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scala3-compiler-bootstrapped/dotty.tools/dotty.tools.dotc/dotty.tools.dotc.transform/TailRec

TailRec

dotty.tools.dotc.transform.TailRec
See theTailRec companion object
class TailRec extends MiniPhase

A Tail Rec Transformer.

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 includes being in tail-position of a return from a Labeled block which is itself in tail-position (which is critical for tail-recursive calls in the cases of a match). To identify tail positions, we recurse over the trees that may contain calls in tail-position (trees that can't contain such calls are not transformed).

When a method contains at least one tail-recursive call, its rhs is wrapped in the following structure:

var localForParam1: T1 = param1
...
while (<empty>) {
  tailResult[ResultType]: {
    return {
      // original rhs with tail recursive calls transformed (see below)
    }
  }
}

Self-recursive calls in tail-position are then replaced by (a) reassigning the local vars substituting formal parameters and (b) a return from the tailResult labeled block, which has the net effect of looping back to the beginning of the method. If the receiver is modifed in a recursive call, an additional var is used to replace this.

As a complete example of the transformation, the classical fact function, defined as:

def fact(n: Int, acc: Int): Int =
  if (n == 0) acc
  else fact(n - 1, acc * n)

is rewritten as:

def fact(n: Int, acc: Int): Int = {
  var acc$tailLocal1: Int = acc
  var n$tailLocal1: Int = n
  while (<empty>) {
    tailLabel1[Unit]: {
      return {
        if (n$tailLocal1 == 0)
          acc$tailLocal1
        else {
          val n$tailLocal1$tmp1: Int = n$tailLocal1 - 1
          val acc$tailLocal1$tmp1: Int = acc$tailLocal1 * n$tailLocal1
          n$tailLocal1 = n$tailLocal1$tmp1
          acc$tailLocal1 = acc$tailLocal1$tmp1
          (return[tailLabel1] ()): Int
        }
      }
    }
  }
}

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). Recursive calls on a different instance are optimized.

This phase has been moved after erasure to allow the use of vars for the parameters combined with a WhileDo. This is also beneficial to support polymorphic tail-recursive calls.

In scalac, if the method had type parameters, the call must contain the same parameters as type arguments. This is no longer the case in dotc thanks to being located after erasure. In scalac, this is named tailCall but it does only provide optimization for self recursive functions, that's why it's renamed to tailrec

Attributes

Authors:

Erik Stenman, Iulian Dragos, ported and heavily modified for dotty by Dmitry Petrashko moved after erasure and adapted to emit Labeled blocks by Sébastien Doeraene

Companion
object
Graph
Supertypes
class MiniPhase
class Phase
class Object
trait Matchable
class Any

Members list

Type members

Classlikes

class TailRecElimination(method: Symbol, enclosingClass: ClassSymbol, paramSyms: List[Symbol], isMandatory: Boolean) extends TreeMap

Attributes

Supertypes
class TreeMap
class Object
trait Matchable
class Any

Value members

Concrete methods

override def description: String

Attributes

Definition Classes
override def phaseName: String

A name given to the Phase that can be used to debug the compiler. For instance, it is possible to print trees after a given phase using:

A name given to the Phase that can be used to debug the compiler. For instance, it is possible to print trees after a given phase using:

$ ./bin/scalac -Xprint:<phaseNameHere> sourceFile.scala

Attributes

Definition Classes
override def runsAfter: Set[String]

List of names of phases that should precede this phase

List of names of phases that should precede this phase

Attributes

Definition Classes
override def transformDefDef(tree: DefDef)(using Context): Tree

Attributes

Definition Classes

Inherited methods

final def <=(that: Phase): Boolean

Attributes

Inherited from:
Phase
def allowsImplicitSearch: Boolean

If set, implicit search is enabled

If set, implicit search is enabled

Attributes

Inherited from:
Phase
final def cancellable(body: Context ?=> Unit)(using Context): Boolean

Do not run if compile progress has been cancelled

Do not run if compile progress has been cancelled

Attributes

Inherited from:
Phase
def changesBaseTypes: Boolean

Can this transform change the base types of a type?

Can this transform change the base types of a type?

Attributes

Inherited from:
Phase
def changesMembers: Boolean

Can this transform create or delete non-private members?

Can this transform create or delete non-private members?

Attributes

Inherited from:
Phase
def changesParents: Boolean

Can this transform change the parents of a class?

Can this transform change the parents of a class?

Attributes

Inherited from:
Phase
def checkPostCondition(tree: Tree)(using Context): Unit

Check what the phase achieves, to be called at any point after it is finished.

Check what the phase achieves, to be called at any point after it is finished.

Attributes

Inherited from:
Phase
def end: PhaseId

Attributes

Inherited from:
Phase
final def erasedTypes: Boolean

Attributes

Inherited from:
Phase
def exists: Boolean

True for all phases except NoPhase

True for all phases except NoPhase

Attributes

Inherited from:
Phase
final def flatClasses: Boolean

Attributes

Inherited from:
Phase
final def hasNext: Boolean

Attributes

Inherited from:
Phase
def id: Int

The sequence position of this phase in the given context where 0 is reserved for NoPhase and the first real phase is at position 1. -1 if the phase is not installed in the context.

The sequence position of this phase in the given context where 0 is reserved for NoPhase and the first real phase is at position 1. -1 if the phase is not installed in the context.

Attributes

Inherited from:
Phase
def initContext(ctx: FreshContext): Unit

Attributes

Inherited from:
Phase
def isCheckable: Boolean

Output should be checkable by TreeChecker

Output should be checkable by TreeChecker

Attributes

Inherited from:
Phase
def isEnabled(using Context): Boolean

This property is queried when phases are first assembled. If it is false, the phase will be dropped from the set of phases to traverse.

This property is queried when phases are first assembled. If it is false, the phase will be dropped from the set of phases to traverse.

Attributes

Inherited from:
Phase
def isRunnable(using Context): Boolean

This property is queried before a phase is run. If it is false, the phase is skipped.

This property is queried before a phase is run. If it is false, the phase is skipped.

Attributes

Inherited from:
Phase
def isTyper: Boolean

Is this phase the standard typerphase? True for TyperPhase, but not for other first phases (such as FromTasty or Parser). The predicate is tested in some places that perform checks and corrections. It's different from ctx.isAfterTyper (and cheaper to test).

Is this phase the standard typerphase? True for TyperPhase, but not for other first phases (such as FromTasty or Parser). The predicate is tested in some places that perform checks and corrections. It's different from ctx.isAfterTyper (and cheaper to test).

Attributes

Inherited from:
Phase
final def iterator: Iterator[Phase]

Attributes

Inherited from:
Phase
final def lambdaLifted: Boolean

Attributes

Inherited from:
Phase
final def megaPhase(using Context): Phase

Attributes

Inherited from:
Phase
final def monitor(doing: String)(body: Context ?=> Unit)(using Context): Boolean

Cancellable region, if not cancelled, run the body in the context of the current compilation unit. Enrich crash messages.

Cancellable region, if not cancelled, run the body in the context of the current compilation unit. Enrich crash messages.

Attributes

Inherited from:
Phase
final def next: Phase

Attributes

Inherited from:
Phase
final def patternTranslated: Boolean

Attributes

Inherited from:
Phase
def period: Period

Attributes

Inherited from:
Phase
def prepareForAlternative(tree: Alternative)(using Context): Context

Attributes

Inherited from:
MiniPhase
def prepareForApply(tree: Apply)(using Context): Context

Attributes

Inherited from:
MiniPhase
def prepareForAssign(tree: Assign)(using Context): Context

Attributes

Inherited from:
MiniPhase
def prepareForBind(tree: Bind)(using Context): Context

Attributes

Inherited from:
MiniPhase
def prepareForBlock(tree: Block)(using Context): Context

Attributes

Inherited from:
MiniPhase
def prepareForCaseDef(tree: CaseDef)(using Context): Context

Attributes

Inherited from:
MiniPhase
def prepareForClosure(tree: Closure)(using Context): Context

Attributes

Inherited from:
MiniPhase
def prepareForDefDef(tree: DefDef)(using Context): Context

Attributes

Inherited from:
MiniPhase
def prepareForIdent(tree: Ident)(using Context): Context

Attributes

Inherited from:
MiniPhase
def prepareForIf(tree: If)(using Context): Context

Attributes

Inherited from:
MiniPhase
def prepareForInlined(tree: Inlined)(using Context): Context

Attributes

Inherited from:
MiniPhase
def prepareForLabeled(tree: Labeled)(using Context): Context

Attributes

Inherited from:
MiniPhase
def prepareForLiteral(tree: Literal)(using Context): Context

Attributes

Inherited from:
MiniPhase
def prepareForMatch(tree: Match)(using Context): Context

Attributes

Inherited from:
MiniPhase
def prepareForNew(tree: New)(using Context): Context

Attributes

Inherited from:
MiniPhase
def prepareForOther(tree: Tree)(using Context): Context

Attributes

Inherited from:
MiniPhase
def prepareForPackageDef(tree: PackageDef)(using Context): Context

Attributes

Inherited from:
MiniPhase
def prepareForQuote(tree: Quote)(using Context): Context

Attributes

Inherited from:
MiniPhase
def prepareForReturn(tree: Return)(using Context): Context

Attributes

Inherited from:
MiniPhase
def prepareForSelect(tree: Select)(using Context): Context

Attributes

Inherited from:
MiniPhase
def prepareForSeqLiteral(tree: SeqLiteral)(using Context): Context

Attributes

Inherited from:
MiniPhase
def prepareForSplice(tree: Splice)(using Context): Context

Attributes

Inherited from:
MiniPhase
def prepareForStats(trees: List[Tree])(using Context): Context

Attributes

Inherited from:
MiniPhase
def prepareForSuper(tree: Super)(using Context): Context

Attributes

Inherited from:
MiniPhase
def prepareForTemplate(tree: Template)(using Context): Context

Attributes

Inherited from:
MiniPhase
def prepareForThis(tree: This)(using Context): Context

Attributes

Inherited from:
MiniPhase
def prepareForTry(tree: Try)(using Context): Context

Attributes

Inherited from:
MiniPhase
def prepareForTypeApply(tree: TypeApply)(using Context): Context

Attributes

Inherited from:
MiniPhase
def prepareForTypeDef(tree: TypeDef)(using Context): Context

Attributes

Inherited from:
MiniPhase
def prepareForTypeTree(tree: TypeTree)(using Context): Context

Attributes

Inherited from:
MiniPhase
def prepareForTyped(tree: Typed)(using Context): Context

Attributes

Inherited from:
MiniPhase
def prepareForUnApply(tree: UnApply)(using Context): Context

Attributes

Inherited from:
MiniPhase
def prepareForUnit(tree: Tree)(using Context): Context

Attributes

Inherited from:
MiniPhase
def prepareForValDef(tree: ValDef)(using Context): Context

Attributes

Inherited from:
MiniPhase
def prepareForWhileDo(tree: WhileDo)(using Context): Context

Attributes

Inherited from:
MiniPhase
final def prev: Phase

Attributes

Inherited from:
Phase
def printingContext(ctx: Context): Context

A hook that allows to transform the usual context passed to the function that prints a compilation unit after a phase

A hook that allows to transform the usual context passed to the function that prints a compilation unit after a phase

Attributes

Inherited from:
Phase
final def refChecked: Boolean

Attributes

Inherited from:
Phase
final override def relaxedTyping: Boolean

If set, allow missing or superfluous arguments in applications and type applications.

If set, allow missing or superfluous arguments in applications and type applications.

Attributes

Definition Classes
Inherited from:
MiniPhase
def relaxedTypingInGroup: Boolean

If set, use relaxed typing for all phases in group

If set, use relaxed typing for all phases in group

Attributes

Inherited from:
MiniPhase
override def run(using Context): Unit

Attributes

Definition Classes
Inherited from:
MiniPhase
def runOn(units: List[CompilationUnit])(using runCtx: Context): List[CompilationUnit]

Attributes

Inherited from:
Phase
inline def runSubPhase[T](id: SubPhase)(inline body: (SubPhase, Context) ?=> T)(using Context): T

Attributes

Inherited from:
Phase
def runsAfterGroupsOf: Set[String]

List of names of phases that should have finished their processing of all compilation units before this phase starts

List of names of phases that should have finished their processing of all compilation units before this phase starts

Attributes

Inherited from:
MiniPhase
final def sameBaseTypesStartId: Int

Attributes

Inherited from:
Phase
final def sameMembersStartId: Int

Attributes

Inherited from:
Phase
final def sameParentsStartId: Int

Attributes

Inherited from:
Phase
def show(tree: Tree)(using Context): String

Convert a compilation unit's tree to a string; can be overridden

Convert a compilation unit's tree to a string; can be overridden

Attributes

Inherited from:
Phase
protected def singletonGroup: MegaPhase

Attributes

Inherited from:
MiniPhase
def skipIfJava(using Context): Boolean

skip the phase for a Java compilation unit, may depend on -Yjava-tasty

skip the phase for a Java compilation unit, may depend on -Yjava-tasty

Attributes

Inherited from:
Phase
def start: Int

Attributes

Inherited from:
Phase
def subPhases: List[SubPhase]

for purposes of progress tracking, overridden in TyperPhase

for purposes of progress tracking, overridden in TyperPhase

Attributes

Inherited from:
Phase
override def toString: String

Returns a string representation of the object.

Returns a string representation of the object.

The default representation is platform dependent.

Attributes

Returns

a string representation of the object.

Definition Classes
Phase -> Any
Inherited from:
Phase
def transformAllDeep(tree: Tree)(using Context): Tree

Transform tree using all transforms of current group (including this one)

Transform tree using all transforms of current group (including this one)

Attributes

Inherited from:
MiniPhase
def transformAlternative(tree: Alternative)(using Context): Tree

Attributes

Inherited from:
MiniPhase
def transformApply(tree: Apply)(using Context): Tree

Attributes

Inherited from:
MiniPhase
def transformAssign(tree: Assign)(using Context): Tree

Attributes

Inherited from:
MiniPhase
def transformBind(tree: Bind)(using Context): Tree

Attributes

Inherited from:
MiniPhase
def transformBlock(tree: Block)(using Context): Tree

Attributes

Inherited from:
MiniPhase
def transformCaseDef(tree: CaseDef)(using Context): Tree

Attributes

Inherited from:
MiniPhase
def transformClosure(tree: Closure)(using Context): Tree

Attributes

Inherited from:
MiniPhase
def transformFollowing(tree: Tree)(using Context): Tree

Transform single node using all transforms following the current one in this group

Transform single node using all transforms following the current one in this group

Attributes

Inherited from:
MiniPhase
def transformFollowingDeep(tree: Tree)(using Context): Tree

Transform tree using all transforms following the current one in this group

Transform tree using all transforms following the current one in this group

Attributes

Inherited from:
MiniPhase
def transformIdent(tree: Ident)(using Context): Tree

Attributes

Inherited from:
MiniPhase
def transformIf(tree: If)(using Context): Tree

Attributes

Inherited from:
MiniPhase
def transformInlined(tree: Inlined)(using Context): Tree

Attributes

Inherited from:
MiniPhase
def transformLabeled(tree: Labeled)(using Context): Tree

Attributes

Inherited from:
MiniPhase
def transformLiteral(tree: Literal)(using Context): Tree

Attributes

Inherited from:
MiniPhase
def transformMatch(tree: Match)(using Context): Tree

Attributes

Inherited from:
MiniPhase
def transformNew(tree: New)(using Context): Tree

Attributes

Inherited from:
MiniPhase
def transformOther(tree: Tree)(using Context): Tree

Attributes

Inherited from:
MiniPhase
def transformPackageDef(tree: PackageDef)(using Context): Tree

Attributes

Inherited from:
MiniPhase
def transformQuote(tree: Quote)(using Context): Tree

Attributes

Inherited from:
MiniPhase
def transformReturn(tree: Return)(using Context): Tree

Attributes

Inherited from:
MiniPhase
def transformSelect(tree: Select)(using Context): Tree

Attributes

Inherited from:
MiniPhase
def transformSeqLiteral(tree: SeqLiteral)(using Context): Tree

Attributes

Inherited from:
MiniPhase
def transformSplice(tree: Splice)(using Context): Tree

Attributes

Inherited from:
MiniPhase
def transformStats(trees: List[Tree])(using Context): List[Tree]

Attributes

Inherited from:
MiniPhase
def transformSuper(tree: Super)(using Context): Tree

Attributes

Inherited from:
MiniPhase
def transformTemplate(tree: Template)(using Context): Tree

Attributes

Inherited from:
MiniPhase
def transformThis(tree: This)(using Context): Tree

Attributes

Inherited from:
MiniPhase
def transformTry(tree: Try)(using Context): Tree

Attributes

Inherited from:
MiniPhase
def transformTypeApply(tree: TypeApply)(using Context): Tree

Attributes

Inherited from:
MiniPhase
def transformTypeDef(tree: TypeDef)(using Context): Tree

Attributes

Inherited from:
MiniPhase
def transformTypeTree(tree: TypeTree)(using Context): Tree

Attributes

Inherited from:
MiniPhase
def transformTyped(tree: Typed)(using Context): Tree

Attributes

Inherited from:
MiniPhase
def transformUnApply(tree: UnApply)(using Context): Tree

Attributes

Inherited from:
MiniPhase
def transformUnit(tree: Tree)(using Context): Tree

Attributes

Inherited from:
MiniPhase
def transformValDef(tree: ValDef)(using Context): Tree

Attributes

Inherited from:
MiniPhase
def transformWhileDo(tree: WhileDo)(using Context): Tree

Attributes

Inherited from:
MiniPhase
final def traversals: Int

Attributes

Inherited from:
Phase

Inherited fields

val cpy: TypedTreeCopier

Attributes

Inherited from:
MiniPhase
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Copyright (c) 2002-2023, LAMP/EPFL
Copyright (c) 2002-2023, LAMP/EPFL