Avoid generating ACONST_NULL; POP; ACONST_NULL when loading null

When loading a value of type scala.runtime.Null$ we need to add
POP; ACONST_NULL, see comment in BCodeBodyBuilder.adapt.
This is however not necessary if the null value is a simple ACONST_NULL.
This patch eliminates that redundancy.

2 files changed, 48 insertions, 1 deletions

diff --git a/test/junit/scala/tools/nsc/backend/jvm/opt/InlinerTest.scala b/test/junit/scala/tools/nsc/backend/jvm/opt/InlinerTest.scala
index 942b62b32c..70f3060027 100644
--- a/test/junit/scala/tools/nsc/backend/jvm/opt/InlinerTest.scala
+++ b/test/junit/scala/tools/nsc/backend/jvm/opt/InlinerTest.scala
@@ -1507,7 +1507,6 @@ class InlinerTest extends ClearAfterClass {
         |}
       """.stripMargin
     val List(c) = compile(code)
-    val t = getSingleMethod(c, "t")
 
     // box-unbox will clean it up
     assertEquals(getSingleMethod(c, "t").instructions.summary,
diff --git a/test/junit/scala/tools/nsc/backend/jvm/opt/UnreachableCodeTest.scala b/test/junit/scala/tools/nsc/backend/jvm/opt/UnreachableCodeTest.scala
index 86c8baa3c6..62ee09e9de 100644
--- a/test/junit/scala/tools/nsc/backend/jvm/opt/UnreachableCodeTest.scala
+++ b/test/junit/scala/tools/nsc/backend/jvm/opt/UnreachableCodeTest.scala
@@ -215,4 +215,52 @@ class UnreachableCodeTest extends ClearAfterClass {
     assertTrue(List(FrameEntry(F_FULL, List(INTEGER, DOUBLE, Label(3)), List("java/lang/Object", Label(4))), Label(3), Label(4)) ===
                List(FrameEntry(F_FULL, List(INTEGER, DOUBLE, Label(1)), List("java/lang/Object", Label(3))), Label(1), Label(3)))
   }
+
+  @Test
+  def loadNullNothingBytecode(): Unit = {
+    val code =
+      """class C {
+        |  def nl: Null = null
+        |  def nt: Nothing = throw new Error("")
+        |  def cons(a: Any) = ()
+        |
+        |  def t1 = cons(null)
+        |  def t2 = cons(nl)
+        |  def t3 = cons(throw new Error(""))
+        |  def t4 = cons(nt)
+        |}
+      """.stripMargin
+    val List(c) = compileClasses(noOptCompiler)(code)
+
+    assertEquals(getSingleMethod(c, "nl").instructions.summary, List(ACONST_NULL, ARETURN))
+
+    assertEquals(getSingleMethod(c, "nt").instructions.summary, List(
+      NEW, DUP, LDC, "<init>", ATHROW))
+
+    assertEquals(getSingleMethod(c, "t1").instructions.summary, List(
+      ALOAD, ACONST_NULL, "cons", RETURN))
+
+    // GenBCode introduces POP; ACONST_NULL after loading an expression of type scala.runtime.Null$,
+    // see comment in BCodeBodyBuilder.adapt
+    assertEquals(getSingleMethod(c, "t2").instructions.summary, List(
+      ALOAD, ALOAD, "nl", POP, ACONST_NULL, "cons", RETURN))
+
+    // the bytecode generated by GenBCode is ... ATHROW; INVOKEVIRTUAL C.cons; RETURN
+    // the ASM classfile writer creates a new basic block (creates a label) right after the ATHROW
+    // and replaces all instructions by NOP*; ATHROW, see comment in BCodeBodyBuilder.adapt
+    // NOTE: DCE is enabled by default and gets rid of the redundant code (tested below)
+    assertEquals(getSingleMethod(c, "t3").instructions.summary, List(
+      ALOAD, NEW, DUP, LDC, "<init>", ATHROW, NOP, NOP, NOP, ATHROW))
+
+    // GenBCode introduces an ATHROW after the invocation of C.nt, see BCodeBodyBuilder.adapt
+    // NOTE: DCE is enabled by default and gets rid of the redundant code (tested below)
+    assertEquals(getSingleMethod(c, "t4").instructions.summary, List(
+      ALOAD, ALOAD, "nt", ATHROW, NOP, NOP, NOP, ATHROW))
+
+    val List(cDCE) = compileClasses(dceCompiler)(code)
+    assertEquals(getSingleMethod(cDCE, "t3").instructions.summary, List(
+      ALOAD, NEW, DUP, LDC, "<init>", ATHROW))
+    assertEquals(getSingleMethod(cDCE, "t4").instructions.summary, List(
+      ALOAD, ALOAD, "nt", ATHROW))
+  }
 }