## MLton 20241230

# 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

- `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.