RefFlatten
==========

<:RefFlatten:> is an optimization pass for the <:SSA2:>
<:IntermediateLanguage:>, invoked from <:SSA2Simplify:>.

== Description ==

This pass flattens a `ref` cell into its containing object.
The idea is to replace, where possible, a type like
----
(int ref * real)
----

with a type like
----
(int[m] * real)
----

where the `[m]` indicates a mutable field of a tuple.

== Implementation ==

* <!ViewGitFile(mlton,master,mlton/ssa/ref-flatten.fun)>

== Details and Notes ==

The savings is obvious, I hope.  We avoid an extra heap-allocated
object for the `ref`, which in the above case saves two words.  We
also save the time and code for the extra indirection at each get and
set.  There are lots of useful data structures (singly-linked and
doubly-linked lists, union-find, Fibonacci heaps, ...) that I believe
we are paying through the nose right now because of the absence of ref
flattening.

The idea is to compute for each occurrence of a `ref` type in the
program whether or not that `ref` can be represented as an offset of
some object (constructor or tuple).  As before, a unification-based
whole-program with deep abstract values makes sure the analysis is
consistent.

The only syntactic part of the analysis that remains is the part that
checks that for a variable bound to a value constructed by `Ref_ref`:

* the object allocation is in the same block.  This is pretty
draconian, and it would be nice to generalize it some day to allow
flattening as long as the `ref` allocation and object allocation "line
up one-to-one" in the same loop-free chunk of code.

* updates occur in the same block (and hence it is safe-for-space
because the containing object is still alive).  It would be nice to
relax this to allow updates as long as it can be provedthat the
container is live.

Prevent flattening of `unit ref`-s.

<:RefFlatten:> is safe for space.  The idea is to prevent a `ref`
being flattened into an object that has a component of unbounded size
(other than possibly the `ref` itself) unless we can prove that at
each point the `ref` is live, then the containing object is live too.
I used a pretty simple approximation to liveness.
