CSE for join points

Join points are hazardous for CSE:

join j = e in ... f e ...

The temptation is of course to rewrite to

join j = e in ... f (jump j) ...

which doesn't typecheck because you can't have a jump in an argument.

Similarly:

join j x = e in
let y = (... join j2 x = e in ...) in ...

can't be rewritten to

join j x = e in
let y = (... join j2 x = jump j x ...) in ...

because you can't jump out of a value binding.

This doesn't seem to come up very often (took a long time for me to trip over it). The simple solution is simply to ignore CSE for join points. That's what we do at the moment. But this is saddening in some cases:

join j x = e in
join j2 x = e in ...

can perfectly well become

join j x = e in
join j2 x = jump j x in ...

We just need to track which join points are still currently valid.

Trac metadata

Trac field	Value
Version	8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

changed weight to 5

added Tbug Trac import join points labels

We just need to track which join points are still currently valid.

Yes, and that would not be hard. Just keep a join-point reverse map in the CSEnv, alongside the existing cs_map; and discard it when going into non-tail positions.

A relatively easy win, but as you say a modest one.

added CSE label and removed join points label

added join points label

added Pnormal label

mentioned in issue #20739