Improve Shl codegen; eliminate Shlw WIR node (#994)

* Improve Shl codegen; eliminate Shlw WIR node

If we emit shl(x, k) as {x, k'h0} instead of (x << k), then there's
no need for Verilog-specific padding in the PadWidths pass.  Avoiding
the redundant padding improves compiler/simulator performance and
renders Shlw unnecessary.

* [skip formal checks] Add test

4 files changed, 13 insertions, 7 deletions

diff --git a/src/main/scala/firrtl/Emitter.scala b/src/main/scala/firrtl/Emitter.scala
index 8547f9b9..67bd1583 100644
--- a/src/main/scala/firrtl/Emitter.scala
+++ b/src/main/scala/firrtl/Emitter.scala
@@ -303,8 +303,7 @@ class VerilogEmitter extends SeqTransform with Emitter {
          case (_: SIntType) => Seq(cast(a0)," >>> ", a1)
          case (_) => Seq(cast(a0), " >> ", a1)
        }
-       case Shlw => Seq(cast(a0), " << ", c0)
-       case Shl => Seq(cast(a0), " << ", c0)
+       case Shl => if (c0 > 0) Seq("{", cast(a0), s", $c0'h0}") else Seq(cast(a0))
        case Shr if c0 >= bitWidth(a0.tpe) =>
          error("Verilog emitter does not support SHIFT_RIGHT >= arg width")
        case Shr => Seq(a0,"[", bitWidth(a0.tpe) - 1, ":", c0, "]")
diff --git a/src/main/scala/firrtl/WIR.scala b/src/main/scala/firrtl/WIR.scala
index a5d2571d..0c5c250f 100644
--- a/src/main/scala/firrtl/WIR.scala
+++ b/src/main/scala/firrtl/WIR.scala
@@ -152,8 +152,6 @@ case object Addw extends PrimOp { override def toString = "addw" }
 case object Subw extends PrimOp { override def toString = "subw" }
 // Resultant width is the same as input argument width
 case object Dshlw extends PrimOp { override def toString = "dshlw" }
-// Resultant width is the same as input argument width
-case object Shlw extends PrimOp { override def toString = "shlw" }
 
 object WrappedExpression {
   def apply(e: Expression) = new WrappedExpression(e)
diff --git a/src/main/scala/firrtl/passes/PadWidths.scala b/src/main/scala/firrtl/passes/PadWidths.scala
index c9aa1539..cbd8250a 100644
--- a/src/main/scala/firrtl/passes/PadWidths.scala
+++ b/src/main/scala/firrtl/passes/PadWidths.scala
@@ -44,9 +44,6 @@ object PadWidths extends Pass {
       case Dshl =>
         // special case as args aren't all same width
         ex copy (op = Dshlw, args = Seq(fixup(width(ex.tpe))(ex.args.head), ex.args(1)))
-      case Shl =>
-        // special case as arg should be same width as result
-        ex copy (op = Shlw, args = Seq(fixup(width(ex.tpe))(ex.args.head)))
       case _ => ex
     }
     case ex => ex
diff --git a/src/test/scala/firrtlTests/UnitTests.scala b/src/test/scala/firrtlTests/UnitTests.scala
index 2cf9c001..b3af920c 100644
--- a/src/test/scala/firrtlTests/UnitTests.scala
+++ b/src/test/scala/firrtlTests/UnitTests.scala
@@ -437,4 +437,16 @@ class UnitTests extends FirrtlFlatSpec {
 
     result should containTree { case Connect(_, `out`, mgen) => true }
   }
+
+  "Shl" should "be emitted in Verilog as concat" in {
+    val input =
+      """circuit Unit :
+        |  module Unit :
+        |    input in : UInt<4>
+        |    output out : UInt<8>
+        |    out <= shl(in, 4)
+        |""".stripMargin
+    val res = (new VerilogCompiler).compileAndEmit(CircuitState(parse(input), ChirrtlForm))
+    res should containLine ("assign out = {in, 4'h0};")
+  }
 }