diff --git a/src/dotty/tools/dotc/core/Contexts.scala b/src/dotty/tools/dotc/core/Contexts.scala
index b0214a631e8b..ac27b74c77b2 100644
--- a/src/dotty/tools/dotc/core/Contexts.scala
+++ b/src/dotty/tools/dotc/core/Contexts.scala
@@ -181,6 +181,11 @@ object Contexts {
     protected def searchHistory_= (searchHistory: SearchHistory) = _searchHistory = searchHistory
     def searchHistory: SearchHistory = _searchHistory
 
+    /** Those fields are used to cache phases created in withPhase.
+      * phasedCtx is first phase with altered phase ever requested.
+      * phasedCtxs is array that uses phaseId's as indexes,
+      * contexts are created only on request and cached in this array
+      */
     private var phasedCtx: Context = _
     private var phasedCtxs: Array[Context] = _
 
diff --git a/src/dotty/tools/dotc/core/Phases.scala b/src/dotty/tools/dotc/core/Phases.scala
index 8b5606da245b..eb3b1d7fc061 100644
--- a/src/dotty/tools/dotc/core/Phases.scala
+++ b/src/dotty/tools/dotc/core/Phases.scala
@@ -67,6 +67,38 @@ object Phases {
       override def lastPhaseId(implicit ctx: Context) = id
     }
 
+    /** Squash TreeTransform's beloning to same sublist to a single TreeTransformer
+      * Each TreeTransform gets own period,
+      * whereas a combined TreeTransformer gets period equal to union of periods of it's TreeTransforms
+      * first TreeTransformer emitted is PostTyperTransformer that simplifies trees, see it's documentation
+      */
+    private def squashPhases(phasess: List[List[Phase]]): Array[Phase] = {
+      val squashedPhases = ListBuffer[Phase]()
+      var postTyperEmmited = false
+      var i = 0
+      while (i < phasess.length) {
+        if (phasess(i).length > 1) {
+          assert(phasess(i).forall(x => x.isInstanceOf[TreeTransform]), "Only tree transforms can be squashed")
+
+          val transforms = phasess(i).asInstanceOf[List[TreeTransform]]
+          val block =
+            if (!postTyperEmmited) {
+              postTyperEmmited = true
+              new PostTyperTransformer {
+                override def name: String = transformations.map(_.name).mkString("TreeTransform:{", ", ", "}")
+                override def transformations: Array[TreeTransform] = transforms.toArray
+              }
+            } else new TreeTransformer {
+              override def name: String = transformations.map(_.name).mkString("TreeTransform:{", ", ", "}")
+              override def transformations: Array[TreeTransform] = transforms.toArray
+            }
+          squashedPhases += block
+          block.init(this, phasess(i).head.id, phasess(i).last.id)
+        } else squashedPhases += phasess(i).head
+        i += 1
+      }
+      (NoPhase :: squashedPhases.toList ::: new TerminalPhase :: Nil).toArray
+    }
 
     /** Use the following phases in the order they are given.
      *  The list should never contain NoPhase.
@@ -94,31 +126,7 @@ object Phases {
       }
 
       if (squash) {
-        val squashedPhases = ListBuffer[Phase]()
-        var postTyperEmmited = false
-        var i = 0
-        while (i < phasess.length) {
-          if (phasess(i).length > 1) {
-            assert(phasess(i).forall(x => x.isInstanceOf[TreeTransform]), "Only tree transforms can be squashed")
-
-            val transforms = phasess(i).asInstanceOf[List[TreeTransform]]
-            val block =
-              if (!postTyperEmmited) {
-                postTyperEmmited = true
-                new PostTyperTransformer {
-                  override def name: String = transformations.map(_.name).mkString("TreeTransform:{", ", ", "}")
-                  override def transformations: Array[TreeTransform] = transforms.toArray
-                }
-              } else new TreeTransformer {
-                override def name: String = transformations.map(_.name).mkString("TreeTransform:{", ", ", "}")
-                override def transformations: Array[TreeTransform] = transforms.toArray
-              }
-            squashedPhases += block
-            block.init(this, phasess(i).head.id, phasess(i).last.id)
-          } else squashedPhases += phasess(i).head
-          i += 1
-        }
-        this.squashedPhases = (NoPhase::squashedPhases.toList :::new TerminalPhase :: Nil).toArray
+        this.squashedPhases = squashPhases(phasess)
       } else {
         this.squashedPhases = this.phases
       }
diff --git a/src/dotty/tools/dotc/core/transform/Erasure.scala b/src/dotty/tools/dotc/core/transform/Erasure.scala
index 842e2f81f7f4..9fb580ae8b95 100644
--- a/src/dotty/tools/dotc/core/transform/Erasure.scala
+++ b/src/dotty/tools/dotc/core/transform/Erasure.scala
@@ -17,7 +17,7 @@ object Erasure {
     (if (isConstructor) 4 else 0) +
     (if (wildcardOK) 8 else 0)
 
-  private var erasures = new Array[Erasure](16)
+  private val erasures = new Array[Erasure](16)
 
   for {
     isJava <- List(false, true)
diff --git a/src/dotty/tools/dotc/transform/TreeTransform.scala b/src/dotty/tools/dotc/transform/TreeTransform.scala
index 425410ae7ed6..0e182a99a8e5 100644
--- a/src/dotty/tools/dotc/transform/TreeTransform.scala
+++ b/src/dotty/tools/dotc/transform/TreeTransform.scala
@@ -23,8 +23,6 @@ object TreeTransforms {
     *
     * If all transforms in a group are NoTransforms, the tree is no longer traversed.
     *
-    * @param group The group this transform belongs to
-    * @param idx The index of this transform in its group
     *
     *            Performance analysis: Taking the dotty compiler frontend as a use case, we are aiming for a warm performance of
     *            about 4000 lines / sec. This means 6 seconds for a codebase of 24'000 lines. Of these the frontend consumes
@@ -346,6 +344,11 @@ object TreeTransforms {
       nxTransStats = indexUpdate(prev.nxTransStats, changedTansformation, transformationIndex, "transformStats", copy)
     }
 
+    /**
+     * Those arrays are used as "execution plan" in order to only execute non-tivial transformations\preparations
+     * for every integer i array(i) contains first non trivial transformation\preparation on particular tree subtype.
+     * If no nontrivial transformation are left stored value is greater than  transformers.size
+     */
     var nxPrepIdent: Array[Int] = _
     var nxPrepSelect: Array[Int] = _
     var nxPrepThis: Array[Int] = _
diff --git a/src/dotty/tools/dotc/util/SimpleMap.scala b/src/dotty/tools/dotc/util/SimpleMap.scala
index 84b9cb25814a..7bd263f0f35e 100644
--- a/src/dotty/tools/dotc/util/SimpleMap.scala
+++ b/src/dotty/tools/dotc/util/SimpleMap.scala
@@ -174,7 +174,7 @@ object SimpleMap {
           return {
             if (v eq bindings(i + 1)) this
             else {
-              val bindings1 = bindings.clone.asInstanceOf[Array[AnyRef]]
+              val bindings1 = bindings.clone
               bindings1(i + 1) = v
               new MapMore(bindings1)
             }
@@ -204,7 +204,7 @@ object SimpleMap {
         val v = value(i)
         val v1 = f(key(i), v)
         if ((v1 ne v) && (bindings1 eq bindings))
-          bindings1 = bindings.clone.asInstanceOf[Array[AnyRef]]
+          bindings1 = bindings.clone
         bindings1(i) = bindings(i)
         bindings1(i + 1) = v1
         i += 2