SI-7006 Simplify jump-only block destination determination

With proper reachability analysis, the code for finding the final
destination of jump-only blocks was more complicated than needed. This
commit simplifies and speeds up that process using a standard
Tortoise and Hare algorithm on a Map from jump-only blocks to their
immediate destinations.

Test t7006 is increased a bit to make sure we don't get stuck on
infinite loops and to make sure we're deleting all but the essential
jump.

2 files changed, 6 insertions, 4 deletions

diff --git a/test/files/jvm/t7006/Foo_1.scala b/test/files/jvm/t7006/Foo_1.scala
index f84269daf2..995619ce6b 100644
--- a/test/files/jvm/t7006/Foo_1.scala
+++ b/test/files/jvm/t7006/Foo_1.scala
@@ -1,9 +1,10 @@
 class Foo_1 {
   def foo {
     try {
-      val x = 3
+      val x = 3 // this will be optimized away, leaving a useless jump only block
     } finally {
       print("hello")
     }
+    while(true){} // ensure infinite loop doesn't break the algoirthm
   }
 }
diff --git a/test/files/jvm/t7006/Test.scala b/test/files/jvm/t7006/Test.scala
index 5cc38e42a2..065a23510e 100644
--- a/test/files/jvm/t7006/Test.scala
+++ b/test/files/jvm/t7006/Test.scala
@@ -7,12 +7,13 @@ object Test extends BytecodeTest {
   def show: Unit = {
     val classNode = loadClassNode("Foo_1")
     val methodNode = getMethod(classNode, "foo")
-    assert(countNops(methodNode.instructions) == 0)
+    assert(count(methodNode.instructions, asm.Opcodes.NOP) == 0)
+    assert(count(methodNode.instructions, asm.Opcodes.GOTO) == 1)
   }
 
-  def countNops(insnList: InsnList): Int = {
+  def count(insnList: InsnList, opcode: Int): Int = {
     def isNop(node: asm.tree.AbstractInsnNode): Boolean =
-      (node.getOpcode == asm.Opcodes.NOP)
+      (node.getOpcode == opcode)
     insnList.iterator.asScala.count(isNop)
   }
 }