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Firrtl-Interpreter

The firrtl interpreter is an experimental circuit simulator that executes low Firrtl IR. It is one of the standard back-ends available as part of the chisel-testers project, and thus one of the tools in the UCB-BAR/chisel hardware synthesis toolbox. This project provides a test harness supporting a peek, poke expect model.
It also provides a interactive simulator shell or repl (see frepl.sh) that allows fine grained incremental execution of a circuit. In combination with a scala debugger such as Eclipse or IntelliJ it can be a very power way of analyzing problematic behavior. The interpreter has not yet been optimzed for performance and may be too slow for emulation of very complex circuits.

Chisel3 is a high-level functional circuit generator. It produces Flexible Intermediate Representation for RTL or FIRRTL. The Firrtl project parses and transforms firrtl. It also provides mechanisms for emitting verilog, for processing by downstream toolchains. This interpreter parse and execute the LoFirrtl subset of Firrtl. The interpreter is useful for a initial debugging of Chisel circuits and is also used for other forms of circuit analysis.

Using the interpreter

Attach it to your project

Most Chisel development projects will have a an SBT file that describes dependencies. To access the firrtl-interpreter in your project add a dependency on

"edu.berkeley.cs" %% "firrtl-interpreter" % "0.1-SNAPSHOT"

There are a number of different ways to specify this dependency in the build.sbt file. If you have based your circuit on the Chisel-template the addition should look like

libraryDependencies ++= Seq(
  "edu.berkeley.cs" %% "chisel3" % chiselVersion,
  "edu.berkeley.cs" %% "chisel-iotesters" % "1.0",
  "edu.berkeley.cs" %% "firrtl-interpreter" % "0.1-SNAPSHOT",
  "org.scalatest" % "scalatest_2.11" % "2.2.4",
  "org.scalacheck" %% "scalacheck" % "1.12.4")

for other usage consult sbt documentation

Use the tester metaphor

The easiest way to invoke the interpreter is through a test based harness. The InterpretiveTester is very similar to the chisel ClassicTester, it's api consists of poke, peek and expect statements. Here is an example of a GCD Circuit

import Chisel._
import firrtl_interpreter.InterpretiveTester
import org.scalatest.{Matchers, FlatSpec}

object GCDCalculator {
  def computeGcd(a: Int, b: Int): (Int, Int) = {
    var x = a
    var y = b
    var depth = 1
    while(y > 0 ) {
      if (x > y) {
        x -= y
      }
      else {
        y -= x
      }
      depth += 1
    }
    (x, depth)
  }
}

class GCD extends Module {
  val io = new Bundle {
    val a  = UInt(INPUT,  16)
    val b  = UInt(INPUT,  16)
    val e  = Bool(INPUT)
    val z  = UInt(OUTPUT, 16)
    val v  = Bool(OUTPUT)
  }
  val x  = Reg(UInt())
  val y  = Reg(UInt())
  when   (x > y) { x := x - y }
  unless (x > y) { y := y - x }
  when (io.e) { x := io.a; y := io.b }
  io.z := x
  io.v := y === UInt(0)
}


class InterpreterUsageSpec extends FlatSpec with Matchers {

  "GCD" should "return correct values for a range of inputs" in {
    val s = Driver.emit(() => new GCD)

    val tester = new InterpretiveTester(s)

    for {
      i <- 1 to 100
      j <- 1 to 100
    } {
      tester.poke("io_a", i)
      tester.poke("io_b", j)
      tester.poke("io_e", 1)
      tester.step()
      tester.poke("io_e", 0)

      var cycles = 0
      while (tester.peek("io_v") != BigInt(1)) {
        tester.step()
        cycles += 1
      }
      tester.expect("io_z", BigInt(GCDCalculator.computeGcd(i, j)._1))
      // uncomment the println to see a lot of output
      // println(f"GCD(${i}%3d, ${j}%3d) => ${interpretiveTester.peek("io_z")}%3d in $cycles%3d cycles")
    }
    tester.report()
  }
}

About ports and names

The firrtl transformations that result in LoFirrtl alter the names of ports. What would be io.a becomes io_a and so forth.

Using the interpreter with a debugger

This section under development