Suppose I write
foo :: (Bool -> a) -> Int# -> a
foo f x = f (tagToEnum# x)
Since tagToEnum#
can fail, GHC compiles this to
foo
= \ (@ a_a10v)
(f_s1by [Occ=Once!] :: GHC.Types.Bool -> a_a10v)
(x_s1bz [Occ=Once] :: GHC.Prim.Int#) ->
let {
sat_s1bA [Occ=Once] :: GHC.Types.Bool
[LclId]
sat_s1bA = GHC.Prim.tagToEnum# @ GHC.Types.Bool x_s1bz } in
f_s1by sat_s1bA
That seems pretty bad! We know that tagToEnum#
is applied to Bool
, so we can transform this to something like
foo f x = case leWord# (intToWord# x) 1## of
1# -> f $! tagToEnum# x
_ -> f (error "tagToEnum# was used at Bool with tag ...")
which avoids an extra closure at the cost of a single Word#
comparison. The same goes for arbitrary known enumeration types. I suspect the right place to fix this up is in CorePrep.