dotty.tools.dotc.transform.VCElideAllocations.scala Maven / Gradle / Ivy

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package dotty.tools.dotc
package transform

import ast.tpd
import core.*
import Contexts.*, Symbols.*, Types.*, Flags.*, Phases.*
import DenotTransformers.*, MegaPhase.*
import TreeExtractors.*, ValueClasses.*

/** This phase elides unnecessary value class allocations
 *
 *  For a value class V defined as:
 *    class V(val underlying: U) extends AnyVal
 *  we avoid unnecessary allocations:
 *     new V(u1) == new V(u2) => u1 == u2   provided V does not redefine `equals`
 *    (new V(u)).underlying() => u
 */
class VCElideAllocations extends MiniPhase with IdentityDenotTransformer {
  import tpd.*

  override def phaseName: String = VCElideAllocations.name

  override def description: String = VCElideAllocations.description

  override def runsAfter: Set[String] = Set(ElimErasedValueType.name)

  override def transformApply(tree: Apply)(using Context): Tree =
    def hasUserDefinedEquals(tp: Type): Boolean =
      val eql = atPhase(erasurePhase) {
        defn.Any_equals.matchingMember(tp.typeSymbol.thisType)
      }
      eql.owner != defn.AnyClass && !eql.is(Synthetic)

    tree match {
      // new V(u1) == new V(u2) => u1 == u2, unless V defines its own equals.
      // (We don't handle != because it has been eliminated by InterceptedMethods)
      case BinaryOp(NewWithArgs(tp1, List(u1)), op, NewWithArgs(tp2, List(u2)))
      if (tp1 eq tp2) && (op eq defn.Any_==)
         && isDerivedValueClass(tp1.typeSymbol)
         && !hasUserDefinedEquals(tp1) =>
        // == is overloaded in primitive classes
        u1.equal(u2)

      // (new V(u)).underlying() => u
      case ValueClassUnbox(NewWithArgs(_, List(u))) =>
        u

      case _ =>
        tree
    }
}

object VCElideAllocations:
  val name: String = "vcElideAllocations"
  val description: String = "peep-hole optimization to eliminate unnecessary value class allocations"