Rework LabelDefs. Fixes boxing of a unit label killing backend.

Though still it would be better if erasure didn't emmit such trees,
but still it's better to have this phase kore robust.

Also made LabelDefs run in linear time on the size of tree
 instead of square on the Block-depth of tree.

Author: DarkDimius

src/dotty/tools/backend/jvm/LabelDefs.scala

@@ -32,6 +32,7 @@ import java.lang.AssertionError
 import dotty.tools.dotc.util.Positions.Position
 import Decorators._
 import tpd._
+import Flags._
 import StdNames.nme
 
 /**
@@ -80,54 +81,68 @@ class LabelDefs extends MiniPhaseTransform {
 
   val queue = new ArrayBuffer[Tree]()
 
-
-
-  override def transformBlock(tree: tpd.Block)(implicit ctx: Context, info: TransformerInfo): tpd.Tree = {
-    collectLabelDefs.clear
-    val newStats = collectLabelDefs.transformStats(tree.stats)
-    val newExpr = collectLabelDefs.transform(tree.expr)
-    val labelCalls = collectLabelDefs.labelCalls
-    val entryPoints = collectLabelDefs.parentLabelCalls
-    val labelDefs = collectLabelDefs.labelDefs
-
-    // make sure that for every label there's a single location it should return and single entry point
-    // if theres already a location that it returns to that's a failure
-    val disallowed = new mutable.HashMap[Symbol, Tree]()
-    queue.sizeHint(labelCalls.size + entryPoints.size)
-    def moveLabels(entryPoint: Tree): List[Tree] = {
-      if((entryPoint.symbol is Flags.Label) && labelDefs.contains(entryPoint.symbol)) {
-        val visitedNow = new mutable.HashMap[Symbol, Tree]()
-        val treesToAppend = new ArrayBuffer[Tree]() // order matters. parents should go first
-        queue.clear()
-
-        var visited = 0
-        queue += entryPoint
-        while (visited < queue.size) {
-          val owningLabelDefSym = queue(visited).symbol
-          val owningLabelDef = labelDefs(owningLabelDefSym)
-          for (call <- labelCalls(owningLabelDefSym))
-            if (disallowed.contains(call.symbol)) {
-              val oldCall = disallowed(call.symbol)
-              ctx.error(s"Multiple return locations for Label $oldCall and $call", call.symbol.pos)
-            } else {
-              if ((!visitedNow.contains(call.symbol)) && labelDefs.contains(call.symbol)) {
-                val df = labelDefs(call.symbol)
-                visitedNow.put(call.symbol, labelDefs(call.symbol))
-                queue += call
+  override def transformDefDef(tree: tpd.DefDef)(implicit ctx: Context, info: TransformerInfo): tpd.Tree = {
+    if (tree.symbol is Flags.Label) tree
+    else {
+      collectLabelDefs.clear
+      val newRhs = collectLabelDefs.transform(tree.rhs)
+      val labelCalls = collectLabelDefs.labelCalls
+      var entryPoints = collectLabelDefs.parentLabelCalls
+      var labelDefs = collectLabelDefs.labelDefs
+
+      // make sure that for every label there's a single location it should return and single entry point
+      // if theres already a location that it returns to that's a failure
+      val disallowed = new mutable.HashMap[Symbol, Tree]()
+      queue.sizeHint(labelCalls.size + entryPoints.size)
+      def moveLabels(entryPoint: Tree): List[Tree] = {
+        if ((entryPoint.symbol is Flags.Label) && labelDefs.contains(entryPoint.symbol)) {
+          val visitedNow = new mutable.HashMap[Symbol, Tree]()
+          val treesToAppend = new ArrayBuffer[Tree]() // order matters. parents should go first
+          queue.clear()
+
+          var visited = 0
+          queue += entryPoint
+          while (visited < queue.size) {
+            val owningLabelDefSym = queue(visited).symbol
+            val owningLabelDef = labelDefs(owningLabelDefSym)
+            for (call <- labelCalls(owningLabelDefSym))
+              if (disallowed.contains(call.symbol)) {
+                val oldCall = disallowed(call.symbol)
+                ctx.error(s"Multiple return locations for Label $oldCall and $call", call.symbol.pos)
+              } else {
+                if ((!visitedNow.contains(call.symbol)) && labelDefs.contains(call.symbol)) {
+                  visitedNow.put(call.symbol, labelDefs(call.symbol))
+                  queue += call
+                }
               }
+            if (!treesToAppend.contains(owningLabelDef)) {
+              treesToAppend += owningLabelDef
             }
-          if(!treesToAppend.contains(owningLabelDef))
-            treesToAppend += owningLabelDef
-          visited += 1
+            visited += 1
+          }
+          disallowed ++= visitedNow
+
+          treesToAppend.toList
+        } else Nil
+      }
+
+      val putLabelDefsNearCallees = new TreeMap() {
+
+        override def transform(tree: tpd.Tree)(implicit ctx: Context): tpd.Tree = {
+          tree match {
+            case t: Apply if (entryPoints.contains(t)) =>
+              entryPoints = entryPoints - t
+              Block(moveLabels(t), t)
+            case _ => if (entryPoints.nonEmpty && labelDefs.nonEmpty) super.transform(tree) else tree
+          }
         }
-        disallowed ++= visitedNow
+      }
 
-        treesToAppend.toList
-      } else Nil
-    }
 
-    cpy.Block(tree)(entryPoints.flatMap(moveLabels).toList ++ newStats, newExpr)
+      val res = cpy.DefDef(tree)(rhs = putLabelDefsNearCallees.transform(newRhs))
 
+      res
+    }
   }
 
   val collectLabelDefs = new TreeMap() {
@@ -137,13 +152,12 @@ class LabelDefs extends MiniPhaseTransform {
     var isInsideLabel = false
     var isInsideBlock = false
 
-    def shouldMoveLabel = !isInsideBlock
+    def shouldMoveLabel = true
 
     // labelSymbol -> Defining tree
     val labelDefs = new mutable.HashMap[Symbol, Tree]()
     // owner -> all calls by this owner
     val labelCalls = new mutable.HashMap[Symbol, mutable.Set[Tree]]()
-    val labelCallCounts = new mutable.HashMap[Symbol, Int]()
 
     def clear = {
       parentLabelCalls.clear()
@@ -175,7 +189,6 @@ class LabelDefs extends MiniPhaseTransform {
         } else r
       case t: Apply if t.symbol is Flags.Label =>
         parentLabelCalls = parentLabelCalls + t
-        labelCallCounts.get(t.symbol)
         super.transform(tree)
       case _ =>
         super.transform(tree)