Fix #15107: Avoid re-emitting a LineNumber after only LabelNodes. (#16813)

There was already some deduplication code to avoid consecutive
`LineNumber` nodes. However, it can happen that `LabelNode`s appear
in-between. In that case, we also want to deduplicate the `LineNumber`s,
since labels do not actually contribute to the final bytecode.

Author: sjrd

compiler/src/dotty/tools/backend/jvm/BCodeSkelBuilder.scala

@@ -556,11 +556,17 @@ trait BCodeSkelBuilder extends BCodeHelpers {
         case _ => false } )
     }
     def lineNumber(tree: Tree): Unit = {
+      @tailrec
+      def getNonLabelNode(a: asm.tree.AbstractInsnNode): asm.tree.AbstractInsnNode = a match {
+        case a: asm.tree.LabelNode => getNonLabelNode(a.getPrevious)
+        case _                     => a
+      }
+
       if (!emitLines || !tree.span.exists) return;
       val nr = ctx.source.offsetToLine(tree.span.point) + 1
       if (nr != lastEmittedLineNr) {
         lastEmittedLineNr = nr
-        lastInsn match {
+        getNonLabelNode(lastInsn) match {
           case lnn: asm.tree.LineNumberNode =>
             // overwrite previous landmark as no instructions have been emitted for it
             lnn.line = nr

compiler/test/dotty/tools/backend/jvm/DottyBytecodeTests.scala

@@ -1622,7 +1622,6 @@ class DottyBytecodeTests extends DottyBytecodeTest {
       val instructions = instructionsFromMethod(method).filter(_.isInstanceOf[LineNumber])
 
       val expected = List(
-        LineNumber(2, Label(0)),
         LineNumber(3, Label(0)),
         LineNumber(4, Label(5)), // case y =>
         LineNumber(5, Label(9)),
@@ -1664,7 +1663,6 @@ class DottyBytecodeTests extends DottyBytecodeTest {
       val instructions = instructionsFromMethod(method).filter(_.isInstanceOf[LineNumber])
 
       val expected = List(
-        LineNumber(2, Label(0)),
         LineNumber(3, Label(0)),
         LineNumber(4, Label(5)), // case a if a == 3 =>
         LineNumber(5, Label(15)),

compiler/test/dotty/tools/backend/jvm/InlineBytecodeTests.scala

@@ -163,28 +163,27 @@ class InlineBytecodeTests extends DottyBytecodeTest {
       val expected =
         List(
           Label(0),
-          LineNumber(6, Label(0)),
           LineNumber(3, Label(0)),
           VarOp(ALOAD, 0),
           Ldc(LDC, "tracking"),
           Invoke(INVOKEVIRTUAL, "Foo", "foo", "(Ljava/lang/String;)V", false),
-          Label(6),
-          LineNumber(8, Label(6)),
+          Label(5),
+          LineNumber(8, Label(5)),
           VarOp(ALOAD, 0),
           Ldc(LDC, "abc"),
           Invoke(INVOKEVIRTUAL, "Foo", "foo", "(Ljava/lang/String;)V", false),
-          Label(11),
-          LineNumber(3, Label(11)),
+          Label(10),
+          LineNumber(3, Label(10)),
           VarOp(ALOAD, 0),
           Ldc(LDC, "tracking"),
           Invoke(INVOKEVIRTUAL, "Foo", "foo", "(Ljava/lang/String;)V", false),
-          Label(16),
-          LineNumber(10, Label(16)),
+          Label(15),
+          LineNumber(10, Label(15)),
           VarOp(ALOAD, 0),
           Ldc(LDC, "inner"),
           Invoke(INVOKEVIRTUAL, "Foo", "foo", "(Ljava/lang/String;)V", false),
           Op(RETURN),
-          Label(22)
+          Label(21)
         )
       assert(instructions == expected,
         "`track` was not properly inlined in `main`\n" + diffInstructions(instructions, expected))
@@ -228,23 +227,22 @@ class InlineBytecodeTests extends DottyBytecodeTest {
       val expected =
         List(
           Label(0),
-          LineNumber(12, Label(0)),
           LineNumber(7, Label(0)),
           VarOp(ALOAD, 0),
           Ldc(LDC, "tracking"),
           Invoke(INVOKEVIRTUAL, "Foo", "foo", "(Ljava/lang/String;)V", false),
-          Label(6),
-          LineNumber(3, Label(6)),
+          Label(5),
+          LineNumber(3, Label(5)),
           VarOp(ALOAD, 0),
           Ldc(LDC, "tracking2"),
           Invoke(INVOKEVIRTUAL, "Foo", "foo", "(Ljava/lang/String;)V", false),
-          Label(11),
-          LineNumber(14, Label(11)),
+          Label(10),
+          LineNumber(14, Label(10)),
           VarOp(ALOAD, 0),
           Ldc(LDC, "abc"),
           Invoke(INVOKEVIRTUAL, "Foo", "foo", "(Ljava/lang/String;)V", false),
           Op(RETURN),
-          Label(17)
+          Label(16)
         )
       assert(instructions == expected,
         "`track` was not properly inlined in `main`\n" + diffInstructions(instructions, expected))
@@ -288,23 +286,22 @@ class InlineBytecodeTests extends DottyBytecodeTest {
       val expected =
         List(
           Label(0),
-          LineNumber(12, Label(0)),
           LineNumber(3, Label(0)),
           VarOp(ALOAD, 0),
           Ldc(LDC, "tracking2"),
           Invoke(INVOKEVIRTUAL, "Foo", "foo", "(Ljava/lang/String;)V", false),
-          Label(6),
-          LineNumber(8, Label(6)),
+          Label(5),
+          LineNumber(8, Label(5)),
           VarOp(ALOAD, 0),
           Ldc(LDC, "fgh"),
           Invoke(INVOKEVIRTUAL, "Foo", "foo", "(Ljava/lang/String;)V", false),
-          Label(11),
-          LineNumber(14, Label(11)),
+          Label(10),
+          LineNumber(14, Label(10)),
           VarOp(ALOAD, 0),
           Ldc(LDC, "abc"),
           Invoke(INVOKEVIRTUAL, "Foo", "foo", "(Ljava/lang/String;)V", false),
           Op(RETURN),
-          Label(17)
+          Label(16)
         )
       assert(instructions == expected,
         "`track` was not properly inlined in `main`\n" + diffInstructions(instructions, expected))
@@ -349,23 +346,22 @@ class InlineBytecodeTests extends DottyBytecodeTest {
       val expected =
         List(
           Label(0),
-          LineNumber(13, Label(0)),
           LineNumber(3, Label(0)),
           VarOp(ALOAD, 0),
           Ldc(LDC, "tracking2"),
           Invoke(INVOKEVIRTUAL, "Foo", "foo", "(Ljava/lang/String;)V", false),
-          Label(6),
-          LineNumber(3, Label(6)),
+          Label(5),
+          LineNumber(3, Label(5)),
           VarOp(ALOAD, 0),
           Ldc(LDC, "tracking2"),
           Invoke(INVOKEVIRTUAL, "Foo", "foo", "(Ljava/lang/String;)V", false),
-          Label(11),
-          LineNumber(15, Label(11)),
+          Label(10),
+          LineNumber(15, Label(10)),
           VarOp(ALOAD, 0),
           Ldc(LDC, "abc"),
           Invoke(INVOKEVIRTUAL, "Foo", "foo", "(Ljava/lang/String;)V", false),
           Op(RETURN),
-          Label(17)
+          Label(16)
         )
       assert(instructions == expected,
         "`track` was not properly inlined in `main`\n" + diffInstructions(instructions, expected))