DFG: Remove topological sort

Cyclic components are now extracted separately, so there is no
functional reason to have to do a topological sort (previously we used it
to detect cyclic graphs). Removing it to gain some speed.

src/V3Dfg.cpp

@@ -46,77 +46,6 @@ void DfgGraph::addGraph(DfgGraph& other) {
     });
 }
 
-bool DfgGraph::sortTopologically(bool reverse) {
-    // Vertices in reverse topological order
-    std::vector<DfgVertex*> order;
-
-    // Markings for algorithm
-    enum class Mark : uint8_t { Scheduled, OnPath, Finished };
-    std::unordered_map<DfgVertex*, Mark> marks;
-
-    // Stack of nodes in depth first search. The second element of the pair is true if the vertex
-    // is on the current DFS path, and false if it's only scheduled for visitation.
-    std::vector<std::pair<DfgVertex*, bool>> stack;
-
-    // Schedule vertex for visitation
-    const auto scheudle = [&](DfgVertex& vtx) {
-        // Nothing to do if already finished
-        if (marks.emplace(&vtx, Mark::Scheduled).first->second == Mark::Finished) return;
-        // Otherwise scheule for visitation
-        stack.emplace_back(&vtx, false);
-    };
-
-    // For each vertex (direct loop, so we can return early)
-    for (DfgVertex* vtxp = m_vertices.begin(); vtxp; vtxp = vtxp->m_verticesEnt.nextp()) {
-        // Initiate DFS from this vertex
-        scheudle(*vtxp);
-        while (!stack.empty()) {
-            // Pick up stack top
-            const auto pair = stack.back();
-            stack.pop_back();
-            DfgVertex* const currp = pair.first;
-            const bool onPath = pair.second;
-            Mark& mark = marks.at(currp);
-
-            if (onPath) {  // Popped node on path
-                // Mark it as done
-                UASSERT_OBJ(mark == Mark::OnPath, currp, "DFS got lost");
-                mark = Mark::Finished;
-                // Add to order
-                order.push_back(currp);
-            } else {  // Otherwise node was scheduled for visitation, so visit it
-                // If already finished, then nothing to do
-                if (mark == Mark::Finished) continue;
-                // If already on path, then not a DAG
-                if (mark == Mark::OnPath) return false;
-                // Push to path and mark as such
-                mark = Mark::OnPath;
-                stack.emplace_back(currp, true);
-                // Schedule children
-                currp->forEachSink(scheudle);
-            }
-        }
-    }
-
-    // Move given vertex to end of vertex list
-    const auto reinsert = [this](DfgVertex& vtx) {
-        // Remove from current location
-        removeVertex(vtx);
-        // 'addVertex' appends to the end of the vertex list, so can do this in one loop
-        addVertex(vtx);
-    };
-
-    // Remember 'order' is in reverse topological order
-    if (!reverse) {
-        for (DfgVertex* vtxp : vlstd::reverse_view(order)) reinsert(*vtxp);
-    } else {
-        for (DfgVertex* vtxp : order) reinsert(*vtxp);
-    }
-
-    // Done
-    return true;
-}
-
 std::vector<std::unique_ptr<DfgGraph>> DfgGraph::splitIntoComponents(std::string label) {
     size_t componentNumber = 0;
     std::unordered_map<const DfgVertex*, unsigned> vertex2component;

src/V3Dfg.h

@@ -179,12 +179,6 @@ public:
     // Add contents of other graph to this graph. Leaves other graph empty.
     void addGraph(DfgGraph& other);
 
-    // Topologically sort the list of vertices in this graph (such that 'forEachVertex' will
-    // iterate in topological order), or reverse topologically if the passed boolean argument is
-    // true. Returns true on success (the graph is acyclic and a topological order exists), false
-    // if the graph is cyclic. If the graph is cyclic, the vertex ordering is not modified.
-    bool sortTopologically(bool reverse = false);
-
     // Split this graph into individual components (unique sub-graphs with no edges between them).
     // Leaves 'this' graph empty.
     std::vector<std::unique_ptr<DfgGraph>> splitIntoComponents(std::string label);

src/V3DfgOptimizer.cpp

@@ -290,9 +290,6 @@ void V3DfgOptimizer::optimize(AstNetlist* netlistp, const string& label) {
         // For each acyclic component
         for (auto& component : acyclicComponents) {
             if (dumpDfg() >= 7) component->dumpDotFilePrefixed(ctx.prefix() + "source");
-            // Reverse topologically sort the component
-            const bool acyclic = component->sortTopologically(/* reverse: */ true);
-            UASSERT_OBJ(acyclic, nodep, "Supposedly acyclic graph is cyclic");
             // Optimize the component
             V3DfgPasses::optimize(*component, ctx);
             // Add back under the main DFG (we will convert everything back in one go)