MLton 20210117

# RSSA

RSSA is an IntermediateLanguage, translated from SSA2 by ToRSSA, optimized by RSSASimplify, and translated by ToMachine to Machine.

## Description

RSSA is a IntermediateLanguage that makes representation decisions explicit.

## Implementation

- rssa.sig

- rssa.fun

## Type Checking

The new type language is aimed at expressing bit-level control over layout and associated packing of data representations. There are singleton types that denote constants, other atomic types for things like integers and reals, and arbitrary sum types and sequence (tuple) types. The big change to the type system is that type checking is now based on subtyping, not type equality. So, for example, the singleton type 0xFFFFEEBB whose only inhabitant is the eponymous constant is a subtype of the type Word32.

## Details and Notes

SSA is an abbreviation for Static Single Assignment. The RSSA IntermediateLanguage is a variant of SSA.