MLton 20210117

# PolyEqual

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

## Description

This pass implements polymorphic equality.

## Implementation

- 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.