Improve performance of graph traversal (#2023)

In complex materials graph and shader graph edge iteration can be extremely slow, because some edges may be visited unnecessarily multiple times. This is especially noticeable in two functions: ShaderGraph::addUpstreamDependencies and ShaderGraph::optimize. This changelist optimizes graph edge iteration by marking and skipping edges that have previously been visited.
AcademySoftwareFoundation · Dec 5, 2024 · f0f9eac · f0f9eac
1 parent bf2804d
commit f0f9eac
Show file tree

Hide file tree

Showing 4 changed files with 37 additions and 5 deletions.
diff --git a/source/MaterialXCore/Traversal.cpp b/source/MaterialXCore/Traversal.cpp
@@ -113,7 +113,7 @@ GraphIterator& GraphIterator::operator++()
         // Traverse to the first upstream edge of this element.
         _stack.emplace_back(_upstreamElem, 0);
         Edge nextEdge = _upstreamElem->getUpstreamEdge(0);
-        if (nextEdge && nextEdge.getUpstreamElement())
+        if (nextEdge && nextEdge.getUpstreamElement() && !skipOrMarkAsVisited(nextEdge))
         {
             extendPathUpstream(nextEdge.getUpstreamElement(), nextEdge.getConnectingElement());
             return *this;
@@ -140,7 +140,7 @@ GraphIterator& GraphIterator::operator++()
         if (parentFrame.second + 1 < parentFrame.first->getUpstreamEdgeCount())
         {
             Edge nextEdge = parentFrame.first->getUpstreamEdge(++parentFrame.second);
-            if (nextEdge && nextEdge.getUpstreamElement())
+            if (nextEdge && nextEdge.getUpstreamElement() && !skipOrMarkAsVisited(nextEdge))
             {
                 extendPathUpstream(nextEdge.getUpstreamElement(), nextEdge.getConnectingElement());
                 return *this;
@@ -177,6 +177,12 @@ void GraphIterator::returnPathDownstream(ElementPtr upstreamElem)
     _connectingElem = ElementPtr();
 }
 
+bool GraphIterator::skipOrMarkAsVisited(const Edge& edge)
+{
+    auto [it, inserted] = _visitedEdges.emplace(edge);
+    return !inserted;
+}
+
 //
 // InheritanceIterator methods
 //

diff --git a/source/MaterialXCore/Traversal.h b/source/MaterialXCore/Traversal.h
@@ -316,12 +316,14 @@ class MX_CORE_API GraphIterator
   private:
     void extendPathUpstream(ElementPtr upstreamElem, ElementPtr connectingElem);
     void returnPathDownstream(ElementPtr upstreamElem);
+    bool skipOrMarkAsVisited(const Edge&);
 
   private:
     ElementPtr _upstreamElem;
     ElementPtr _connectingElem;
     ElementSet _pathElems;
     vector<StackFrame> _stack;
+    std::set<Edge> _visitedEdges;
     bool _prune;
     size_t _holdCount;
 };

diff --git a/source/MaterialXGenShader/ShaderGraph.cpp b/source/MaterialXGenShader/ShaderGraph.cpp
@@ -916,7 +916,7 @@ void ShaderGraph::optimize()
             ShaderOutput* upstreamPort = outputSocket->getConnection();
             if (upstreamPort && upstreamPort->getNode() != this)
             {
-                for (ShaderGraphEdge edge : ShaderGraph::traverseUpstream(upstreamPort))
+                for (ShaderGraphEdge edge : traverseUpstream(upstreamPort))
                 {
                     ShaderNode* node = edge.upstream->getNode();
                     if (usedNodesSet.count(node) == 0)
@@ -1206,7 +1206,7 @@ ShaderGraphEdgeIterator& ShaderGraphEdgeIterator::operator++()
         ShaderInput* input = _upstream->getNode()->getInput(0);
         ShaderOutput* output = input->getConnection();
 
-        if (output && !output->getNode()->isAGraph())
+        if (output && !output->getNode()->isAGraph() && !skipOrMarkAsVisited({ output, input }))
         {
             extendPathUpstream(output, input);
             return *this;
@@ -1234,7 +1234,7 @@ ShaderGraphEdgeIterator& ShaderGraphEdgeIterator::operator++()
             ShaderInput* input = parentFrame.first->getNode()->getInput(++parentFrame.second);
             ShaderOutput* output = input->getConnection();
 
-            if (output && !output->getNode()->isAGraph())
+            if (output && !output->getNode()->isAGraph() && !skipOrMarkAsVisited({ output, input }))
             {
                 extendPathUpstream(output, input);
                 return *this;
@@ -1275,4 +1275,10 @@ void ShaderGraphEdgeIterator::returnPathDownstream(ShaderOutput* upstream)
     _downstream = nullptr;
 }
 
+bool ShaderGraphEdgeIterator::skipOrMarkAsVisited(ShaderGraphEdge edge)
+{
+    auto [it, inserted] = _visitedEdges.emplace(edge);
+    return !inserted;
+}
+
 MATERIALX_NAMESPACE_END
diff --git a/source/MaterialXGenShader/ShaderGraph.h b/source/MaterialXGenShader/ShaderGraph.h
@@ -209,6 +209,22 @@ class MX_GENSHADER_API ShaderGraphEdge
         downstream(down)
     {
     }
+
+    bool operator==(const ShaderGraphEdge& rhs) const
+    {
+        return upstream == rhs.upstream && downstream == rhs.downstream;
+    }
+
+    bool operator!=(const ShaderGraphEdge& rhs) const
+    {
+        return !(*this == rhs);
+    }
+
+    bool operator<(const ShaderGraphEdge& rhs) const
+    {
+        return std::tie(upstream, downstream) < std::tie(rhs.upstream, rhs.downstream);
+    }
+
     ShaderOutput* upstream;
     ShaderInput* downstream;
 };
@@ -254,12 +270,14 @@ class MX_GENSHADER_API ShaderGraphEdgeIterator
   private:
     void extendPathUpstream(ShaderOutput* upstream, ShaderInput* downstream);
     void returnPathDownstream(ShaderOutput* upstream);
+    bool skipOrMarkAsVisited(ShaderGraphEdge);
 
     ShaderOutput* _upstream;
     ShaderInput* _downstream;
     using StackFrame = std::pair<ShaderOutput*, size_t>;
     std::vector<StackFrame> _stack;
     std::set<ShaderOutput*> _path;
+    std::set<ShaderGraphEdge> _visitedEdges;
 };
 
 MATERIALX_NAMESPACE_END