Useless
=======

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

== Description ==

This pass:

* builds some kind of dependence graph where
** a value of ground type is useful if it is an arg to a primitive
** a tuple is useful if it contains a useful component
** a constructor is useful if it contains a useful component or is used in a `Case` transfer

If a useful tuple is coerced to another useful tuple, then all of
their components must agree (exactly).  It is trivial to convert a
useful value to a useless one.

== Implementation ==

* <!ViewGitFile(mlton,master,mlton/ssa/useless.fun)>

== Details and Notes ==

It is also trivial to convert a useful tuple to one of its useful
components -- but this seems hard.

Suppose that you have a `ref`/`array`/`vector` that is useful, but the
components aren't -- then the components are converted to type `unit`,
and any primitive args must be as well.
