1 files changed, 41 insertions, 0 deletions
diff --git a/compiler/src/dotty/tools/dotc/transform/VCElideAllocations.scala b/compiler/src/dotty/tools/dotc/transform/VCElideAllocations.scala
new file mode 100644
index 000000000..1582158ac
--- /dev/null
+++ b/compiler/src/dotty/tools/dotc/transform/VCElideAllocations.scala
@@ -0,0 +1,41 @@
+package dotty.tools.dotc
+package transform
+
+import ast.{Trees, tpd}
+import core._, core.Decorators._
+import Contexts._, Trees._, StdNames._, Symbols._
+import DenotTransformers._, TreeTransforms._, Phases.Phase
+import ExtensionMethods._, 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
+ *    (new V(u)).underlying() => u
+ */
+class VCElideAllocations extends MiniPhaseTransform with IdentityDenotTransformer {
+  import tpd._
+
+  override def phaseName: String = "vcElideAllocations"
+
+  override def runsAfter: Set[Class[_ <: Phase]] = Set(classOf[ElimErasedValueType])
+
+  override def transformApply(tree: Apply)(implicit ctx: Context, info: TransformerInfo): Tree =
+    tree match {
+      // new V(u1) == new V(u2) => u1 == u2
+      // (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) =>
+        // == is overloaded in primitive classes
+        applyOverloaded(u1, nme.EQ, List(u2), Nil, defn.BooleanType)
+
+      // (new V(u)).underlying() => u
+      case ValueClassUnbox(NewWithArgs(_, List(u))) =>
+        u
+
+      case _ =>
+        tree
+    }
+}