Don't const-prop a register's self-init (#1441)

* Fixes #1214

Co-authored-by: Jack Koenig <koenig@sifive.com>

2 files changed, 45 insertions, 13 deletions

diff --git a/src/main/scala/firrtl/transforms/ConstantPropagation.scala b/src/main/scala/firrtl/transforms/ConstantPropagation.scala
index c11bc44d..18577147 100644
--- a/src/main/scala/firrtl/transforms/ConstantPropagation.scala
+++ b/src/main/scala/firrtl/transforms/ConstantPropagation.scala
@@ -501,17 +501,24 @@ class ConstantPropagation extends Transform with ResolvedAnnotationPaths {
       propagated
     }
 
-    def backPropStmt(stmt: Statement): Statement = stmt map backPropExpr match {
-      case decl: IsDeclaration if swapMap.contains(decl.name) =>
-        val newName = swapMap(decl.name)
-        nPropagated += 1
-        decl match {
-          case node: DefNode => node.copy(name = newName)
-          case wire: DefWire => wire.copy(name = newName)
-          case reg: DefRegister => reg.copy(name = newName)
-          case other => throwInternalError()
-        }
-      case other => other map backPropStmt
+    def backPropStmt(stmt: Statement): Statement = stmt match {
+      case reg: DefRegister if (WrappedExpression.weq(reg.init, WRef(reg))) =>
+        // Self-init reset is an idiom for "no reset," and must be handled separately
+        swapMap.get(reg.name)
+               .map(newName => reg.copy(name = newName, init = WRef(reg).copy(name = newName)))
+               .getOrElse(reg)
+      case s => s map backPropExpr match {
+        case decl: IsDeclaration if swapMap.contains(decl.name) =>
+          val newName = swapMap(decl.name)
+          nPropagated += 1
+          decl match {
+            case node: DefNode => node.copy(name = newName)
+            case wire: DefWire => wire.copy(name = newName)
+            case reg: DefRegister => reg.copy(name = newName)
+            case other => throwInternalError()
+          }
+        case other => other map backPropStmt
+      }
     }
 
     // When propagating a reference, check if we want to keep the name that would be deleted
diff --git a/src/test/scala/firrtlTests/ConstantPropagationTests.scala b/src/test/scala/firrtlTests/ConstantPropagationTests.scala
index 189809a6..3296b13b 100644
--- a/src/test/scala/firrtlTests/ConstantPropagationTests.scala
+++ b/src/test/scala/firrtlTests/ConstantPropagationTests.scala
@@ -5,6 +5,7 @@ package firrtlTests
 import firrtl._
 import firrtl.passes._
 import firrtl.transforms._
+import firrtl.annotations.Annotation
 
 class ConstantPropagationSpec extends FirrtlFlatSpec {
   val transforms = Seq(
@@ -14,8 +15,8 @@ class ConstantPropagationSpec extends FirrtlFlatSpec {
       ResolveFlows,
       new InferWidths,
       new ConstantPropagation)
-  protected def exec(input: String) = {
-    transforms.foldLeft(CircuitState(parse(input), UnknownForm)) {
+  protected def exec(input: String, annos: Seq[Annotation] = Nil) = {
+    transforms.foldLeft(CircuitState(parse(input), UnknownForm, AnnotationSeq(annos))) {
       (c: CircuitState, t: Transform) => t.runTransform(c)
     }.circuit.serialize
   }
@@ -751,6 +752,30 @@ class ConstantPropagationSingleModule extends ConstantPropagationSpec {
     (parse(exec(input))) should be(parse(check))
   }
 
+  "ConstProp" should "NOT touch self-inits" in {
+    val input =
+      """circuit Top :
+        |  module Top :
+        |    input clk : Clock
+        |    input rst : UInt<1>
+        |    output z : UInt<4>
+        |    reg selfinit : UInt<1>, clk with : (reset => (UInt<1>(0), selfinit))
+        |    selfinit <= UInt<1>(0)
+        |    z <= mux(UInt(1), UInt<2>(0), UInt<4>(0))
+     """.stripMargin
+    val check =
+      """circuit Top :
+        |  module Top :
+        |    input clk : Clock
+        |    input rst : UInt<1>
+        |    output z : UInt<4>
+        |    reg selfinit : UInt<1>, clk with : (reset => (UInt<1>(0), selfinit))
+        |    selfinit <= UInt<1>(0)
+        |    z <= UInt<4>(0)
+     """.stripMargin
+    (parse(exec(input, Seq(NoDCEAnnotation)))) should be(parse(check))
+  }
+
   def castCheck(tpe: String, cast: String): Unit = {
     val input =
      s"""circuit Top :