Add optionalPrerequisites to Dependency API

Signed-off-by: Schuyler Eldridge <schuyler.eldridge@ibm.com>

2 files changed, 18 insertions, 2 deletions

diff --git a/src/main/scala/firrtl/options/DependencyManager.scala b/src/main/scala/firrtl/options/DependencyManager.scala
index dc23d3ce..4880ab8f 100644
--- a/src/main/scala/firrtl/options/DependencyManager.scala
+++ b/src/main/scala/firrtl/options/DependencyManager.scala
@@ -126,6 +126,14 @@ trait DependencyManager[A, B <: TransformLike[A] with DependencyAPI[B]] extends
     DiGraph(new LinkedHashMap() ++ v.map(vv => vv -> (v & (vv.dependents.toSet).map(dToO)))).reverse
   }
 
+  /** A directed graph of *optional* prerequisites. Each optional prerequisite is promoted to a full prerequisite if the
+    * optional prerequisite is already a node in the prerequisite graph.
+    */
+  private lazy val optionalPrerequisitesGraph: DiGraph[B] = {
+    val v = new LinkedHashSet() ++ prerequisiteGraph.getVertices
+    DiGraph(new LinkedHashMap() ++ v.map(vv => vv -> (v & (vv.optionalPrerequisites.toSet).map(dToO))))
+  }
+
   /** A directed graph consisting of prerequisites derived from ALL targets. This is necessary for defining targets for
     * [[DependencyManager]] sub-problems.
     */
@@ -143,7 +151,7 @@ trait DependencyManager[A, B <: TransformLike[A] with DependencyAPI[B]] extends
   }
 
   /** A directed graph consisting of all prerequisites, including prerequisites derived from dependents */
-  lazy val dependencyGraph: DiGraph[B] = prerequisiteGraph + dependentsGraph
+  lazy val dependencyGraph: DiGraph[B] = prerequisiteGraph + dependentsGraph + optionalPrerequisitesGraph
 
   /** A directed graph consisting of invalidation edges */
   lazy val invalidateGraph: DiGraph[B] = {
@@ -282,7 +290,8 @@ trait DependencyManager[A, B <: TransformLike[A] with DependencyAPI[B]] extends
     val connections =
       Seq( (prerequisiteGraph, "edge []"),
            (dependentsGraph,   """edge [style=bold color="#4292c6"]"""),
-           (invalidateGraph,   """edge [minlen=2 style=dashed constraint=false color="#fb6a4a"]""") )
+           (invalidateGraph,   """edge [minlen=2 style=dashed constraint=false color="#fb6a4a"]"""),
+           (optionalPrerequisitesGraph, """edge [style=dotted color="#a1d99b"]""") )
         .flatMap{ case (a, b) => toGraphviz(a, b) }
         .mkString("\n")
 
diff --git a/src/main/scala/firrtl/options/Phase.scala b/src/main/scala/firrtl/options/Phase.scala
index 0f29d165..0e534ec8 100644
--- a/src/main/scala/firrtl/options/Phase.scala
+++ b/src/main/scala/firrtl/options/Phase.scala
@@ -103,6 +103,13 @@ trait DependencyAPI[A <: DependencyAPI[A]] { this: TransformLike[_] =>
   def prerequisites: Seq[Dependency[A]] = Seq.empty
   private[options] lazy val _prerequisites: LinkedHashSet[Dependency[A]] = new LinkedHashSet() ++ prerequisites.toSet
 
+  /** All transforms that, if a prerequisite of *another* transform, will run before this transform.
+    * $seqNote
+    */
+  def optionalPrerequisites: Seq[Dependency[A]] = Seq.empty
+  private[options] lazy val _optionalPrerquisites: LinkedHashSet[Dependency[A]] =
+    new LinkedHashSet() ++ optionalPrerequisites.toSet
+
   /** All transforms that must run ''after'' this transform
     *
     * ''This is a means of prerequisite injection into some other transform.'' Normally a transform will define its own