Verilog emitter transform InlineNots (#1270)

[skip formal checks]

* ConstProp FoldEqual/FoldNotEqual propagate boolean (non-)equality with true/false
* transform InlineNots
* transform back-to-back Nots into straight rename
* swap mux with inverted select

Co-authored-by: Jack Koenig <jack.koenig3@gmail.com>

3 files changed, 95 insertions, 3 deletions

diff --git a/src/main/scala/firrtl/Emitter.scala b/src/main/scala/firrtl/Emitter.scala
index c87928c5..95c762ae 100644
--- a/src/main/scala/firrtl/Emitter.scala
+++ b/src/main/scala/firrtl/Emitter.scala
@@ -237,7 +237,10 @@ class VerilogEmitter extends SeqTransform with Emitter {
         if (e.tpe == AsyncResetType) {
           throw EmitterException("Cannot emit async reset muxes directly")
         }
-        emit(Seq(e.cond," ? ",cast(e.tval)," : ",cast(e.fval)),top + 1)
+        e.cond match {
+          case DoPrim(Not, Seq(sel), _,_) => emit(Seq(sel," ? ",cast(e.fval)," : ",cast(e.tval)),top + 1)
+          case _ => emit(Seq(e.cond," ? ",cast(e.tval)," : ",cast(e.fval)),top + 1)
+        }
       }
       case (e: ValidIf) => emit(Seq(cast(e.value)),top + 1)
       case (e: WRef) => w write e.serialize
@@ -307,13 +310,15 @@ class VerilogEmitter extends SeqTransform with Emitter {
      def c0: Int = doprim.consts.head.toInt
      def c1: Int = doprim.consts(1).toInt
 
-     def checkArgumentLegality(e: Expression) = e match {
+     def checkArgumentLegality(e: Expression): Unit = e match {
        case _: UIntLiteral | _: SIntLiteral | _: WRef | _: WSubField =>
+       case DoPrim(Not, args, _,_) => args.foreach(checkArgumentLegality)
        case _ => throw EmitterException(s"Can't emit ${e.getClass.getName} as PrimOp argument")
      }
 
      def checkCatArgumentLegality(e: Expression): Unit = e match {
        case _: UIntLiteral | _: SIntLiteral | _: WRef | _: WSubField =>
+       case DoPrim(Not, args, _,_) => args.foreach(checkArgumentLegality)
        case DoPrim(Cat, args, _, _) => args foreach(checkCatArgumentLegality)
        case _ => throw EmitterException(s"Can't emit ${e.getClass.getName} as PrimOp argument")
      }
@@ -378,7 +383,7 @@ class VerilogEmitter extends SeqTransform with Emitter {
          case (_: UIntType) => Seq("{1'b0,", cast(a0), "}")
          case (_: SIntType) => Seq(cast(a0))
        }
-       case Not => Seq("~ ", a0)
+       case Not => Seq("~", a0)
        case And => Seq(cast_as(a0), " & ", cast_as(a1))
        case Or => Seq(cast_as(a0), " | ", cast_as(a1))
        case Xor => Seq(cast_as(a0), " ^ ", cast_as(a1))
@@ -961,6 +966,7 @@ class VerilogEmitter extends SeqTransform with Emitter {
   def transforms = Seq(
     new BlackBoxSourceHelper,
     new ReplaceTruncatingArithmetic,
+    new InlineNotsTransform,
     new FlattenRegUpdate,
     new DeadCodeElimination,
     passes.VerilogModulusCleanup,
diff --git a/src/main/scala/firrtl/transforms/ConstantPropagation.scala b/src/main/scala/firrtl/transforms/ConstantPropagation.scala
index f224546b..a008a4d3 100644
--- a/src/main/scala/firrtl/transforms/ConstantPropagation.scala
+++ b/src/main/scala/firrtl/transforms/ConstantPropagation.scala
@@ -155,6 +155,7 @@ class ConstantPropagation extends Transform with ResolvedAnnotationPaths {
     def fold(c1: Literal, c2: Literal) = UIntLiteral(if (c1.value == c2.value) 1 else 0, IntWidth(1))
     def simplify(e: Expression, lhs: Literal, rhs: Expression) = lhs match {
       case UIntLiteral(v, IntWidth(w)) if v == BigInt(1) && w == BigInt(1) && bitWidth(rhs.tpe) == BigInt(1) => rhs
+      case UIntLiteral(v, IntWidth(w)) if v == BigInt(0) && w == BigInt(1) && bitWidth(rhs.tpe) == BigInt(1) => DoPrim(Not, Seq(rhs), Nil, e.tpe)
       case _ => e
     }
   }
@@ -163,6 +164,7 @@ class ConstantPropagation extends Transform with ResolvedAnnotationPaths {
     def fold(c1: Literal, c2: Literal) = UIntLiteral(if (c1.value != c2.value) 1 else 0, IntWidth(1))
     def simplify(e: Expression, lhs: Literal, rhs: Expression) = lhs match {
       case UIntLiteral(v, IntWidth(w)) if v == BigInt(0) && w == BigInt(1) && bitWidth(rhs.tpe) == BigInt(1) => rhs
+      case UIntLiteral(v, IntWidth(w)) if v == BigInt(1) && w == BigInt(1) && bitWidth(rhs.tpe) == BigInt(1) => DoPrim(Not, Seq(rhs), Nil, e.tpe)
       case _ => e
     }
   }
diff --git a/src/main/scala/firrtl/transforms/InlineNots.scala b/src/main/scala/firrtl/transforms/InlineNots.scala
new file mode 100644
index 00000000..3dab5168
--- /dev/null
+++ b/src/main/scala/firrtl/transforms/InlineNots.scala
@@ -0,0 +1,84 @@
+package firrtl
+package transforms
+
+import firrtl.ir._
+import firrtl.Mappers._
+import firrtl.PrimOps.Not
+import firrtl.Utils.isTemp
+import firrtl.WrappedExpression._
+
+import scala.collection.mutable
+
+object InlineNotsTransform {
+
+  /** Returns true if Expression is a Not PrimOp, false otherwise */
+  private def isNot(expr: Expression): Boolean = expr match {
+    case DoPrim(Not, args,_,_) => args.forall(isSimpleExpr)
+    case _ => false
+  }
+
+  // Checks if an Expression is made up of only Nots terminated by a Literal or Reference.
+  // private because it's not clear if this definition of "Simple Expression" would be useful elsewhere.
+  // Note that this can have false negatives but MUST NOT have false positives.
+  private def isSimpleExpr(expr: Expression): Boolean = expr match {
+    case _: WRef | _: Literal | _: WSubField => true
+    case DoPrim(Not, args, _,_) => args.forall(isSimpleExpr)
+    case _ => false
+  }
+
+  /** Mapping from references to the [[firrtl.ir.Expression Expression]]s that drive them */
+  type Netlist = mutable.HashMap[WrappedExpression, Expression]
+
+  /** Recursively replace [[WRef]]s with new [[Expression]]s
+    *
+    * @param netlist a '''mutable''' HashMap mapping references to [[firrtl.ir.DefNode DefNode]]s to their connected
+    * [[firrtl.ir.Expression Expression]]s. It is '''not''' mutated in this function
+    * @param expr the Expression being transformed
+    * @return Returns expr with Nots inlined
+    */
+  def onExpr(netlist: Netlist)(expr: Expression): Expression = {
+    expr.map(onExpr(netlist)) match {
+      case e @ WRef(name, _,_,_) =>
+        netlist.get(we(e))
+               .filter(isNot)
+               .getOrElse(e)
+      // replace back-to-back inversions with a straight rename
+      case lhs @ DoPrim(Not, Seq(inv), _,_) if isSimpleExpr(inv) =>
+        netlist.getOrElse(we(inv), inv) match {
+          case DoPrim(Not, Seq(rhs), _,_) if isSimpleExpr(inv) => rhs
+          case _ => lhs  // Not a candiate
+        }
+      case other => other // Not a candidate
+    }
+  }
+
+  /** Inline nots in a Statement
+    *
+    * @param netlist a '''mutable''' HashMap mapping references to [[firrtl.ir.DefNode DefNode]]s to their connected
+    * [[firrtl.ir.Expression Expression]]s. This function '''will''' mutate it if stmt is a [[firrtl.ir.DefNode
+    * DefNode]] with a value that is a [[PrimOp]] Not
+    * @param stmt the Statement being searched for nodes and transformed
+    * @return Returns stmt with nots inlined
+    */
+  def onStmt(netlist: Netlist)(stmt: Statement): Statement =
+    stmt.map(onStmt(netlist)).map(onExpr(netlist)) match {
+      case node @ DefNode(_, name, value) if isTemp(name) =>
+        netlist(we(WRef(name))) = value
+        node
+      case other => other
+    }
+
+  /** Inline nots in a Module */
+  def onMod(mod: DefModule): DefModule = mod.map(onStmt(new Netlist))
+}
+
+/** Inline nodes that are simple nots */
+class InlineNotsTransform extends Transform {
+  def inputForm = LowForm
+  def outputForm = LowForm
+
+  def execute(state: CircuitState): CircuitState = {
+    val modulesx = state.circuit.modules.map(InlineNotsTransform.onMod(_))
+    state.copy(circuit = state.circuit.copy(modules = modulesx))
+  }
+}