Motivation: to make progress in the new compiler stack, we have
to preserve join points during lambda lifting. Right now, they are
lifted as regular lambdas. So, to keep them, we need some basic
support for them in the old VM. The implementation here is quick and
dirty. This is not an issue since this code will be deleted soon.
This is not a perfect solution yet. I have identified two problems:
1) The heuristic used at `mark_to_reset_fields` is too weak.
Suppose we have
```
let ...
_x := prod.mk field_1 0
in some _x
```
The current implementation will mark `field_1` to be reset since it
occurs in a non tail call `prod.mk field_1 0`. However, we can
assume `prod.mk field_1 0` is part of the return value.
2) The current semantics of the reset instruction may produce
unnecessary memory allocation. Consider the following example
```
let _x_1 := x.1,
_x_2 := x.2,
_x_3 := _reset.2 x
in @bool.cases_on y
(_cnstr.0.0)
(let _x_4 := f _x_2,
in _updt.2 _x_3 _x_4)
```
The memory cell `_x_3` is only used in the second branch of
the `bool.cases_on`. So, if `y` is `ff`, and `x` is a shared
object, we will allocate memory at `_reset.2 x` for nothing.
There are still some cases like errors in nested equations that leak
metavariables, but as long as we report at least one error, that probably isn't
a high-priority issue.
This commit also add two new instructions to the old VM: `updt` and
`updt_cidx`.
They allow us to test the new new memory reuse technique.
TODO: We need to reset fields after we project. Otherwise, we prevent
destructive updates on nested objects.
Now, the nodes in a `rbmap` contain the key and value, and we avoid
one level of indirection. `rbmap`s are more common than `rbtree`.
We implement `rbtree A` as `rbmap A unit`.
Remove transformations such as
```
prod.cases_on M (\fun a b, t)
```
==>
```
let a := M.0 in
let b := M.1 in
t
```
We will perform this kind of transformation in a later stage.
Motivation: explicit control flow graph
TODO: disabled `split_entries` for now.
I believe the new feature exposed a bug at `move_to_entries`.
I will fix this new issue in another commit.
