= PolyEqual

<<PolyEqual#>> is an optimization pass for the <<SSA#>>
<<IntermediateLanguage#>>, invoked from <<SSASimplify#>>.

== Description

This pass implements polymorphic equality.

== Implementation

* https://github.com/MLton/mlton/blob/master/mlton/ssa/poly-equal.fun[`poly-equal.fun`]

== Details and Notes

For each datatype, tycon, and vector type, it builds and equality
function and translates calls to `MLton_equal` into calls to that
function.

Also generates calls to `Word_equal`.

For tuples, it does the equality test inline; i.e., it does not create
a separate equality function for each tuple type.

All equality functions are created only if necessary, i.e., if
equality is actually used at a type.

Optimizations:

* for datatypes that are enumerations, do not build a case dispatch,
just use `MLton_eq`, as the backend will represent these as ints

* deep equality always does an `MLton_eq` test first

* If one argument to `=` is a constant and the type will get
translated to an `IntOrPointer`, then just use `eq` instead of the
full equality.  This is important for implementing code like the
following efficiently:
+
----
if x = 0  ...    (* where x is of type IntInf.int *)
----

* Also convert pointer equality on scalar types to type specific
primitives.
