[PBQP] Do not add an edge between nodes with totally disjoint allowed registers

Such edges are zero matrix, and they bring no additional info to the
allocation problem, apart from contributing to nodes' degree. Removing
those edges is expected to improve allocation time.

Tune the spill cost comparison, as this gives better average performances
now that the nodes' degrees has changed.

llvm-svn: 230904

llvm/include/llvm/CodeGen/RegAllocPBQP.h

@@ -544,8 +544,10 @@ private:
   public:
     SpillCostComparator(const Graph& G) : G(G) {}
     bool operator()(NodeId N1Id, NodeId N2Id) {
-      PBQPNum N1SC = G.getNodeCosts(N1Id)[0] / G.getNodeDegree(N1Id);
-      PBQPNum N2SC = G.getNodeCosts(N2Id)[0] / G.getNodeDegree(N2Id);
+      PBQPNum N1SC = G.getNodeCosts(N1Id)[0];
+      PBQPNum N2SC = G.getNodeCosts(N2Id)[0];
+      if (N1SC == N2SC)
+        return G.getNodeDegree(N1Id) < G.getNodeDegree(N2Id);
       return N1SC < N2SC;
     }
   private:

llvm/lib/CodeGen/RegAllocPBQP.cpp

@@ -178,8 +178,38 @@ class Interference : public PBQPRAConstraint {
 private:
 
   typedef const PBQP::RegAlloc::AllowedRegVector* AllowedRegVecPtr;
-  typedef std::pair<AllowedRegVecPtr, AllowedRegVecPtr> IMatrixKey;
-  typedef DenseMap<IMatrixKey, PBQPRAGraph::MatrixPtr> IMatrixCache;
+  typedef std::pair<AllowedRegVecPtr, AllowedRegVecPtr> IKey;
+  typedef DenseMap<IKey, PBQPRAGraph::MatrixPtr> IMatrixCache;
+  typedef DenseSet<IKey> DisjointAllowedRegsCache;
+
+  bool haveDisjointAllowedRegs(const PBQPRAGraph &G, PBQPRAGraph::NodeId NId,
+                               PBQPRAGraph::NodeId MId,
+                               const DisjointAllowedRegsCache &D) const {
+    const auto *NRegs = &G.getNodeMetadata(NId).getAllowedRegs();
+    const auto *MRegs = &G.getNodeMetadata(MId).getAllowedRegs();
+
+    if (NRegs == MRegs)
+      return false;
+
+    if (NRegs < MRegs)
+      return D.count(IKey(NRegs, MRegs)) > 0;
+    else
+      return D.count(IKey(MRegs, NRegs)) > 0;
+  }
+
+  void setDisjointAllowedRegs(const PBQPRAGraph &G, PBQPRAGraph::NodeId NId,
+                              PBQPRAGraph::NodeId MId,
+                              DisjointAllowedRegsCache &D) {
+    const auto *NRegs = &G.getNodeMetadata(NId).getAllowedRegs();
+    const auto *MRegs = &G.getNodeMetadata(MId).getAllowedRegs();
+
+    assert(NRegs != MRegs && "AllowedRegs can not be disjoint with itself");
+
+    if (NRegs < MRegs)
+      D.insert(IKey(NRegs, MRegs));
+    else
+      D.insert(IKey(MRegs, NRegs));
+  }
 
   // Holds (Interval, CurrentSegmentID, and NodeId). The first two are required
   // for the fast interference graph construction algorithm. The last is there
@@ -247,6 +277,9 @@ public:
     // and uniquing them.
     IMatrixCache C;
 
+    // Cache known disjoint allowed registers pairs
+    DisjointAllowedRegsCache D;
+
     typedef std::set<IntervalInfo, decltype(&lowestEndPoint)> IntervalSet;
     typedef std::priority_queue<IntervalInfo, std::vector<IntervalInfo>,
                                 decltype(&lowestStartPoint)> IntervalQueue;
@@ -290,6 +323,11 @@ public:
       for (const auto &A : Active) {
         PBQP::GraphBase::NodeId MId = getNodeId(A);
 
+        // Do not add an edge when the nodes' allowed registers do not
+        // intersect: there is obviously no interference.
+        if (haveDisjointAllowedRegs(G, NId, MId, D))
+          continue;
+
         // Check that we haven't already added this edge
         // FIXME: findEdge is expensive in the worst case (O(max_clique(G))).
         //        It might be better to replace this with a local bit-matrix.
@@ -297,7 +335,8 @@ public:
           continue;
 
         // This is a new edge - add it to the graph.
-        createInterferenceEdge(G, NId, MId, C);
+        if (!createInterferenceEdge(G, NId, MId, C))
+          setDisjointAllowedRegs(G, NId, MId, D);
       }
 
       // Finally, add Cur to the Active set.
@@ -307,35 +346,48 @@ public:
 
 private:
 
-  void createInterferenceEdge(PBQPRAGraph &G, PBQPRAGraph::NodeId NId,
-                              PBQPRAGraph::NodeId MId, IMatrixCache &C) {
+  // Create an Interference edge and add it to the graph, unless it is
+  // a null matrix, meaning the nodes' allowed registers do not have any
+  // interference. This case occurs frequently between integer and floating
+  // point registers for example.
+  // return true iff both nodes interferes.
+  bool createInterferenceEdge(PBQPRAGraph &G,
+                              PBQPRAGraph::NodeId NId, PBQPRAGraph::NodeId MId,
+                              IMatrixCache &C) {
 
     const TargetRegisterInfo &TRI =
         *G.getMetadata().MF.getSubtarget().getRegisterInfo();
-
     const auto &NRegs = G.getNodeMetadata(NId).getAllowedRegs();
     const auto &MRegs = G.getNodeMetadata(MId).getAllowedRegs();
 
     // Try looking the edge costs up in the IMatrixCache first.
-    IMatrixKey K(&NRegs, &MRegs);
+    IKey K(&NRegs, &MRegs);
     IMatrixCache::iterator I = C.find(K);
     if (I != C.end()) {
       G.addEdgeBypassingCostAllocator(NId, MId, I->second);
-      return;
+      return true;
     }
 
     PBQPRAGraph::RawMatrix M(NRegs.size() + 1, MRegs.size() + 1, 0);
+    bool NodesInterfere = false;
     for (unsigned I = 0; I != NRegs.size(); ++I) {
       unsigned PRegN = NRegs[I];
       for (unsigned J = 0; J != MRegs.size(); ++J) {
         unsigned PRegM = MRegs[J];
-        if (TRI.regsOverlap(PRegN, PRegM))
+        if (TRI.regsOverlap(PRegN, PRegM)) {
           M[I + 1][J + 1] = std::numeric_limits<PBQP::PBQPNum>::infinity();
+          NodesInterfere = true;
+        }
       }
     }
 
+    if (!NodesInterfere)
+      return false;
+
     PBQPRAGraph::EdgeId EId = G.addEdge(NId, MId, std::move(M));
     C[K] = G.getEdgeCostsPtr(EId);
+
+    return true;
   }
 };