Make Constant Propagation respect dontTouch on registers

2 files changed, 16 insertions, 1 deletions

diff --git a/src/main/scala/firrtl/transforms/ConstantPropagation.scala b/src/main/scala/firrtl/transforms/ConstantPropagation.scala
index b767ef65..d08a7e6b 100644
--- a/src/main/scala/firrtl/transforms/ConstantPropagation.scala
+++ b/src/main/scala/firrtl/transforms/ConstantPropagation.scala
@@ -367,7 +367,7 @@ class ConstantPropagation extends Transform {
         // Const prop registers that are fed only a constant or a mux between and constant and the
         // register itself
         // This requires that reset has been made explicit
-        case Connect(_, lref @ WRef(lname, ltpe, RegKind, _), expr) => expr match {
+        case Connect(_, lref @ WRef(lname, ltpe, RegKind, _), expr) if !dontTouches.contains(lname) => expr match {
           case lit: Literal =>
             nodeMap(lname) = constPropExpression(pad(lit, ltpe))
           case Mux(_, tval: WRef, fval: Literal, _) if weq(lref, tval) =>
diff --git a/src/test/scala/firrtlTests/ConstantPropagationTests.scala b/src/test/scala/firrtlTests/ConstantPropagationTests.scala
index 2aad4f21..e143f853 100644
--- a/src/test/scala/firrtlTests/ConstantPropagationTests.scala
+++ b/src/test/scala/firrtlTests/ConstantPropagationTests.scala
@@ -712,6 +712,21 @@ class ConstantPropagationIntegrationSpec extends LowTransformSpec {
     execute(input, check, Seq(dontTouch("Top.z")))
   }
 
+  "ConstProp" should "NOT optimize across dontTouch on registers" in {
+      val input =
+        """circuit Top :
+          |  module Top :
+          |    input clk : Clock
+          |    input reset : UInt<1>
+          |    output y : UInt<1>
+          |    reg z : UInt<1>, clk
+          |    y <= z
+          |    z <= mux(reset, UInt<1>("h0"), z)""".stripMargin
+      val check = input
+    execute(input, check, Seq(dontTouch("Top.z")))
+  }
+
+
   it should "NOT optimize across dontTouch on wires" in {
       val input =
         """circuit Top :