Verilog rem fix (#404)

* Add pass that fixes up widths with modulus operator for verilog

* Add basic test for Verilog emission of Rem

* Oops, left in some printlns.

3 files changed, 106 insertions, 0 deletions

diff --git a/src/main/scala/firrtl/Emitter.scala b/src/main/scala/firrtl/Emitter.scala
index e40a18a4..a290ced4 100644
--- a/src/main/scala/firrtl/Emitter.scala
+++ b/src/main/scala/firrtl/Emitter.scala
@@ -562,6 +562,7 @@ class VerilogEmitter extends Emitter with PassBased {
    }
 
   def passSeq = Seq(
+    passes.VerilogModulusCleanup,
     passes.VerilogWrap,
     passes.VerilogRename,
     passes.VerilogPrep)
diff --git a/src/main/scala/firrtl/passes/VerilogModulusCleanup.scala b/src/main/scala/firrtl/passes/VerilogModulusCleanup.scala
new file mode 100644
index 00000000..b4df534f
--- /dev/null
+++ b/src/main/scala/firrtl/passes/VerilogModulusCleanup.scala
@@ -0,0 +1,84 @@
+// See LICENSE for license details.
+
+package firrtl
+package passes
+
+import firrtl.ir._
+import firrtl.Mappers._
+import firrtl.PrimOps.{Bits, Rem}
+import firrtl.Utils._
+
+import scala.collection.mutable
+
+/**
+ * Verilog has the width of (a % b) = Max(W(a), W(b))
+ * FIRRTL has the width of (a % b) = Min(W(a), W(b)), which makes more sense,
+ * but nevertheless is a problem when emitting verilog
+ *
+ * This pass finds every instance of (a % b) and:
+ *   1) adds a temporary node equal to (a % b) with width Max(W(a), W(b))
+ *   2) replaces the reference to (a % b) with a bitslice of the temporary node
+ *      to get back down to width Min(W(a), W(b))
+ *
+ *  This is technically incorrect firrtl, but allows the verilog emitter
+ *  to emit correct verilog without needing to add temporary nodes
+ */
+object VerilogModulusCleanup extends Pass {
+  def name = "Add temporary nodes with verilog widths for modulus"
+
+  private def onModule(m: Module): Module = {
+    val namespace = Namespace(m)
+    def onStmt(s: Statement): Statement = {
+      val v = mutable.ArrayBuffer[Statement]()
+
+      def getWidth(e: Expression): Width = e.tpe match {
+        case t: GroundType => t.width
+        case t => UnknownWidth
+      }
+
+      def maxWidth(ws: Seq[Width]): Width = ws reduceLeft { (x,y) => (x,y) match {
+        case (IntWidth(x), IntWidth(y)) => IntWidth(x max y)
+        case (x, y) => UnknownWidth
+      }}
+
+      def verilogRemWidth(e: DoPrim)(tpe: Type): Type = {
+        val newWidth = maxWidth(e.args.map(exp => getWidth(exp)))
+        tpe mapWidth (w => newWidth)
+      }
+
+      def removeRem(e: Expression): Expression = e match {
+        case e: DoPrim => e.op match {
+          case Rem => 
+            val name = namespace.newTemp
+            val newType = e mapType verilogRemWidth(e)
+            v += DefNode(get_info(s), name, e mapType verilogRemWidth(e))
+            val remRef = WRef(name, newType.tpe, kind(e), gender(e))
+            val remWidth = bitWidth(e.tpe)
+            DoPrim(Bits, Seq(remRef), Seq(remWidth - 1, BigInt(0)), e.tpe)
+          case _ => e
+        }
+        case _ => e
+      }
+
+      s map removeRem match {
+        case x: Block => x map onStmt
+        case EmptyStmt => EmptyStmt
+        case x =>
+          v += x
+          v.size match {
+            case 1 => v.head
+            case _ => Block(v.toSeq)
+          }
+      }
+    }
+    Module(m.info, m.name, m.ports, onStmt(m.body))
+  }
+
+  def run(c: Circuit): Circuit = {
+    val modules = c.modules map {
+      case m: Module => onModule(m)
+      case m: ExtModule => m
+    }
+    Circuit(c.info, modules, c.main)
+  }
+}
diff --git a/src/test/scala/firrtlTests/VerilogEmitterTests.scala b/src/test/scala/firrtlTests/VerilogEmitterTests.scala
index 39592269..41fd6e41 100644
--- a/src/test/scala/firrtlTests/VerilogEmitterTests.scala
+++ b/src/test/scala/firrtlTests/VerilogEmitterTests.scala
@@ -74,4 +74,25 @@ class DoPrimVerilog extends FirrtlFlatSpec {
         |""".stripMargin.split("\n") map normalized
     executeTest(input, check, compiler)
   }
+  "Rem" should "emit correctly" in {
+    val compiler = new VerilogCompiler
+    val input =
+      """circuit Test :
+        |  module Test :
+        |    input in : UInt<8>
+        |    output out : UInt<1>
+        |    out <= rem(in, UInt<1>("h1"))
+        |""".stripMargin
+    val check =
+      """module Test(
+        |  input  [7:0] in, 
+        |  output  out 
+        |);
+        |  wire [7:0] _GEN_0;
+        |  assign out = _GEN_0[0];
+        |  assign _GEN_0 = in % 8'h1;
+        |endmodule
+        |""".stripMargin.split("\n") map normalized
+    executeTest(input, check, compiler)
+  }
 }