Fix bug in ConstProp where module dependency edges were dropped (#696)

This resulted in parent modules sometimes being constant proppagated
before a child module. If the child module has a constant driving one of
its outputs, the parent module would thus not see the constant. This
resulted in strange unstable constant propagation behavior where
sometimes constant outputs would not propagate.

Also add test illustrating why this occurs with uses of InstanceGraph

3 files changed, 43 insertions, 1 deletions

diff --git a/src/main/scala/firrtl/graph/DiGraph.scala b/src/main/scala/firrtl/graph/DiGraph.scala
index 7e56919c..b869982d 100644
--- a/src/main/scala/firrtl/graph/DiGraph.scala
+++ b/src/main/scala/firrtl/graph/DiGraph.scala
@@ -302,7 +302,8 @@ class DiGraph[T] private[graph] (private[graph] val edges: LinkedHashMap[T, Link
     * @return a transformed DiGraph[Q]
     */
   def transformNodes[Q](f: (T) => Q): DiGraph[Q] = {
-    val eprime = edges.map({ case (k, v) => (f(k), v.map(f(_))) })
+    val eprime = edges.map({ case (k, _) => (f(k), new LinkedHashSet[Q]) })
+    edges.foreach({ case (k, v) => eprime(f(k)) ++= v.map(f(_)) })
     new DiGraph(eprime)
   }
 
diff --git a/src/test/scala/firrtlTests/analyses/InstanceGraphTests.scala b/src/test/scala/firrtlTests/analyses/InstanceGraphTests.scala
index 3e517079..f01de1f4 100644
--- a/src/test/scala/firrtlTests/analyses/InstanceGraphTests.scala
+++ b/src/test/scala/firrtlTests/analyses/InstanceGraphTests.scala
@@ -65,4 +65,34 @@ circuit Top :
     val graph = new InstanceGraph(circuit).graph.transformNodes(_.module)
     getEdgeSet(graph) shouldBe Map("Top" -> Set("Child1"), "Top2" -> Set("Child2", "Child3"), "Child2" -> Set("Child2a", "Child2b"), "Child1" -> Set(), "Child2a" -> Set(), "Child2b" -> Set(), "Child3" -> Set())
   }
+
+  it should "not drop duplicate nodes when they collide as a result of transformNodes" in {
+    val input =
+"""circuit Top :
+  module Buzz :
+    skip
+  module Fizz :
+    inst b of Buzz
+  module Foo :
+    inst f1 of Fizz
+  module Bar :
+    inst f2 of Fizz
+  module Top :
+    inst f of Foo
+    inst b of Bar
+"""
+    val circuit = ToWorkingIR.run(parse(input))
+    val graph = (new InstanceGraph(circuit)).graph
+
+    // Create graphs with edges from child to parent module
+    // g1 has collisions on parents to children, ie. it combines:
+    //   (f1, Fizz) -> (b, Buzz) and (f2, Fizz) -> (b, Buzz)
+    val g1 = graph.transformNodes(_.module).reverse
+    g1.getEdges("Fizz") shouldBe Set("Foo", "Bar")
+
+    val g2 = graph.reverse.transformNodes(_.module)
+    // g2 combines
+    //   (f1, Fizz) -> (f, Foo) and (f2, Fizz) -> (b, Bar)
+    g2.getEdges("Fizz") shouldBe Set("Foo", "Bar")
+  }
 }
diff --git a/src/test/scala/firrtlTests/graph/DiGraphTests.scala b/src/test/scala/firrtlTests/graph/DiGraphTests.scala
index 84122f83..b9f51699 100644
--- a/src/test/scala/firrtlTests/graph/DiGraphTests.scala
+++ b/src/test/scala/firrtlTests/graph/DiGraphTests.scala
@@ -29,6 +29,13 @@ class DiGraphTests extends FirrtlFlatSpec {
     "c" -> Set("d"),
     "d" -> Set("a")))
 
+  val tupleGraph = DiGraph(Map(
+    ("a", 0) -> Set(("b", 2)),
+    ("a", 1) -> Set(("c", 3)),
+    ("b", 2) -> Set.empty[(String, Int)],
+    ("c", 3) -> Set.empty[(String, Int)]
+  ))
+
   val degenerateGraph = DiGraph(Map("a" -> Set.empty[String]))
 
   acyclicGraph.findSCCs.filter(_.length > 1) shouldBe empty
@@ -47,4 +54,8 @@ class DiGraphTests extends FirrtlFlatSpec {
 
   degenerateGraph.getEdgeMap should equal (degenerateGraph.reverse.getEdgeMap)
 
+  "transformNodes" should "combine vertices that collide, not drop them" in {
+    tupleGraph.transformNodes(_._1).getEdgeMap should contain ("a" -> Set("b", "c"))
+  }
+
 }