Package gnu.expr

Class FindTailCalls

  • All Implemented Interfaces:
    SourceLocator, SourceLocator, Locator
    public class FindTailCalls
extends ExpExpVisitor<Expression>
    A visitor that checks for tails-calls; also notes read/write/call accesses.

    The main goal of this pass is to figure of which functions can and should be inlined. We inline a function where possible to avoid closure allocation and related overheads, and also (more importantly) to enable tail-call-optiomization: A tail-call can be implemented cheaply and correctly using a goto bytecode instruction - but only for functions inlined in the same JVM method.

    We currently restrict inlining to cases when we can do so without code duplication: When the function is only called once, not counting tail-calls. Because passing a "return link" is difficult, we require that all calls to the function have the same "return continuation".

    The extra visitor parameter is the returnContinuation - the expression we "return to" - i.e. when done evaluating an expression, we're also done with the returnContinuation. Normally it is is same Expression as we are visiting, but (for example) when visiting the last expression of a BeginExp the returnContinuation is the same as that of the containing BeginExp. We're in a tail-context (in the sense of the Scheme reports) iff the current returnContinuation is the body of the current LambdaExp.

    For each non-inlined function f we define inlineSet(f) as the set of functions g such that g.inlinedIn(f). There are various requirements for a function g to be inlined; for example we require that it have a fixed number of arguments. Because of the no-duplication policy, all calls to g have to be known, and all calls have to be from other inlined functions: If h calls g, then h==f || h.inlinedIn(f). In addition all calls must have the same returnContinuation.

    This analysis is done in two parts: First the main expression-walker, and at the end (in visitModuleExp) we check each procedure using the data from the main pass.

    When this vistor is done, it has set the returnContinuation, tailCallers, and inlineHome fields of a LambdaExp. If function f tail-calls g, then f is added to the set of g's tailCallers.

    If there is a non-tail-call to g then we try to set g's returnContinuation to the current (context) returnContinuation, if the former hasn't been set yet; otherwise we set it to the special unknownContinuation value. We also construct tailCallers as the list of functions h that make a tail-call to g. At the end of this pass, if a function's returnContinuation is is non-null and not unknown then it has a unique continuation, and the function can potentially be inlined at the location of the continuation. However, that depends on if any tail-calls also have same return-continuation; that analysis happens later in checkInlineable.

    (In addition a validate method (executed during the earlier InlinedCalls pass may pre-initialized the returnContinuation inlineHome fields, but only for a LambdaExp that will be inlined during code generation in a custom compile method.)