GenBCode: Eliminate redundant labels and line number nodes

Cleanup optimizations
 - remove line number nodes that describe no executable instructions
 - squash sequences of label nodes, remove unreferenced labels

Command-line flags that allow enabling these transformations are added
in a later comimt.

1 files changed, 101 insertions, 0 deletions

diff --git a/test/junit/scala/tools/nsc/backend/jvm/opt/EmptyLabelsAndLineNumbersTest.scala b/test/junit/scala/tools/nsc/backend/jvm/opt/EmptyLabelsAndLineNumbersTest.scala
new file mode 100644
index 0000000000..213af4bcc1
--- /dev/null
+++ b/test/junit/scala/tools/nsc/backend/jvm/opt/EmptyLabelsAndLineNumbersTest.scala
@@ -0,0 +1,101 @@
+package scala.tools.nsc
+package backend.jvm
+package opt
+
+import org.junit.runner.RunWith
+import org.junit.runners.JUnit4
+import org.junit.Test
+import scala.tools.asm.Opcodes._
+import org.junit.Assert._
+import scala.tools.testing.AssertUtil._
+
+import CodeGenTools._
+import scala.tools.partest.ASMConverters
+import ASMConverters._
+
+@RunWith(classOf[JUnit4])
+class EmptyLabelsAndLineNumbersTest {
+  import UnreachableCodeTest._ // ".dead" extension method on instructions
+
+  @Test
+  def removeEmptyLineNumbers(): Unit = {
+    val ops = List[(Instruction, Boolean)](
+      Label(1),
+      LineNumber(1, Label(1)),
+      Label(2),
+      Label(3),
+      Op(RETURN),
+
+      Label(4),
+      LineNumber(4, Label(4)).dead,
+      LineNumber(5, Label(4)),
+      Op(RETURN),
+
+      Label(5),
+      LineNumber(6, Label(5)).dead,
+      Label(6),
+      Label(7),
+      LineNumber(7, Label(7)),
+      Op(RETURN),
+
+      Label(9),
+      LineNumber(8, Label(9)).dead,
+      Label(10)
+    )
+
+    val method = genMethod()(ops.map(_._1): _*)
+    assertTrue(LocalOpt.removeEmptyLineNumbers(method))
+    assertSameCode(instructionsFromMethod(method), ops.filter(_._2).map(_._1))
+  }
+
+  @Test
+  def badlyLocatedLineNumbers(): Unit = {
+    def t(ops: Instruction*) =
+      assertThrows[AssertionError](LocalOpt.removeEmptyLineNumbers(genMethod()(ops: _*)))
+
+    // line numbers have to be right after their referenced label node
+    t(LineNumber(0, Label(1)), Label(1))
+    t(Label(0), Label(1), LineNumber(0, Label(0)))
+  }
+
+  @Test
+  def removeEmptyLabels(): Unit = {
+    val handler = List(ExceptionHandler(Label(4), Label(5), Label(6), Some("java/lang/Throwable")))
+    def ops(target1: Int, target2: Int, target3: Int, target4: Int, target5: Int, target6: Int) = List[(Instruction, Boolean)](
+      Label(1),
+      Label(2).dead,
+      Label(3).dead,
+      LineNumber(3, Label(target1)),
+      VarOp(ILOAD, 1),
+      Jump(IFGE, Label(target2)),
+
+      Label(4),
+      Label(5).dead,
+      Label(6).dead,
+      VarOp(ILOAD, 2),
+      Jump(IFGE, Label(target3)),
+
+      Label(7),
+      Label(8).dead,
+      Label(9).dead,
+      Op(RETURN),
+
+      LookupSwitch(LOOKUPSWITCH, Label(target4), List(1,2), List(Label(target4), Label(target5))),
+      TableSwitch(TABLESWITCH, 1, 2, Label(target4), List(Label(target4), Label(target5))),
+
+      Label(10),
+      LineNumber(10, Label(10)),
+      Label(11).dead,
+      LineNumber(12, Label(target6))
+    )
+
+    val method = genMethod(handlers = handler)(ops(2, 3, 8, 8, 9, 11).map(_._1): _*)
+    assertTrue(LocalOpt.removeEmptyLabelNodes(method))
+    val m = convertMethod(method)
+    assertSameCode(m.instructions, ops(1, 1, 7, 7, 7, 10).filter(_._2).map(_._1))
+    assertTrue(m.handlers match {
+      case List(ExceptionHandler(Label(4), Label(4), Label(4), _)) => true
+      case _ => false
+    })
+  }
+}