Avoid creating most trees in GenBCode

GenBCode generates trees when it desugars Idents based on their type.
We now cache the generated tree, avoiding generating the same tree several times.

In the Dotty bootstrap, this reduced generated trees in backend from 606K (about 20%
of total) to 49K.

Author: odersky

compiler/src/dotty/tools/backend/jvm/DottyBackendInterface.scala

@@ -424,18 +424,26 @@ class DottyBackendInterface(outputDirectory: AbstractFile, val superCallsMap: Ma
   }.bits
 
   def isQualifierSafeToElide(qual: Tree): Boolean = tpd.isIdempotentExpr(qual)
+
+  private val desugared = new java.util.IdentityHashMap[Type, tpd.Select]
+
   def desugarIdent(i: Ident): Option[tpd.Select] = {
-    i.tpe match {
-      case TermRef(prefix: TermRef, name) =>
-        Some(tpd.ref(prefix).select(i.symbol))
-      case TermRef(prefix: ThisType, name) =>
-        Some(tpd.This(prefix.cls).select(i.symbol))
-      case TermRef(NoPrefix, name) =>
-        if (i.symbol is Flags.Method) Some(This(i.symbol.topLevelClass).select(i.symbol)) // workaround #342 todo: remove after fixed
-        else None
-      case _ => None
+    var found = desugared.get(i.tpe)
+    if (found == null) {
+      i.tpe match {
+        case TermRef(prefix: TermRef, name) =>
+          found = tpd.ref(prefix).select(i.symbol)
+        case TermRef(prefix: ThisType, name) =>
+          found = tpd.This(prefix.cls).select(i.symbol)
+        case TermRef(NoPrefix, name) =>
+          if (i.symbol is Flags.Method) found = This(i.symbol.topLevelClass).select(i.symbol) // workaround #342 todo: remove after fixed
+        case _ =>
+      }
+      if (found != null) desugared.put(i.tpe, found)
     }
+    if (found == null) None else Some(found)
   }
+
   def getLabelDefOwners(tree: Tree): Map[Tree, List[LabelDef]] = {
     // for each rhs of a defdef returns LabelDefs inside this DefDef
     val res = new collection.mutable.HashMap[Tree, List[LabelDef]]()
@@ -892,8 +900,6 @@ class DottyBackendInterface(outputDirectory: AbstractFile, val superCallsMap: Ma
     def parents: List[Type] = tp.parents
   }
 
-
-
   object Assign extends AssignDeconstructor {
     def _1: Tree = field.lhs
     def _2: Tree = field.rhs

compiler/src/dotty/tools/dotc/Run.scala

@@ -128,11 +128,11 @@ class Run(comp: Compiler, ictx: Context) {
           }
         }
         ctx.informTime(s"$phase ", start)
+        Stats.record(s"total trees at end of $phase", ast.Trees.ntrees)
+        for (unit <- units)
+          Stats.record(s"retained typed trees at end of $phase", unit.tpdTree.treeSize)
       }
     if (!ctx.reporter.hasErrors) Rewrites.writeBack()
-    for (unit <- units)
-      Stats.record("retained typed trees at end", unit.tpdTree.treeSize)
-    Stats.record("total trees at end", ast.Trees.ntrees)
   }
 
   private sealed trait PrintedTree