SD-233 synchronized blocks are JIT-friendly again

GenBCode, the new backend in Scala 2.12, subtly changed
the way that synchronized blocks are emitted.

It used `java/lang/Throwable` as an explicitly named exception
type, rather than implying the same by omitting this in bytecode.

This appears to confuse HotSpot JIT, which reports a error
parsing the bytecode into its IR which leaves the enclosing method
stuck in interpreted mode.

This commit passes a `null` descriptor to restore the old pattern
(the same one used by javac.) I've checked that the JIT warnings
are gone and that the method can be compiled again.

1 files changed, 8 insertions, 0 deletions

diff --git a/test/junit/scala/tools/nsc/backend/jvm/BytecodeTest.scala b/test/junit/scala/tools/nsc/backend/jvm/BytecodeTest.scala
index b09a41969e..00b6d1cc42 100644
--- a/test/junit/scala/tools/nsc/backend/jvm/BytecodeTest.scala
+++ b/test/junit/scala/tools/nsc/backend/jvm/BytecodeTest.scala
@@ -187,4 +187,12 @@ class BytecodeTest extends BytecodeTesting {
       List(Label(0), LineNumber(2, Label(0)), VarOp(ALOAD, 0), Invoke(INVOKESPECIAL, "T", "t", "()V", true), Op(RETURN), Label(4))
     )
   }
+
+  @Test
+  def sd233(): Unit = {
+    val code = "def f = { println(1); synchronized(println(2)) }"
+    val m = compileMethod(code)
+    val List(ExceptionHandler(_, _, _, desc)) = m.handlers
+    assert(desc == None, desc)
+  }
 }