Initial support for anti-dependence breaking. Currently this code does not

introduce any new spilling; it just uses unused registers.

Refactor the SUnit topological sort code out of the RRList scheduler and
make use of it to help with the post-pass scheduler.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@59999 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/ScheduleDAG.cpp

@@ -263,3 +263,204 @@ void ScheduleDAG::VerifySchedule(bool isBottomUp) {
          "The number of nodes scheduled doesn't match the expected number!");
 }
 #endif
+
+/// InitDAGTopologicalSorting - create the initial topological 
+/// ordering from the DAG to be scheduled.
+///
+/// The idea of the algorithm is taken from 
+/// "Online algorithms for managing the topological order of
+/// a directed acyclic graph" by David J. Pearce and Paul H.J. Kelly
+/// This is the MNR algorithm, which was first introduced by 
+/// A. Marchetti-Spaccamela, U. Nanni and H. Rohnert in  
+/// "Maintaining a topological order under edge insertions".
+///
+/// Short description of the algorithm: 
+///
+/// Topological ordering, ord, of a DAG maps each node to a topological
+/// index so that for all edges X->Y it is the case that ord(X) < ord(Y).
+///
+/// This means that if there is a path from the node X to the node Z, 
+/// then ord(X) < ord(Z).
+///
+/// This property can be used to check for reachability of nodes:
+/// if Z is reachable from X, then an insertion of the edge Z->X would 
+/// create a cycle.
+///
+/// The algorithm first computes a topological ordering for the DAG by
+/// initializing the Index2Node and Node2Index arrays and then tries to keep
+/// the ordering up-to-date after edge insertions by reordering the DAG.
+///
+/// On insertion of the edge X->Y, the algorithm first marks by calling DFS
+/// the nodes reachable from Y, and then shifts them using Shift to lie
+/// immediately after X in Index2Node.
+void ScheduleDAGTopologicalSort::InitDAGTopologicalSorting() {
+  unsigned DAGSize = SUnits.size();
+  std::vector<SUnit*> WorkList;
+  WorkList.reserve(DAGSize);
+
+  Index2Node.resize(DAGSize);
+  Node2Index.resize(DAGSize);
+
+  // Initialize the data structures.
+  for (unsigned i = 0, e = DAGSize; i != e; ++i) {
+    SUnit *SU = &SUnits[i];
+    int NodeNum = SU->NodeNum;
+    unsigned Degree = SU->Succs.size();
+    // Temporarily use the Node2Index array as scratch space for degree counts.
+    Node2Index[NodeNum] = Degree;
+
+    // Is it a node without dependencies?
+    if (Degree == 0) {
+      assert(SU->Succs.empty() && "SUnit should have no successors");
+      // Collect leaf nodes.
+      WorkList.push_back(SU);
+    }
+  }  
+
+  int Id = DAGSize;
+  while (!WorkList.empty()) {
+    SUnit *SU = WorkList.back();
+    WorkList.pop_back();
+    Allocate(SU->NodeNum, --Id);
+    for (SUnit::const_pred_iterator I = SU->Preds.begin(), E = SU->Preds.end();
+         I != E; ++I) {
+      SUnit *SU = I->Dep;
+      if (!--Node2Index[SU->NodeNum])
+        // If all dependencies of the node are processed already,
+        // then the node can be computed now.
+        WorkList.push_back(SU);
+    }
+  }
+
+  Visited.resize(DAGSize);
+
+#ifndef NDEBUG
+  // Check correctness of the ordering
+  for (unsigned i = 0, e = DAGSize; i != e; ++i) {
+    SUnit *SU = &SUnits[i];
+    for (SUnit::const_pred_iterator I = SU->Preds.begin(), E = SU->Preds.end();
+         I != E; ++I) {
+      assert(Node2Index[SU->NodeNum] > Node2Index[I->Dep->NodeNum] && 
+      "Wrong topological sorting");
+    }
+  }
+#endif
+}
+
+/// AddPred - Updates the topological ordering to accomodate an edge
+/// to be added from SUnit X to SUnit Y.
+void ScheduleDAGTopologicalSort::AddPred(SUnit *Y, SUnit *X) {
+  int UpperBound, LowerBound;
+  LowerBound = Node2Index[Y->NodeNum];
+  UpperBound = Node2Index[X->NodeNum];
+  bool HasLoop = false;
+  // Is Ord(X) < Ord(Y) ?
+  if (LowerBound < UpperBound) {
+    // Update the topological order.
+    Visited.reset();
+    DFS(Y, UpperBound, HasLoop);
+    assert(!HasLoop && "Inserted edge creates a loop!");
+    // Recompute topological indexes.
+    Shift(Visited, LowerBound, UpperBound);
+  }
+}
+
+/// RemovePred - Updates the topological ordering to accomodate an
+/// an edge to be removed from the specified node N from the predecessors
+/// of the current node M.
+void ScheduleDAGTopologicalSort::RemovePred(SUnit *M, SUnit *N) {
+  // InitDAGTopologicalSorting();
+}
+
+/// DFS - Make a DFS traversal to mark all nodes reachable from SU and mark
+/// all nodes affected by the edge insertion. These nodes will later get new
+/// topological indexes by means of the Shift method.
+void ScheduleDAGTopologicalSort::DFS(const SUnit *SU, int UpperBound, bool& HasLoop) {
+  std::vector<const SUnit*> WorkList;
+  WorkList.reserve(SUnits.size()); 
+
+  WorkList.push_back(SU);
+  while (!WorkList.empty()) {
+    SU = WorkList.back();
+    WorkList.pop_back();
+    Visited.set(SU->NodeNum);
+    for (int I = SU->Succs.size()-1; I >= 0; --I) {
+      int s = SU->Succs[I].Dep->NodeNum;
+      if (Node2Index[s] == UpperBound) {
+        HasLoop = true; 
+        return;
+      }
+      // Visit successors if not already and in affected region.
+      if (!Visited.test(s) && Node2Index[s] < UpperBound) {
+        WorkList.push_back(SU->Succs[I].Dep);
+      } 
+    } 
+  }
+}
+
+/// Shift - Renumber the nodes so that the topological ordering is 
+/// preserved.
+void ScheduleDAGTopologicalSort::Shift(BitVector& Visited, int LowerBound, 
+                              int UpperBound) {
+  std::vector<int> L;
+  int shift = 0;
+  int i;
+
+  for (i = LowerBound; i <= UpperBound; ++i) {
+    // w is node at topological index i.
+    int w = Index2Node[i];
+    if (Visited.test(w)) {
+      // Unmark.
+      Visited.reset(w);
+      L.push_back(w);
+      shift = shift + 1;
+    } else {
+      Allocate(w, i - shift);
+    }
+  }
+
+  for (unsigned j = 0; j < L.size(); ++j) {
+    Allocate(L[j], i - shift);
+    i = i + 1;
+  }
+}
+
+
+/// WillCreateCycle - Returns true if adding an edge from SU to TargetSU will
+/// create a cycle.
+bool ScheduleDAGTopologicalSort::WillCreateCycle(SUnit *SU, SUnit *TargetSU) {
+  if (IsReachable(TargetSU, SU))
+    return true;
+  for (SUnit::pred_iterator I = SU->Preds.begin(), E = SU->Preds.end();
+       I != E; ++I)
+    if (I->Cost < 0 && IsReachable(TargetSU, I->Dep))
+      return true;
+  return false;
+}
+
+/// IsReachable - Checks if SU is reachable from TargetSU.
+bool ScheduleDAGTopologicalSort::IsReachable(const SUnit *SU, const SUnit *TargetSU) {
+  // If insertion of the edge SU->TargetSU would create a cycle
+  // then there is a path from TargetSU to SU.
+  int UpperBound, LowerBound;
+  LowerBound = Node2Index[TargetSU->NodeNum];
+  UpperBound = Node2Index[SU->NodeNum];
+  bool HasLoop = false;
+  // Is Ord(TargetSU) < Ord(SU) ?
+  if (LowerBound < UpperBound) {
+    Visited.reset();
+    // There may be a path from TargetSU to SU. Check for it. 
+    DFS(TargetSU, UpperBound, HasLoop);
+  }
+  return HasLoop;
+}
+
+/// Allocate - assign the topological index to the node n.
+void ScheduleDAGTopologicalSort::Allocate(int n, int index) {
+  Node2Index[n] = index;
+  Index2Node[index] = n;
+}
+
+ScheduleDAGTopologicalSort::ScheduleDAGTopologicalSort(
+                                                     std::vector<SUnit> &sunits)
+ : SUnits(sunits) {}