Fix ConstProp bug where multiple names would swap with one

Fixes issue in https://github.com/freechipsproject/rocket-chip/pull/848

2 files changed, 30 insertions, 1 deletions

diff --git a/src/main/scala/firrtl/transforms/ConstantPropagation.scala b/src/main/scala/firrtl/transforms/ConstantPropagation.scala
index bf8b1a55..46c12b2d 100644
--- a/src/main/scala/firrtl/transforms/ConstantPropagation.scala
+++ b/src/main/scala/firrtl/transforms/ConstantPropagation.scala
@@ -304,7 +304,9 @@ class ConstantPropagation extends Transform {
     // When propagating a reference, check if we want to keep the name that would be deleted
     def propagateRef(lname: String, value: Expression): Unit = {
       value match {
-        case WRef(rname,_,_,_) if betterName(lname, rname) =>
+        case WRef(rname,_,_,_) if betterName(lname, rname) && !swapMap.contains(rname) =>
+          assert(!swapMap.contains(lname)) // <- Shouldn't be possible because lname is either a
+          // node declaration or the single connection to a wire or register
           swapMap += (lname -> rname, rname -> lname)
         case _ =>
       }
diff --git a/src/test/scala/firrtlTests/ConstantPropagationTests.scala b/src/test/scala/firrtlTests/ConstantPropagationTests.scala
index 75c43cf2..380d53e5 100644
--- a/src/test/scala/firrtlTests/ConstantPropagationTests.scala
+++ b/src/test/scala/firrtlTests/ConstantPropagationTests.scala
@@ -451,6 +451,33 @@ class ConstantPropagationSpec extends FirrtlFlatSpec {
 """
       (parse(exec(input))) should be (parse(check))
    }
+
+   // =============================
+   "ConstProp" should "only swap a given name with one other name" in {
+      val input =
+"""circuit Top :
+  module Top :
+    input x : UInt<1>
+    input y : UInt<1>
+    output z : UInt<3>
+    node _T_1 = add(x, y)
+    node n = _T_1
+    node m = _T_1
+    z <= add(n, m)
+"""
+      val check =
+"""circuit Top :
+  module Top :
+    input x : UInt<1>
+    input y : UInt<1>
+    output z : UInt<3>
+    node n = add(x, y)
+    node _T_1 = n
+    node m = n
+    z <= add(n, n)
+"""
+      (parse(exec(input))) should be (parse(check))
+   }
 }
 
 // More sophisticated tests of the full compiler