Performance improvement ideas for Zinc 1.4.x

[retronym]

I'm profiling a build using the latest Zinc. I'll use this ticket to record the areas where Zinc could be improved.

JarUtils.ClassInJar

The build is using straight-to-JAR compilation. This seems to be an allocation hotspot within:

   jo2o(output.getSingleOutputAsPath) match {
      case Some(so) if so.getFileName.toString.endsWith(".jar") =>
        knownProducts foreach { product =>
          new JarUtils.ClassInJar(product.id).toClassFilePath foreach { path =>
            ps.add(path.replace('\\', '/'))
          }
        }

The main optimization would be to avoid repeatedly calling new File in ClassInJar.splitJarReference on what I presume is always the same path (the path of the JAR), and is anyway discarded by def toClassFilePath: Option[ClassFilePath] = splitJarReference._2!

new File internally incurs String allocations, at least on Windows, in WinNTFileSystem.normalize.

I am also inclined to remove use of parallel collections should be used in knownProducts.

This is also inefficient:

val Array(jar, cls) = toString.split("!")