MLton 20241230

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

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.