## MLton 20241230

# 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.