LocalRef
========

<:LocalRef:> is an optimization pass for the <:SSA:>
<:IntermediateLanguage:>, invoked from <:SSASimplify:>.

== Description ==

This pass optimizes `ref` cells local to a <:SSA:> function:

* global `ref`-s only used in one function are moved to the function

* `ref`-s only created, read from, and written to (i.e., don't escape)
are converted into function local variables

Uses <:Multi:> and <:Restore:>.

== Implementation ==

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

== Details and Notes ==

Moving a global `ref` requires the <:Multi:> analysis, because a
global `ref` can only be moved into a function that is executed at
most once.

Conversion of non-escaping `ref`-s is structured in three phases:

* analysis -- a variable `r = Ref_ref x` escapes if
** `r` is used in any context besides `Ref_assign (r, _)` or `Ref_deref r`
** all uses `r` reachable from a (direct or indirect) call to `Thread_copyCurrent` are of the same flavor (either `Ref_assign` or `Ref_deref`); this also requires the <:Multi:> analysis.

* transformation
+
--
** rewrites `r = Ref_ref x` to `r = x`
** rewrites `_ = Ref_assign (r, y)` to `r = y`
** rewrites `z = Ref_deref r` to `z = r`
--
+
Note that the resulting program violates the SSA condition.

* <:Restore:> -- restore the SSA condition.