scala.tools.nsc.transform.TailCalls.scala Maven / Gradle / Ivy
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/*
* Scala (https://www.scala-lang.org)
*
* Copyright EPFL and Lightbend, Inc.
*
* Licensed under Apache License 2.0
* (http://www.apache.org/licenses/LICENSE-2.0).
*
* See the NOTICE file distributed with this work for
* additional information regarding copyright ownership.
*/
package scala
package tools.nsc
package transform
import symtab.Flags
import Flags.SYNTHETIC
import scala.annotation._
/** Perform tail recursive call elimination.
*
* @author Iulian Dragos
*/
abstract class TailCalls extends Transform {
import global._ // the global environment
import definitions._ // standard classes and methods
import typer.typedPos // methods to type trees
val phaseName: String = "tailcalls"
override def enabled = settings.debuginfo.value != "notailcalls"
def newTransformer(unit: CompilationUnit): AstTransformer =
new TailCallElimination(unit)
import treeInfo.hasSynthCaseSymbol
/**
* A Tail Call Transformer
*
* @author Erik Stenman, Iulian Dragos
*
* What it does:
*
* Finds method calls in tail-position and replaces them with jumps.
* A call is in a tail-position if it is the last instruction to be
* executed in the body of a method. This is done by recursing over
* the trees that may contain calls in tail-position (trees that can't
* contain such calls are not transformed). However, they are not that
* many.
*
*
* Self-recursive calls in tail-position are replaced by jumps to a
* label at the beginning of the method. As the JVM provides no way to
* jump from a method to another one, non-recursive calls in
* tail-position are not optimized.
*
*
* A method call is self-recursive if it calls the current method and
* the method is final (otherwise, it could
* be a call to an overridden method in a subclass). Furthermore, if
* the method has `@specialized` annotated type parameters, the recursive
* call must contain these parameters as type arguments.
* Recursive calls on a different instance are optimized.
* Since 'this' is not a local variable, a dummy local val
* is added and used as a label parameter. The backend knows to load
* the corresponding argument in the 'this' (local at index 0). This dummy local
* is never used and should be cleaned up by dead code elimination (when enabled).
*
*
* This phase has been moved before pattern matching to catch more
* of the common cases of tail recursive functions. This means that
* more cases should be taken into account (like nested function, and
* pattern cases).
*
*
* If a method contains self-recursive calls, a label is added to at
* the beginning of its body and the calls are replaced by jumps to
* that label.
*
*
* Assumes: `Uncurry` has been run already, and no multiple
* parameter lists exist.
*
*/
class TailCallElimination(@unused unit: CompilationUnit) extends AstTransformer {
private def defaultReason = "it contains a recursive call not in tail position"
private val failPositions = perRunCaches.newMap[TailContext, Position]() withDefault (_.methodPos)
private val failReasons = perRunCaches.newMap[TailContext, String]() withDefaultValue defaultReason
private def tailrecFailure(ctx: TailContext): Unit = {
val method = ctx.method
val failReason = failReasons(ctx)
val failPos = failPositions(ctx)
reporter.error(failPos, s"could not optimize @tailrec annotated $method: $failReason")
}
/** Has the label been accessed? Then its symbol is in this set. */
private val accessed = perRunCaches.newSet[Symbol]()
// `accessed` was stored as boolean in the current context -- this is no longer tenable
// with jumps to labels in tailpositions now considered in tailposition,
// a downstream context may access the label, and the upstream one will be none the wiser
// this is necessary because tail-calls may occur in places where syntactically they seem impossible
// (since we now consider jumps to labels that are in tailposition, such as matchEnd(x) {x})
sealed trait TailContext {
def method: Symbol // current method
def tparams: List[Symbol] // type parameters
def methodPos: Position // default position for failure reporting
def tailPos: Boolean // context is in tail position
def label: Symbol // new label, tail call target
def tailLabels: Set[Symbol]
def enclosingType = method.enclClass.typeOfThis
def isEligible = method.isEffectivelyFinalOrNotOverridden
def isMandatory = method.hasAnnotation(TailrecClass)
def isTransformed = isEligible && accessed(label)
def newThis(pos: Position) = {
def msg = "Creating new `this` during tailcalls\n method: %s\n current class: %s".format(
method.ownerChain.mkString(" -> "),
currentClass.ownerChain.mkString(" -> ")
)
logResult(msg)(method.newValue(nme.THIS, pos, SYNTHETIC) setInfo currentClass.typeOfThis)
}
override def toString = s"${method.name} tparams=$tparams tailPos=$tailPos label=$label label info=${label.info}"
final def noTailContext() = clonedTailContext(tailPos = false)
final def yesTailContext() = clonedTailContext(tailPos = true)
@tailrec
protected final def clonedTailContext(tailPos: Boolean): TailContext = this match {
case _ if this.tailPos == tailPos => this
case clone: ClonedTailContext => clone.that.clonedTailContext(tailPos)
case _ => new ClonedTailContext(this, tailPos)
}
}
object EmptyTailContext extends TailContext {
def method = NoSymbol
def tparams = Nil
def methodPos = NoPosition
def tailPos = false
def label = NoSymbol
def tailLabels = Set.empty[Symbol]
}
class DefDefTailContext(dd: DefDef) extends TailContext {
def method = dd.symbol
def tparams = dd.tparams map (_.symbol)
def methodPos = dd.pos
def tailPos = true
lazy val label = mkLabel()
lazy val tailLabels = {
// labels are local to a method, so only traverse the rhs of a defdef
val collector = new TailPosLabelsTraverser
collector traverse dd.rhs
collector.tailLabels.toSet
}
private def mkLabel() = {
val label = method.newLabel(newTermName("_" + method.name), method.pos)
val thisParam = method.newSyntheticValueParam(currentClass.typeOfThis)
label setInfo MethodType(thisParam :: method.tpe.params, method.tpe_*.finalResultType)
if (isEligible)
label.substInfo(method.tpe.typeParams, tparams)
label
}
private def isRecursiveCall(t: Tree) = {
val receiver = t.symbol
( (receiver != null)
&& receiver.isMethod
&& (method.name == receiver.name)
&& (method.enclClass isSubClass receiver.enclClass)
)
}
def containsRecursiveCall(t: Tree) = t exists isRecursiveCall
}
class ClonedTailContext(val that: TailContext, override val tailPos: Boolean) extends TailContext {
def method = that.method
def tparams = that.tparams
def methodPos = that.methodPos
def tailLabels = that.tailLabels
def label = that.label
}
private var ctx: TailContext = EmptyTailContext
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
ctx = nctx
try transform(tree)
finally this.ctx = saved
}
def yesTailTransform(tree: Tree): Tree = transform(tree, ctx.yesTailContext())
def noTailTransform(tree: Tree): Tree = transform(tree, ctx.noTailContext())
def noTailTransforms(trees: List[Tree]) = {
val nctx = ctx.noTailContext()
trees mapConserve (t => transform(t, nctx))
}
override def transform(tree: Tree): Tree = {
/* A possibly polymorphic apply to be considered for tail call transformation. */
def rewriteApply(target: Tree, fun: Tree, targs: List[Tree], args: List[Tree], mustTransformArgs: Boolean = true) = {
val receiver: Tree = fun match {
case Select(qual, _) => qual
case _ => EmptyTree
}
def receiverIsSame = ctx.enclosingType.widen =:= receiver.tpe.widen
def receiverIsSuper = ctx.enclosingType.widen <:< receiver.tpe.widen
def isRecursiveCall = (ctx.method eq fun.symbol) && ctx.tailPos
def transformArgs = if (mustTransformArgs) noTailTransforms(args) else args
def matchesTypeArgs = (ctx.tparams corresponds targs)((p, a) => !isSpecialized(p) || p == a.tpe.typeSymbol)
def isSpecialized(tparam: Symbol) =
tparam.hasAnnotation(SpecializedClass)
/* Records failure reason in Context for reporting.
* Position is unchanged (by default, the method definition.)
*/
def fail(reason: String) = {
debuglog("Cannot rewrite recursive call at: " + fun.pos + " because: " + 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) = {
if (ctx.isMandatory) failPositions(ctx) = fun.pos
fail(reason)
}
def rewriteTailCall(recv: Tree): Tree = {
debuglog("Rewriting tail recursive call: " + fun.pos.lineContent.trim)
accessed += ctx.label
typedPos(fun.pos) {
val args = mapWithIndex(transformArgs)((arg, i) => mkAttributedCastHack(arg, ctx.label.info.params(i + 1).tpe))
Apply(Ident(ctx.label), noTailTransform(recv) :: args)
}
}
if (!ctx.isEligible) fail("it is neither private nor final so can be overridden")
else if (!isRecursiveCall) {
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 specialized 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)
}
// intentionally shadowing imports from definitions for performance
val runDefinitions = currentRun.runDefinitions
import runDefinitions.{Boolean_or, Boolean_and}
tree match {
case _: DefDef if tree.symbol.isLazy && tree.symbol.hasAnnotation(TailrecClass) =>
reporter.error(tree.pos, "lazy vals are not tailcall transformed")
tree.transform(this)
case dd @ DefDef(_, name, _, vparamss0, _, rhs0) if isEligible(dd) =>
val newCtx = new DefDefTailContext(dd)
if (newCtx.isMandatory && !(newCtx containsRecursiveCall rhs0))
reporter.error(tree.pos, "@tailrec annotated method contains no recursive calls")
debuglog(s"Considering $name for tailcalls, with labels in tailpos: ${newCtx.tailLabels}")
val newRHS = transform(rhs0, newCtx)
deriveDefDef(tree) { _ =>
if (newCtx.isTransformed) {
/* We have rewritten the tree, but there may be nested recursive calls remaining.
* If @tailrec is given we need to fail those now.
*/
if (newCtx.isMandatory) {
for (t @ Apply(fn, _) <- newRHS ; if fn.symbol == newCtx.method) {
failPositions(newCtx) = t.pos
tailrecFailure(newCtx)
}
}
val newThis = newCtx.newThis(tree.pos)
val vpSyms = vparamss0.flatten map (_.symbol)
typedPos(tree.pos)(Block(
List(ValDef(newThis, This(currentClass))),
LabelDef(newCtx.label, newThis :: vpSyms, mkAttributedCastHack(newRHS, newCtx.label.tpe.resultType))
))
}
else {
if (newCtx.isMandatory && (newCtx containsRecursiveCall newRHS))
tailrecFailure(newCtx)
newRHS
}
}
// a translated match
case Block(stats, expr) if stats forall hasSynthCaseSymbol =>
// the assumption is once we encounter a case, the remainder of the block will consist of cases
// the prologue may be empty, usually it is the valdef that stores the scrut
val (prologue, cases) = stats span (s => !s.isInstanceOf[LabelDef])
val transformedPrologue = noTailTransforms(prologue)
val transformedCases = transformTrees(cases)
val transformedStats =
if ((prologue eq transformedPrologue) && (cases eq transformedCases)) stats // allow reuse of `tree` if the subtransform was an identity
else transformedPrologue ++ transformedCases
treeCopy.Block(tree,
transformedStats,
transform(expr)
)
// a translated casedef
case LabelDef(_, _, _) if hasSynthCaseSymbol(tree) =>
deriveLabelDef(tree)(transform)
case Block(stats, expr) =>
treeCopy.Block(tree,
noTailTransforms(stats),
transform(expr)
)
case CaseDef(pat, guard, body) =>
// CaseDefs are already translated and guards were moved into the body.
// If this was not the case, guards would have to be transformed here as well.
assert(guard.isEmpty, "empty guard")
deriveCaseDef(tree)(transform)
case If(cond, thenp, elsep) =>
treeCopy.If(tree,
noTailTransform(cond),
transform(thenp),
transform(elsep)
)
case Match(selector, cases) =>
treeCopy.Match(tree,
noTailTransform(selector),
transformTrees(cases).asInstanceOf[List[CaseDef]]
)
case Try(block, catches, finalizer @ EmptyTree) =>
// scala/bug#1672 Catches are in tail position when there is no finalizer
treeCopy.Try(tree,
noTailTransform(block),
transformTrees(catches).asInstanceOf[List[CaseDef]],
EmptyTree
)
case Try(block, catches, finalizer) =>
// no calls inside a try are in tail position if there is a finalizer, but keep recursing for nested functions
treeCopy.Try(tree,
noTailTransform(block),
noTailTransforms(catches).asInstanceOf[List[CaseDef]],
noTailTransform(finalizer)
)
case Apply(tapply @ TypeApply(fun, targs), vargs) =>
rewriteApply(tapply, fun, targs, vargs)
case Apply(fun, args) if fun.symbol == Boolean_or || fun.symbol == Boolean_and =>
treeCopy.Apply(tree, noTailTransform(fun), transformTrees(args))
// this is to detect tailcalls in translated matches
// it's a one-argument call to a label that is in a tailposition and that looks like label(x) {x}
// thus, the argument to the call is in tailposition
case Apply(fun, args @ (arg :: Nil)) if fun.symbol.isLabel && ctx.tailLabels(fun.symbol) =>
debuglog(s"in tailpos label: $arg")
val res = yesTailTransform(arg)
// we tail-called -- TODO: shield from false-positives where we rewrite but don't tail-call
// must leave the jump to the original tailpos-label (fun)!
// there might be *a* tailcall *in* res, but it doesn't mean res *always* tailcalls
if (res ne arg)
treeCopy.Apply(tree, fun, res :: Nil)
else
rewriteApply(fun, fun, Nil, args, mustTransformArgs = false)
case Apply(fun, args) =>
rewriteApply(fun, fun, Nil, args)
case Alternative(_) | Star(_) | Bind(_, _) =>
assert(false, "We should've never gotten inside a pattern")
tree
case Select(qual, name) =>
treeCopy.Select(tree, noTailTransform(qual), name)
case EmptyTree | Super(_, _) | This(_) | Ident(_) | Literal(_) | Function(_, _) | TypeTree() =>
tree
case _ =>
tree.transform(this)
}
}
// Workaround for scala/bug#6900. Uncurry installs an InfoTransformer and a tree Transformer.
// These leave us with conflicting view on method signatures; the parameter symbols in
// the MethodType can be clones of the ones originally found on the parameter ValDef, and
// consequently appearing in the typechecked RHS of the method.
private def mkAttributedCastHack(tree: Tree, tpe: Type) =
gen.mkAttributedCast(tree, tpe)
}
// collect the LabelDefs (generated by the pattern matcher) in a DefDef that are in tail position
// the labels all look like: matchEnd(x) {x}
// then, in a forward jump `matchEnd(expr)`, `expr` is considered in tail position (and the matchEnd jump is replaced by the jump generated by expr)
class TailPosLabelsTraverser extends Traverser {
val tailLabels = new scala.collection.mutable.HashSet[Symbol]()
private var maybeTail: Boolean = true // since we start in the rhs of a DefDef
def traverse(tree: Tree, maybeTailNew: Boolean): Unit = {
val saved = maybeTail
maybeTail = maybeTailNew
try traverse(tree)
finally maybeTail = saved
}
def traverseNoTail(tree: Tree) = traverse(tree, maybeTailNew = false)
def traverseTreesNoTail(trees: List[Tree]) = trees foreach traverseNoTail
// intentionally shadowing imports from definitions for performance
private val runDefinitions = currentRun.runDefinitions
import runDefinitions.{Boolean_or, Boolean_and}
override def traverse(tree: Tree) = tree match {
// we're looking for label(x){x} in tail position, since that means `a` is in tail position in a call `label(a)`
case LabelDef(_, List(arg), body@Ident(_)) if arg.symbol == body.symbol =>
if (maybeTail) tailLabels += tree.symbol
// jumps to matchEnd are transparent; need this case for nested matches
// (and the translated match case below does things in reverse for this case's sake)
case Apply(fun, arg :: Nil) if hasSynthCaseSymbol(fun) && tailLabels(fun.symbol) =>
traverse(arg)
case Apply(fun, args) if (fun.symbol == Boolean_or || fun.symbol == Boolean_and) =>
traverseTrees(args)
// a translated casedef
case LabelDef(_, _, body) if hasSynthCaseSymbol(tree) =>
traverse(body)
// a translated match
case Block(stats, expr) if stats forall hasSynthCaseSymbol =>
// the assumption is once we encounter a case, the remainder of the block will consist of cases
// the prologue may be empty, usually it is the valdef that stores the scrut
val (prologue, cases) = stats span (s => !s.isInstanceOf[LabelDef])
traverse(expr)
traverseTrees(cases.reverse) // reverse so that we enter the matchEnd LabelDef before we see jumps to it
traverseTreesNoTail(prologue) // selector (may be absent)
case CaseDef(pat, guard, body) =>
traverse(body)
case Match(selector, cases) =>
traverseNoTail(selector)
traverseTrees(cases)
case DefDef(_, _, _, _, _, _) => // we are run per-method
case Block(stats, expr) =>
traverseTreesNoTail(stats)
traverse(expr)
case If(cond, thenp, elsep) =>
traverse(thenp)
traverse(elsep)
case Try(block, catches, finalizer) =>
traverseNoTail(block)
traverseTreesNoTail(catches)
traverseNoTail(finalizer)
case Apply(_, _) | EmptyTree | Super(_, _) | This(_) | Select(_, _) | Ident(_) | Literal(_) | Function(_, _) | TypeTree() =>
case _ => super.traverse(tree)
}
}
}
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