Faust: Feature Application Using Scala Trees

Faust is a metaprogramming framework for applying features in Scala code. It provides a small domain specific language for feature designation, feature dependency checking, and automatic application of features to a code directory.

Creating a new Feature

To create a new feature in Faust, one first needs to extend the Feature class. A Feature class represents one isolated feature that can be optionally applied to the code base.

Feature Building Language

Inside the Feature class a feature is built using a special syntax that we provide. A feature is made of advice. Advice tells the framework where and what code needs to be applied to code to implement the feature. There are three types of advice that can be added to a feature: before, after, and extend. One of these keywords begin all advice. All code must be wrapped in quasiquotes as we operate upon trees in the background.

Keyword	Use
before	Designates advice that will come before a statement.
after	Designates advice that will come after a statement.
extend	Designates advice that will extend a class.
insert	Designates what code will be inserted by the advice.
register	Designates the end of the advice statement.

Example:

package faust

import scala.meta._
import scala.meta.contrib._
import java.io.File
import java.io._

class CounterSystemFeature () extends Feature {
  val numPerfCounters = 28
  val haveBasicCounters = true

  //modifying Rocket Core
  val RocketCoreContext = q"class RocketImpl"
  after (q"hookUpCore()") insert (q"csr.io.counters foreach { c => c.inc := RegNext(perfEvents.evaluate(c.eventSel)) }") in RocketCoreContext register

  //modifying CSR
  val CSRContext = q"class CSRFile"

  val stat = q"${mod"override"} val counters = Vec(${numPerfCounters}, new PerfCounterIO)"
  extend (init"CSRFileIO") insert (q"{ $stat }") in CSRContext register

  before (q"buildMappings()") insert (q"val performanceCounters = new PerformanceCounters(perfEventSets, this, ${numPerfCounters}, ${haveBasicCounters})") in CSRContext register

  after(q"buildMappings()") insert q"performanceCounters.buildMappings()" in CSRContext register

  before (q"buildDecode()") insert (q"performanceCounters.buildDecode()") in CSRContext register
}

Adding Dependencies

Currently users must manually add the dependency relationships to the system. However, we have plans to automate this in the future. New dependency information must be added to feature.json and the correct class containing your custom feature must be added to DependencyChecher.scala.

Applying Features

The directory you wish to apply features to must be designated in the enviroment variable SCALADIR.

A particular set of features can be requested by adding them in request.json. To apply features in sbt call run apply. To return your code to its original state call run undo.

Included Features

Currently, our example features are for Rocket Chip which is written in Chisel. However, this framework should work with any Scala code or any DSL embedded in Scala.

Currently we include the following features for Rocket Chip

• Base Performance Counter System
• Instruction Events
• Microarchitecture Events
• System Events