new bytecode emitter, GenBCode, for now under a flag

GenBCode is a drop-in replacement for GenASM with several advantages:

  - faster: ICode isn't necessary anymore.
    Instead, the ASTs delivered by CleanUp (an expression language)
    are translated directly into a stack-language (ASM Tree nodes)

  - future-proofing for Java 8 (MethodHandles, invokedynamic).

  - documentation included, shared mutable state kept to a minimum,
    all contributing to making GenBCode more maintainable
    than its counterpart (its counterpart being GenICode + GenASM).

A few tests are modified in this commit, for reasons given below.

(1) files/neg/case-collision

    Just like GenASM, GenBCode also detects output classfiles
    differing only in case. However the error message differs
    from that of GenASM (collisions may be show in different order).
    Thus the original test now has a flags file containing -neo:GenASM
    and a new test (files/neg/case-collision2) has been added
    for GenBCode. The .check files in each case show expected output.

(2) files/pos/t5031_3

    Currently the test above doesn't work with GenBCode
    (try with -neo:GenBCode in the flags file)
    The root cause lies in the fix to
    https://issues.scala-lang.org/browse/SI-5031
    which weakened an assertion in GenASM
    (GenBCode keeps the original assertion).
    Actually that ticket mentions the fix is a "workaround"

(3) files/run/t7008-scala-defined

    This test also passes only under GenASM and not GenBCode,
    thus the flags file. GenASM turns a bling eye to:

      An AbstractTypeSymbol (SI-7122) has reached the bytecode emitter,
      for which no JVM-level internal name can be found:
      ScalaClassWithCheckedExceptions_1.E1

    The error message above (shown by GenBCode) highlights
    there's no ScalaClassWithCheckedExceptions_1.E1 class,
    thus shouldn't show up in the emitted bytecode
    (GenASM emits bytecode that mentions the inexistent class).

src/asm/scala/tools/asm/tree/MethodNode.java

@@ -458,7 +458,7 @@ public void visitEnd() {
      */
     protected LabelNode getLabelNode(final Label l) {
         if (!(l.info instanceof LabelNode)) {
-            l.info = new LabelNode();
+            l.info = new LabelNode(l);
         }
         return (LabelNode) l.info;
     }

src/compiler/scala/tools/nsc/Global.scala

@@ -26,6 +26,7 @@ import transform.patmat.PatternMatching
 import transform._
 import backend.icode.{ ICodes, GenICode, ICodeCheckers }
 import backend.{ ScalaPrimitives, Platform, JavaPlatform }
+import backend.jvm.GenBCode
 import backend.jvm.GenASM
 import backend.opt.{ Inliners, InlineExceptionHandlers, ConstantOptimization, ClosureElimination, DeadCodeElimination }
 import backend.icode.analysis._
@@ -617,6 +618,13 @@ class Global(var currentSettings: Settings, var reporter: Reporter)
     val runsRightAfter = None
   } with GenASM
 
+  // phaseName = "bcode"
+  object genBCode extends {
+    val global: Global.this.type = Global.this
+    val runsAfter = List("dce")
+    val runsRightAfter = None
+  } with GenBCode
+
   // phaseName = "terminal"
   object terminal extends {
     val global: Global.this.type = Global.this

src/compiler/scala/tools/nsc/backend/JavaPlatform.scala

@@ -38,9 +38,13 @@ trait JavaPlatform extends Platform {
     // replaces the tighter abstract definition here. If we had DOT typing rules, the two
     // types would be conjoined and everything would work out. Yet another reason to push for DOT.
 
+  private def classEmitPhase =
+    if (settings.isBCodeActive) genBCode
+    else genASM
+
   def platformPhases = List(
     flatten,        // get rid of inner classes
-    genASM  // generate .class files
+    classEmitPhase  // generate .class files
   )
 
   lazy val externalEquals          = getDecl(BoxesRunTimeClass, nme.equals_)

src/compiler/scala/tools/nsc/backend/icode/GenICode.scala

@@ -52,6 +52,7 @@ abstract class GenICode extends SubComponent  {
     }
 
     override def apply(unit: CompilationUnit): Unit = {
+      if (settings.isBCodeActive) { return }
       this.unit = unit
       unit.icode.clear()
       informProgress("Generating icode for " + unit)