Improve floating for join points

Having looked at the code in SetLevels, am very uncomfortable. My nose tells me that there is far too much chuffing about; it all makes my head spin.

Question 1. Why do we ever "ruin" a join point? See

Note [When to ruin a join point]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Generally, we protect join points zealously. However, there are two situations
in which it can pay to promote a join point to a function:

1. If the join point has no value arguments, then floating it outward will make
   it a *thunk*, not a function, so we might get increased sharing.
2. If we float the join point all the way to the top level, it still won't be
   allocated, so the cost is much less.

Refusing to lose a join point in either of these cases can be disastrous---for
instance, allocation in imaginary/x2n1 *triples* because $w$s^ becomes too big
to inline, which prevents Float In from making a particular binding strictly
demanded.

But I don't agree at all. If we have

let-join j = e in b

then we can leave j in place as a join point. We can float e (via lvlMFE) if doing so would increase sharing. Indeed this applies uniformly to all join points. For example

f x = let g y = let-join j z1 z2 = expensive x
                in case y of
                   A p q -> j p q
                   B r   -> j r r
                   C     -> True

Here it would make sense to float (expensive x) out of the \y abstrction:

f x = let lvl = expensive x
          g y = let-join j z1 z2 = lvl
                in case y of
                   A p q -> j p q
                   B r   -> j r r
                   C     -> True

But doing so does not affect the join point j. Nullary join points are no different.

This includes floating to the top level. Incidentally the RHS of the join point then becomes tiny, so the join point will be inlined.

In short, I think we can just delete all this special code.

Question 2: Note [Join points and MFEs]. Whe do we ever float out a MFE that has a free join variable? SLPJ claim: if there is a free join variable, do not float it anywhere.

Question 3: Do we actually need to float join points at all? Why?

I thik the reason is just to make them small

let-join j1 x = let-join j2 y = y+1
                in ...

Here perhaps if we float j2 out of j1 that might make j1 small enough to inline. But if that is the only motivation (unlike most of FloatOut which is about saving work) we'd better say so loud and clear. And the question is a bit more complicated; e.g. we might want to abstract over Ids to achieve this. e.g.

let-join j1 x = let-join j2 y = x+y
                in ...
===>
let-join j2' x y = x+y
         j1 x = let-join j2 y = j2' x y
                in ...

Now we can inline j2. (The example would be more realistic if the x+y was a big expression.) It's not just join parameters; we can abstract over any free variables:

let-join j1 x = let p = x+y in
                let-join j2 z = p+z
                in ....

Here we could abstract j2 over p in order to float it.

It is not clear how best to do this; but I worry that we are asking FloatOut to do too much.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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