MLton 20210117

# Restore

Restore is a rewrite pass for the SSA and SSA2 IntermediateLanguages, invoked from KnownCase and LocalRef.

## Description

This pass restores the SSA condition for a violating SSA or SSA2 program; the program must satisfy:

Every path from the root to a use of a variable (excluding globals) passes through a def of that variable.

## Implementation

- restore.sig

- restore.fun

- restore2.sig

- restore2.fun

## Details and Notes

Based primarily on Section 19.1 of Modern Compiler Implementation in ML.

The main deviation is the calculation of liveness of the violating variables, which is used to predicate the insertion of phi arguments. This is due to the algorithm’s bias towards imperative languages, for which it makes the assumption that all variables are defined in the start block and all variables are "used" at exit.

This is "optimized" for restoration of functions with small numbers of violating variables — use bool vectors to represent sets of violating variables.

Also, we use a Promise.t to suspend part of the dominance frontier computation.