path: root/src/main/scala/firrtl/passes/ConstProp.scala

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package firrtl.passes

import firrtl._
import firrtl.ir._
import firrtl.Utils._
import firrtl.Mappers._
import firrtl.PrimOps._

import annotation.tailrec

object ConstProp extends Pass {
  def name = "Constant Propagation"

  trait FoldLogicalOp {
    def fold(c1: UIntLiteral, c2: UIntLiteral): UIntLiteral
    def simplify(e: Expression, lhs: UIntLiteral, rhs: Expression): Expression

    def apply(e: DoPrim): Expression = (e.args(0), e.args(1)) match {
      case (lhs: UIntLiteral, rhs: UIntLiteral) => fold(lhs, rhs)
      case (lhs: UIntLiteral, rhs) => simplify(e, lhs, rhs)
      case (lhs, rhs: UIntLiteral) => simplify(e, rhs, lhs)
      case _ => e
    }
  }

  object FoldAND extends FoldLogicalOp {
    def fold(c1: UIntLiteral, c2: UIntLiteral) = UIntLiteral(c1.value & c2.value, c1.width max c2.width)
    def simplify(e: Expression, lhs: UIntLiteral, rhs: Expression) = lhs.width match {
      case IntWidth(w) if long_BANG(tpe(rhs)) == w =>
        if (lhs.value == 0) lhs // and(x, 0) => 0
        else if (lhs.value == (BigInt(1) << w.toInt) - 1) rhs // and(x, 1) => x
        else e
      case _ => e
    }
  }

  object FoldOR extends FoldLogicalOp {
    def fold(c1: UIntLiteral, c2: UIntLiteral) = UIntLiteral(c1.value | c2.value, c1.width max c2.width)
    def simplify(e: Expression, lhs: UIntLiteral, rhs: Expression) = lhs.width match {
      case IntWidth(w) if long_BANG(tpe(rhs)) == w =>
        if (lhs.value == 0) rhs // or(x, 0) => x
        else if (lhs.value == (BigInt(1) << w.toInt) - 1) lhs // or(x, 1) => 1
        else e
      case _ => e
    }
  }

  object FoldXOR extends FoldLogicalOp {
    def fold(c1: UIntLiteral, c2: UIntLiteral) = UIntLiteral(c1.value ^ c2.value, c1.width max c2.width)
    def simplify(e: Expression, lhs: UIntLiteral, rhs: Expression) = lhs.width match {
      case IntWidth(w) if long_BANG(tpe(rhs)) == w =>
        if (lhs.value == 0) rhs // xor(x, 0) => x
        else e
      case _ => e
    }
  }

  object FoldEqual extends FoldLogicalOp {
    def fold(c1: UIntLiteral, c2: UIntLiteral) = UIntLiteral(if (c1.value == c2.value) 1 else 0, IntWidth(1))
    def simplify(e: Expression, lhs: UIntLiteral, rhs: Expression) = lhs.width match {
      case IntWidth(w) if w == 1 && long_BANG(tpe(rhs)) == 1 =>
        if (lhs.value == 1) rhs // eq(x, 1) => x
        else e
      case _ => e
    }
  }

  object FoldNotEqual extends FoldLogicalOp {
    def fold(c1: UIntLiteral, c2: UIntLiteral) = UIntLiteral(if (c1.value != c2.value) 1 else 0, IntWidth(1))
    def simplify(e: Expression, lhs: UIntLiteral, rhs: Expression) = lhs.width match {
      case IntWidth(w) if w == 1 && long_BANG(tpe(rhs)) == w =>
        if (lhs.value == 0) rhs // neq(x, 0) => x
        else e
      case _ => e
    }
  }

  private def foldConcat(e: DoPrim) = (e.args(0), e.args(1)) match {
    case (UIntLiteral(xv, IntWidth(xw)), UIntLiteral(yv, IntWidth(yw))) => UIntLiteral(xv << yw.toInt | yv, IntWidth(xw + yw))
    case _ => e
  }

  private def foldShiftLeft(e: DoPrim) = e.consts(0).toInt match {
    case 0 => e.args(0)
    case x => e.args(0) match {
      case UIntLiteral(v, IntWidth(w)) => UIntLiteral(v << x, IntWidth(w + x))
      case SIntLiteral(v, IntWidth(w)) => SIntLiteral(v << x, IntWidth(w + x))
      case _ => e
    }
  }

  private def foldShiftRight(e: DoPrim) = e.consts(0).toInt match {
    case 0 => e.args(0)
    case x => e.args(0) match {
      // TODO when amount >= x.width, return a zero-width wire
      case UIntLiteral(v, IntWidth(w)) => UIntLiteral(v >> x, IntWidth((w - x) max 1))
      // take sign bit if shift amount is larger than arg width
      case SIntLiteral(v, IntWidth(w)) => SIntLiteral(v >> x, IntWidth((w - x) max 1))
      case _ => e
    }
  }

  private def foldComparison(e: DoPrim) = {
    def foldIfZeroedArg(x: Expression): Expression = {
      def isUInt(e: Expression): Boolean = tpe(e) match {
        case UIntType(_) => true
        case _ => false
      }
      def isZero(e: Expression) = e match {
          case UIntLiteral(value, _) => value == BigInt(0)
          case SIntLiteral(value, _) => value == BigInt(0)
          case _ => false
        }
      x match {
        case DoPrim(Lt,  Seq(a,b),_,_) if(isUInt(a) && isZero(b)) => zero
        case DoPrim(Leq, Seq(a,b),_,_) if(isZero(a) && isUInt(b)) => one
        case DoPrim(Gt,  Seq(a,b),_,_) if(isZero(a) && isUInt(b)) => zero
        case DoPrim(Geq, Seq(a,b),_,_) if(isUInt(a) && isZero(b)) => one
        case e => e
      }
    }

    def foldIfOutsideRange(x: Expression): Expression = {
      //Note, only abides by a partial ordering
      case class Range(min: BigInt, max: BigInt) {
        def === (that: Range) =
          Seq(this.min, this.max, that.min, that.max)
            .sliding(2,1)
            .map(x => x(0) == x(1))
            .reduce(_ && _)
        def > (that: Range) = this.min > that.max
        def >= (that: Range) = this.min >= that.max
        def < (that: Range) = this.max < that.min
        def <= (that: Range) = this.max <= that.min
      }
      def range(e: Expression): Range = e match {
        case UIntLiteral(value, _) => Range(value, value)
        case SIntLiteral(value, _) => Range(value, value)
        case _ => tpe(e) match {
          case SIntType(IntWidth(width)) => Range(
            min = BigInt(0) - BigInt(2).pow(width.toInt - 1),
            max = BigInt(2).pow(width.toInt - 1) - BigInt(1)
          )
          case UIntType(IntWidth(width)) => Range(
            min = BigInt(0),
            max = BigInt(2).pow(width.toInt) - BigInt(1)
          )
        }
      }
      // Calculates an expression's range of values
      x match {
        case e: DoPrim => {
          def r0 = range(e.args(0))
          def r1 = range(e.args(1))
          e.op match {
            // Always true
            case Lt  if (r0 < r1) => one
            case Leq if (r0 <= r1) => one
            case Gt  if (r0 > r1) => one
            case Geq if (r0 >= r1) => one
            // Always false
            case Lt  if (r0 >= r1) => zero
            case Leq if (r0 > r1) => zero
            case Gt  if (r0 <= r1) => zero
            case Geq if (r0 < r1) => zero
            case _ => e
          }
        }
        case e => e
      }
    }
    foldIfZeroedArg(foldIfOutsideRange(e))
  }

  private def constPropPrim(e: DoPrim): Expression = e.op match {
    case Shl => foldShiftLeft(e)
    case Shr => foldShiftRight(e)
    case Cat => foldConcat(e)
    case And => FoldAND(e)
    case Or => FoldOR(e)
    case Xor => FoldXOR(e)
    case Eq => FoldEqual(e)
    case Neq => FoldNotEqual(e)
    case (Lt | Leq | Gt | Geq) => foldComparison(e)
    case Not => e.args(0) match {
      case UIntLiteral(v, IntWidth(w)) => UIntLiteral(v ^ ((BigInt(1) << w.toInt) - 1), IntWidth(w))
      case _ => e
    }
    case Bits => e.args(0) match {
      case UIntLiteral(v, _) => {
        val hi = e.consts(0).toInt
        val lo = e.consts(1).toInt
        require(hi >= lo)
        UIntLiteral((v >> lo) & ((BigInt(1) << (hi - lo + 1)) - 1), widthBANG(tpe(e)))
      }
      case x if long_BANG(tpe(e)) == long_BANG(tpe(x)) => tpe(x) match {
        case t: UIntType => x
        case _ => DoPrim(AsUInt, Seq(x), Seq(), tpe(e))
      }
      case _ => e
    }
    case _ => e
  }

  private def constPropMuxCond(m: Mux) = {
    // Only propagate a value if its width matches the mux width
    def propagate(e: Expression, muxWidth: BigInt) = e match {
      case UIntLiteral(v, _) => UIntLiteral(v, IntWidth(muxWidth))
      case _ => tpe(e) match {
        case UIntType(IntWidth(w)) if muxWidth == w => e
        case _ => m
      }
    }
    (m.cond, m.tpe) match {
      case (UIntLiteral(c, _), UIntType(IntWidth(w))) => propagate(if (c == 1) m.tval else m.fval, w)
      case _ => m
    }
  }

  private def constPropMux(m: Mux): Expression = (m.tval, m.fval) match {
    case _ if m.tval == m.fval => m.tval
    case (t: UIntLiteral, f: UIntLiteral) =>
      if (t.value == 1 && f.value == 0 && long_BANG(m.tpe) == 1) m.cond
      else constPropMuxCond(m)
    case _ => constPropMuxCond(m)
  }

  private def constPropNodeRef(r: WRef, e: Expression) = e match {
    case _: UIntLiteral | _: SIntLiteral | _: WRef => e
    case _ => r
  }

  @tailrec
  private def constPropModule(m: Module): Module = {
    var nPropagated = 0L
    val nodeMap = collection.mutable.HashMap[String, Expression]()

    def constPropExpression(e: Expression): Expression = {
      val old = e map constPropExpression
      val propagated = old match {
        case p: DoPrim => constPropPrim(p)
        case m: Mux => constPropMux(m)
        case r: WRef if nodeMap contains r.name => constPropNodeRef(r, nodeMap(r.name))
        case x => x
      }
      if (old ne propagated)
        nPropagated += 1
      propagated
    }

    def constPropStmt(s: Statement): Statement = {
      s match {
        case x: DefNode => nodeMap(x.name) = x.value
        case _ =>
      }
      s map constPropStmt map constPropExpression
    }

    val res = Module(m.info, m.name, m.ports, constPropStmt(m.body))
    if (nPropagated > 0) constPropModule(res) else res
  }

  def run(c: Circuit): Circuit = {
    val modulesx = c.modules.map {
      case m: ExtModule => m
      case m: Module => constPropModule(m)
    }
    Circuit(c.info, modulesx, c.main)
  }
}