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

Description

This pas optimizes ref cells local to a SSA function:

• global refs only used in one function are moved to the function

• refs 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 refs 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.