lib/CodeGen/SelectionDAG/ScheduleDAGList.cpp

   1 //===---- ScheduleDAGList.cpp - Implement a list scheduler for isel DAG ---===//
   2 //
   3 //                     The LLVM Compiler Infrastructure
   4 //
   5 // This file is distributed under the University of Illinois Open Source
   6 // License. See LICENSE.TXT for details.
   7 //
   8 //===----------------------------------------------------------------------===//
   9 //
  10 // This implements a top-down list scheduler, using standard algorithms.
  11 // The basic approach uses a priority queue of available nodes to schedule.
  12 // One at a time, nodes are taken from the priority queue (thus in priority
  13 // order), checked for legality to schedule, and emitted if legal.
  14 //
  15 // Nodes may not be legal to schedule either due to structural hazards (e.g.
  16 // pipeline or resource constraints) or because an input to the instruction has
  17 // not completed execution.
  18 //
  19 //===----------------------------------------------------------------------===//
  20
  21 #define DEBUG_TYPE "pre-RA-sched"
  22 #include "ScheduleDAGSDNodes.h"
  23 #include "llvm/CodeGen/LatencyPriorityQueue.h"
  24 #include "llvm/CodeGen/ScheduleHazardRecognizer.h"
  25 #include "llvm/CodeGen/SchedulerRegistry.h"
  26 #include "llvm/CodeGen/SelectionDAGISel.h"
  27 #include "llvm/Target/TargetRegisterInfo.h"
  28 #include "llvm/Target/TargetData.h"
  29 #include "llvm/Target/TargetInstrInfo.h"
  30 #include "llvm/Support/Debug.h"
  31 #include "llvm/Support/Compiler.h"
  32 #include "llvm/ADT/PriorityQueue.h"
  33 #include "llvm/ADT/Statistic.h"
  34 #include <climits>
  35 using namespace llvm;
  36
  37 STATISTIC(NumNoops , "Number of noops inserted");
  38 STATISTIC(NumStalls, "Number of pipeline stalls");
  39
  40 static RegisterScheduler
  41   tdListDAGScheduler("list-td", "Top-down list scheduler",
  42                      createTDListDAGScheduler);
  43
  44 namespace {
  45 //===----------------------------------------------------------------------===//
  46 /// ScheduleDAGList - The actual list scheduler implementation.  This supports
  47 /// top-down scheduling.
  48 ///
  49 class VISIBILITY_HIDDEN ScheduleDAGList : public ScheduleDAGSDNodes {
  50 private:
  51   /// AvailableQueue - The priority queue to use for the available SUnits.
  52   ///
  53   SchedulingPriorityQueue *AvailableQueue;
  54
  55   /// PendingQueue - This contains all of the instructions whose operands have
  56   /// been issued, but their results are not ready yet (due to the latency of
  57   /// the operation).  Once the operands become available, the instruction is
  58   /// added to the AvailableQueue.
  59   std::vector<SUnit*> PendingQueue;
  60
  61   /// HazardRec - The hazard recognizer to use.
  62   ScheduleHazardRecognizer *HazardRec;
  63
  64 public:
  65   ScheduleDAGList(MachineFunction &mf,
  66                   SchedulingPriorityQueue *availqueue,
  67                   ScheduleHazardRecognizer *HR)
  68     : ScheduleDAGSDNodes(mf),
  69       AvailableQueue(availqueue), HazardRec(HR) {
  70     }
  71
  72   ~ScheduleDAGList() {
  73     delete HazardRec;
  74     delete AvailableQueue;
  75   }
  76
  77   void Schedule();
  78
  79 private:
  80   void ReleaseSucc(SUnit *SU, const SDep &D);
  81   void ReleaseSuccessors(SUnit *SU);
  82   void ScheduleNodeTopDown(SUnit *SU, unsigned CurCycle);
  83   void ListScheduleTopDown();
  84 };
  85 }  // end anonymous namespace
  86
  87 /// Schedule - Schedule the DAG using list scheduling.
  88 void ScheduleDAGList::Schedule() {
  89   DOUT << "********** List Scheduling **********\n";
  90
  91   // Build the scheduling graph.
  92   BuildSchedGraph();
  93
  94   AvailableQueue->initNodes(SUnits);
  95
  96   ListScheduleTopDown();
  97
  98   AvailableQueue->releaseState();
  99 }
 100
 101 //===----------------------------------------------------------------------===//
 102 //  Top-Down Scheduling
 103 //===----------------------------------------------------------------------===//
 104
 105 /// ReleaseSucc - Decrement the NumPredsLeft count of a successor. Add it to
 106 /// the PendingQueue if the count reaches zero. Also update its cycle bound.
 107 void ScheduleDAGList::ReleaseSucc(SUnit *SU, const SDep &D) {
 108   SUnit *SuccSU = D.getSUnit();
 109   --SuccSU->NumPredsLeft;
 110
 111 #ifndef NDEBUG
 112   if (SuccSU->NumPredsLeft < 0) {
 113     cerr << "*** Scheduling failed! ***\n";
 114     SuccSU->dump(this);
 115     cerr << " has been released too many times!\n";
 116     assert(0);
 117   }
 118 #endif
 119
 120   SuccSU->setDepthToAtLeast(SU->getDepth() + D.getLatency());
 121
 122   // If all the node's predecessors are scheduled, this node is ready
 123   // to be scheduled. Ignore the special ExitSU node.
 124   if (SuccSU->NumPredsLeft == 0 && SuccSU != &ExitSU)
 125     PendingQueue.push_back(SuccSU);
 126 }
 127
 128 void ScheduleDAGList::ReleaseSuccessors(SUnit *SU) {
 129   // Top down: release successors.
 130   for (SUnit::succ_iterator I = SU->Succs.begin(), E = SU->Succs.end();
 131        I != E; ++I) {
 132     assert(!I->isAssignedRegDep() &&
 133            "The list-td scheduler doesn't yet support physreg dependencies!");
 134
 135     ReleaseSucc(SU, *I);
 136   }
 137 }
 138
 139 /// ScheduleNodeTopDown - Add the node to the schedule. Decrement the pending
 140 /// count of its successors. If a successor pending count is zero, add it to
 141 /// the Available queue.
 142 void ScheduleDAGList::ScheduleNodeTopDown(SUnit *SU, unsigned CurCycle) {
 143   DOUT << "*** Scheduling [" << CurCycle << "]: ";
 144   DEBUG(SU->dump(this));
 145
 146   Sequence.push_back(SU);
 147   assert(CurCycle >= SU->getDepth() && "Node scheduled above its depth!");
 148   SU->setDepthToAtLeast(CurCycle);
 149
 150   ReleaseSuccessors(SU);
 151   SU->isScheduled = true;
 152   AvailableQueue->ScheduledNode(SU);
 153 }
 154
 155 /// ListScheduleTopDown - The main loop of list scheduling for top-down
 156 /// schedulers.
 157 void ScheduleDAGList::ListScheduleTopDown() {
 158   unsigned CurCycle = 0;
 159
 160   // Release any successors of the special Entry node.
 161   ReleaseSuccessors(&EntrySU);
 162
 163   // All leaves to Available queue.
 164   for (unsigned i = 0, e = SUnits.size(); i != e; ++i) {
 165     // It is available if it has no predecessors.
 166     if (SUnits[i].Preds.empty()) {
 167       AvailableQueue->push(&SUnits[i]);
 168       SUnits[i].isAvailable = true;
 169     }
 170   }
 171
 172   // While Available queue is not empty, grab the node with the highest
 173   // priority. If it is not ready put it back.  Schedule the node.
 174   std::vector<SUnit*> NotReady;
 175   Sequence.reserve(SUnits.size());
 176   while (!AvailableQueue->empty() || !PendingQueue.empty()) {
 177     // Check to see if any of the pending instructions are ready to issue.  If
 178     // so, add them to the available queue.
 179     for (unsigned i = 0, e = PendingQueue.size(); i != e; ++i) {
 180       if (PendingQueue[i]->getDepth() == CurCycle) {
 181         AvailableQueue->push(PendingQueue[i]);
 182         PendingQueue[i]->isAvailable = true;
 183         PendingQueue[i] = PendingQueue.back();
 184         PendingQueue.pop_back();
 185         --i; --e;
 186       } else {
 187         assert(PendingQueue[i]->getDepth() > CurCycle && "Negative latency?");
 188       }
 189     }
 190
 191     // If there are no instructions available, don't try to issue anything, and
 192     // don't advance the hazard recognizer.
 193     if (AvailableQueue->empty()) {
 194       ++CurCycle;
 195       continue;
 196     }
 197
 198     SUnit *FoundSUnit = 0;
 199
 200     bool HasNoopHazards = false;
 201     while (!AvailableQueue->empty()) {
 202       SUnit *CurSUnit = AvailableQueue->pop();
 203
 204       ScheduleHazardRecognizer::HazardType HT =
 205         HazardRec->getHazardType(CurSUnit);
 206       if (HT == ScheduleHazardRecognizer::NoHazard) {
 207         FoundSUnit = CurSUnit;
 208         break;
 209       }
 210
 211       // Remember if this is a noop hazard.
 212       HasNoopHazards |= HT == ScheduleHazardRecognizer::NoopHazard;
 213
 214       NotReady.push_back(CurSUnit);
 215     }
 216
 217     // Add the nodes that aren't ready back onto the available list.
 218     if (!NotReady.empty()) {
 219       AvailableQueue->push_all(NotReady);
 220       NotReady.clear();
 221     }
 222
 223     // If we found a node to schedule, do it now.
 224     if (FoundSUnit) {
 225       ScheduleNodeTopDown(FoundSUnit, CurCycle);
 226       HazardRec->EmitInstruction(FoundSUnit);
 227
 228       // If this is a pseudo-op node, we don't want to increment the current
 229       // cycle.
 230       if (FoundSUnit->Latency)  // Don't increment CurCycle for pseudo-ops!
 231         ++CurCycle;
 232     } else if (!HasNoopHazards) {
 233       // Otherwise, we have a pipeline stall, but no other problem, just advance
 234       // the current cycle and try again.
 235       DOUT << "*** Advancing cycle, no work to do\n";
 236       HazardRec->AdvanceCycle();
 237       ++NumStalls;
 238       ++CurCycle;
 239     } else {
 240       // Otherwise, we have no instructions to issue and we have instructions
 241       // that will fault if we don't do this right.  This is the case for
 242       // processors without pipeline interlocks and other cases.
 243       DOUT << "*** Emitting noop\n";
 244       HazardRec->EmitNoop();
 245       Sequence.push_back(0);   // NULL here means noop
 246       ++NumNoops;
 247       ++CurCycle;
 248     }
 249   }
 250
 251 #ifndef NDEBUG
 252   VerifySchedule(/*isBottomUp=*/false);
 253 #endif
 254 }
 255
 256 //===----------------------------------------------------------------------===//
 257 //                         Public Constructor Functions
 258 //===----------------------------------------------------------------------===//
 259
 260 /// createTDListDAGScheduler - This creates a top-down list scheduler with a
 261 /// new hazard recognizer. This scheduler takes ownership of the hazard
 262 /// recognizer and deletes it when done.
 263 ScheduleDAGSDNodes *
 264 llvm::createTDListDAGScheduler(SelectionDAGISel *IS, CodeGenOpt::Level) {
 265   return new ScheduleDAGList(*IS->MF,
 266                              new LatencyPriorityQueue(),
 267                              IS->CreateTargetHazardRecognizer());
 268 }