LocalRef - MLton Standard ML Compiler (SML Compiler)

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

## Description

This pas 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.sig 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.

Last edited on 2006-11-02 17:46:41 by MatthewFluet.