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Optimizations in tail calls #3448

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merged 3 commits into from Feb 2, 2014
Merged

Optimizations in tail calls #3448

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eb2627c
Don't try to eliminate tail calls in constructors.
retronym Nov 5, 2013
67f7df2
Optimize tailcall elimination
retronym Nov 5, 2013
c30cf0e
Optimization in TailCalls
retronym Nov 2, 2013
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27 changes: 23 additions & 4 deletions src/compiler/scala/tools/nsc/transform/TailCalls.scala
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Expand Up @@ -128,6 +128,7 @@ abstract class TailCalls extends Transform {
logResult(msg)(method.newValue(nme.THIS, pos, SYNTHETIC) setInfo currentClass.typeOfThis)
}
override def toString = s"${method.name} tparams=$tparams tailPos=$tailPos label=$label label info=${label.info}"

}

object EmptyTailContext extends TailContext {
Expand Down Expand Up @@ -185,6 +186,18 @@ abstract class TailCalls extends Transform {
private def noTailContext() = new ClonedTailContext(ctx, tailPos = false)
private def yesTailContext() = new ClonedTailContext(ctx, tailPos = true)


override def transformUnit(unit: CompilationUnit): Unit = {
try {
super.transformUnit(unit)
} finally {
// OPT clear these after each compilation unit
failPositions.clear()
failReasons.clear()
accessed.clear()
}
}

/** Rewrite this tree to contain no tail recursive calls */
def transform(tree: Tree, nctx: TailContext): Tree = {
val saved = ctx
Expand Down Expand Up @@ -218,12 +231,12 @@ abstract class TailCalls extends Transform {
*/
def fail(reason: String) = {
debuglog("Cannot rewrite recursive call at: " + fun.pos + " because: " + reason)
failReasons(ctx) = reason
if (ctx.isMandatory) failReasons(ctx) = reason
treeCopy.Apply(tree, noTailTransform(target), transformArgs)
}
/* Position of failure is that of the tree being considered. */
def failHere(reason: String) = {
failPositions(ctx) = fun.pos
if (ctx.isMandatory) failPositions(ctx) = fun.pos
fail(reason)
}
def rewriteTailCall(recv: Tree): Tree = {
Expand All @@ -237,14 +250,20 @@ abstract class TailCalls extends Transform {

if (!ctx.isEligible) fail("it is neither private nor final so can be overridden")
else if (!isRecursiveCall) {
if (receiverIsSuper) failHere("it contains a recursive call targeting a supertype")
if (ctx.isMandatory && receiverIsSuper) // OPT expensive check, avoid unless we will actually report the error
failHere("it contains a recursive call targeting a supertype")
else failHere(defaultReason)
}
else if (!matchesTypeArgs) failHere("it is called recursively with different type arguments")
else if (receiver == EmptyTree) rewriteTailCall(This(currentClass))
else if (!receiverIsSame) failHere("it changes type of 'this' on a polymorphic recursive call")
else rewriteTailCall(receiver)
}

def isEligible(tree: DefDef) = {
val sym = tree.symbol
!(sym.hasAccessorFlag || sym.isConstructor)
}

tree match {
case ValDef(_, _, _, _) =>
Expand All @@ -253,7 +272,7 @@ abstract class TailCalls extends Transform {

super.transform(tree)

case dd @ DefDef(_, name, _, vparamss0, _, rhs0) if !dd.symbol.hasAccessorFlag =>
case dd @ DefDef(_, name, _, vparamss0, _, rhs0) if isEligible(dd) =>
val newCtx = new DefDefTailContext(dd)
if (newCtx.isMandatory && !(newCtx containsRecursiveCall rhs0))
unit.error(tree.pos, "@tailrec annotated method contains no recursive calls")
Expand Down