## MLton 20241230

# LocalRef

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

## Description

This pass optimizes `ref` cells local to a SSA function:

- global `ref`-s only used in one function are moved to the function

- `ref`-s only created, read from, and written to (i.e., don’t escape) are converted into function local variables

Uses Multi and Restore.

## Implementation

- `local-ref.fun`

## Details and Notes

Moving a global `ref` requires the Multi analysis, because a global `ref` can only be moved into a function that is executed at most once.

Conversion of non-escaping `ref`-s is structured in three phases:

- analysis — a variable `r = Ref_ref x` escapes if

  - `r` is used in any context besides `Ref_assign (r, _)` or `Ref_deref r`

  - all uses `r` reachable from a (direct or indirect) call to `Thread_copyCurrent` are of the same flavor (either `Ref_assign` or `Ref_deref`); this also requires the Multi analysis.

- transformation

  - rewrites `r = Ref_ref x` to `r = x`

  - rewrites `_ = Ref_assign (r, y)` to `r = y`

  - rewrites `z = Ref_deref r` to `z = r`

  Note that the resulting program violates the SSA condition.

- Restore— restore the SSA condition.