Do Shr constant propagation in Legalize

This uses the foldShiftRight method of the ConstantPropagation
Transform when legalizing Shr PrimOps. This has the effect of removing
literals with bit extracts from the MinimumVerilogCompiler.

This makes the formerly private foldShiftRight method of a public
method of the ConstantPropagation companion object.

Tests in the MimimumVerilogCompilerSpec are updated to check that Shr
is handled as intended.

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>

1 files changed, 17 insertions, 13 deletions

diff --git a/src/main/scala/firrtl/passes/Passes.scala b/src/main/scala/firrtl/passes/Passes.scala
index bb65201b..04bfb19c 100644
--- a/src/main/scala/firrtl/passes/Passes.scala
+++ b/src/main/scala/firrtl/passes/Passes.scala
@@ -183,19 +183,23 @@ object ExpandConnects extends Pass {
 object Legalize extends Pass {
   private def legalizeShiftRight(e: DoPrim): Expression = {
     require(e.op == Shr)
-    val amount = e.consts.head.toInt
-    val width = bitWidth(e.args.head.tpe)
-    lazy val msb = width - 1
-    if (amount >= width) {
-      e.tpe match {
-        case UIntType(_) => zero
-        case SIntType(_) =>
-          val bits = DoPrim(Bits, e.args, Seq(msb, msb), BoolType)
-          DoPrim(AsSInt, Seq(bits), Seq.empty, SIntType(IntWidth(1)))
-        case t => error(s"Unsupported type $t for Primop Shift Right")
-      }
-    } else {
-      e
+    e.args.head match {
+      case _: UIntLiteral | _: SIntLiteral => ConstantPropagation.foldShiftRight(e)
+      case _ =>
+        val amount = e.consts.head.toInt
+        val width = bitWidth(e.args.head.tpe)
+        lazy val msb = width - 1
+        if (amount >= width) {
+          e.tpe match {
+            case UIntType(_) => zero
+            case SIntType(_) =>
+              val bits = DoPrim(Bits, e.args, Seq(msb, msb), BoolType)
+              DoPrim(AsSInt, Seq(bits), Seq.empty, SIntType(IntWidth(1)))
+            case t => error(s"Unsupported type $t for Primop Shift Right")
+          }
+        } else {
+          e
+        }
     }
   }
   private def legalizeBitExtract(expr: DoPrim): Expression = {