Make sure not to DCE input-only extmodules unless specified (#590)

Fixes #589

1 files changed, 4 insertions, 1 deletions

diff --git a/src/main/scala/firrtl/transforms/DeadCodeElimination.scala b/src/main/scala/firrtl/transforms/DeadCodeElimination.scala
index 5199276c..bf7ff7eb 100644
--- a/src/main/scala/firrtl/transforms/DeadCodeElimination.scala
+++ b/src/main/scala/firrtl/transforms/DeadCodeElimination.scala
@@ -151,6 +151,9 @@ class DeadCodeElimination extends Transform {
       case ext: ExtModule =>
         // Connect all inputs to all outputs
         val node = LogicNode(ext)
+        // Don't touch external modules *unless* they are specifically marked as doTouch
+        // Simply marking the extmodule itself is sufficient to prevent inputs from being removed
+        if (!doTouchExtMods.contains(ext.name)) depGraph.addEdge(circuitSink, node)
         ext.ports.foreach {
           case Port(_, pname, _, AnalogType(_)) =>
             depGraph.addEdge(LogicNode(ext.name, pname), node)
@@ -158,7 +161,7 @@ class DeadCodeElimination extends Transform {
           case Port(_, pname, Output, _) =>
             val portNode = LogicNode(ext.name, pname)
             depGraph.addEdge(portNode, node)
-            // Don't touch external modules *unless* they are specifically marked as doTouch
+            // Also mark all outputs as circuit sinks (unless marked doTouch obviously)
             if (!doTouchExtMods.contains(ext.name)) depGraph.addEdge(circuitSink, portNode)
           case Port(_, pname, Input, _) => depGraph.addEdge(node, LogicNode(ext.name, pname))
         }