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/Support/ErrorHandling.h"
  33 #include "llvm/ADT/PriorityQueue.h"
  34 #include "llvm/ADT/Statistic.h"
  35 #include <climits>
  36 using namespace llvm;
  37
  38 STATISTIC(NumNoops , "Number of noops inserted");
  39 STATISTIC(NumStalls, "Number of pipeline stalls");
  40
  41 static RegisterScheduler
  42   tdListDAGScheduler("list-td", "Top-down list scheduler",
  43                      createTDListDAGScheduler);
  44
  45 namespace {
  46 //===----------------------------------------------------------------------===//
  47 /// ScheduleDAGList - The actual list scheduler implementation.  This supports
  48 /// top-down scheduling.
  49 ///
  50 class VISIBILITY_HIDDEN ScheduleDAGList : public ScheduleDAGSDNodes {
  51 private:
  52   /// AvailableQueue - The priority queue to use for the available SUnits.
  53   ///
  54   SchedulingPriorityQueue *AvailableQueue;
  55
  56   /// PendingQueue - This contains all of the instructions whose operands have
  57   /// been issued, but their results are not ready yet (due to the latency of
  58   /// the operation).  Once the operands become available, the instruction is
  59   /// added to the AvailableQueue.
  60   std::vector<SUnit*> PendingQueue;
  61
  62   /// HazardRec - The hazard recognizer to use.
  63   ScheduleHazardRecognizer *HazardRec;
  64
  65 public:
  66   ScheduleDAGList(MachineFunction &mf,
  67                   SchedulingPriorityQueue *availqueue,
  68                   ScheduleHazardRecognizer *HR)
  69     : ScheduleDAGSDNodes(mf),
  70       AvailableQueue(availqueue), HazardRec(HR) {
  71     }
  72
  73   ~ScheduleDAGList() {
  74     delete HazardRec;
  75     delete AvailableQueue;
  76   }
  77
  78   void Schedule();
  79
  80 private:
  81   void ReleaseSucc(SUnit *SU, const SDep &D);
  82   void ReleaseSuccessors(SUnit *SU);
  83   void ScheduleNodeTopDown(SUnit *SU, unsigned CurCycle);
  84   void ListScheduleTopDown();
  85 };
  86 }  // end anonymous namespace
  87
  88 /// Schedule - Schedule the DAG using list scheduling.
  89 void ScheduleDAGList::Schedule() {
  90   DOUT << "********** List Scheduling **********\n";
  91
  92   // Build the scheduling graph.
  93   BuildSchedGraph();
  94
  95   AvailableQueue->initNodes(SUnits);
  96
  97   ListScheduleTopDown();
  98
  99   AvailableQueue->releaseState();
 100 }
 101
 102 //===----------------------------------------------------------------------===//
 103 //  Top-Down Scheduling
 104 //===----------------------------------------------------------------------===//
 105
 106 /// ReleaseSucc - Decrement the NumPredsLeft count of a successor. Add it to
 107 /// the PendingQueue if the count reaches zero. Also update its cycle bound.
 108 void ScheduleDAGList::ReleaseSucc(SUnit *SU, const SDep &D) {
 109   SUnit *SuccSU = D.getSUnit();
 110   --SuccSU->NumPredsLeft;
 111
 112 #ifndef NDEBUG
 113   if (SuccSU->NumPredsLeft < 0) {
 114     cerr << "*** Scheduling failed! ***\n";
 115     SuccSU->dump(this);
 116     cerr << " has been released too many times!\n";
 117     llvm_unreachable(0);
 118   }
 119 #endif
 120
 121   SuccSU->setDepthToAtLeast(SU->getDepth() + D.getLatency());
 122
 123   // If all the node's predecessors are scheduled, this node is ready
 124   // to be scheduled. Ignore the special ExitSU node.
 125   if (SuccSU->NumPredsLeft == 0 && SuccSU != &ExitSU)
 126     PendingQueue.push_back(SuccSU);
 127 }
 128
 129 void ScheduleDAGList::ReleaseSuccessors(SUnit *SU) {
 130   // Top down: release successors.
 131   for (SUnit::succ_iterator I = SU->Succs.begin(), E = SU->Succs.end();
 132        I != E; ++I) {
 133     assert(!I->isAssignedRegDep() &&
 134            "The list-td scheduler doesn't yet support physreg dependencies!");
 135
 136     ReleaseSucc(SU, *I);
 137   }
 138 }
 139
 140 /// ScheduleNodeTopDown - Add the node to the schedule. Decrement the pending
 141 /// count of its successors. If a successor pending count is zero, add it to
 142 /// the Available queue.
 143 void ScheduleDAGList::ScheduleNodeTopDown(SUnit *SU, unsigned CurCycle) {
 144   DOUT << "*** Scheduling [" << CurCycle << "]: ";
 145   DEBUG(SU->dump(this));
 146
 147   Sequence.push_back(SU);
 148   assert(CurCycle >= SU->getDepth() && "Node scheduled above its depth!");
 149   SU->setDepthToAtLeast(CurCycle);
 150
 151   ReleaseSuccessors(SU);
 152   SU->isScheduled = true;
 153   AvailableQueue->ScheduledNode(SU);
 154 }
 155
 156 /// ListScheduleTopDown - The main loop of list scheduling for top-down
 157 /// schedulers.
 158 void ScheduleDAGList::ListScheduleTopDown() {
 159   unsigned CurCycle = 0;
 160
 161   // Release any successors of the special Entry node.
 162   ReleaseSuccessors(&EntrySU);
 163
 164   // All leaves to Available queue.
 165   for (unsigned i = 0, e = SUnits.size(); i != e; ++i) {
 166     // It is available if it has no predecessors.
 167     if (SUnits[i].Preds.empty()) {
 168       AvailableQueue->push(&SUnits[i]);
 169       SUnits[i].isAvailable = true;
 170     }
 171   }
 172
 173   // While Available queue is not empty, grab the node with the highest
 174   // priority. If it is not ready put it back.  Schedule the node.
 175   std::vector<SUnit*> NotReady;
 176   Sequence.reserve(SUnits.size());
 177   while (!AvailableQueue->empty() || !PendingQueue.empty()) {
 178     // Check to see if any of the pending instructions are ready to issue.  If
 179     // so, add them to the available queue.
 180     for (unsigned i = 0, e = PendingQueue.size(); i != e; ++i) {
 181       if (PendingQueue[i]->getDepth() == CurCycle) {
 182         AvailableQueue->push(PendingQueue[i]);
 183         PendingQueue[i]->isAvailable = true;
 184         PendingQueue[i] = PendingQueue.back();
 185         PendingQueue.pop_back();
 186         --i; --e;
 187       } else {
 188         assert(PendingQueue[i]->getDepth() > CurCycle && "Negative latency?");
 189       }
 190     }
 191
 192     // If there are no instructions available, don't try to issue anything, and
 193     // don't advance the hazard recognizer.
 194     if (AvailableQueue->empty()) {
 195       ++CurCycle;
 196       continue;
 197     }
 198
 199     SUnit *FoundSUnit = 0;
 200
 201     bool HasNoopHazards = false;
 202     while (!AvailableQueue->empty()) {
 203       SUnit *CurSUnit = AvailableQueue->pop();
 204
 205       ScheduleHazardRecognizer::HazardType HT =
 206         HazardRec->getHazardType(CurSUnit);
 207       if (HT == ScheduleHazardRecognizer::NoHazard) {
 208         FoundSUnit = CurSUnit;
 209         break;
 210       }
 211
 212       // Remember if this is a noop hazard.
 213       HasNoopHazards |= HT == ScheduleHazardRecognizer::NoopHazard;
 214
 215       NotReady.push_back(CurSUnit);
 216     }
 217
 218     // Add the nodes that aren't ready back onto the available list.
 219     if (!NotReady.empty()) {
 220       AvailableQueue->push_all(NotReady);
 221       NotReady.clear();
 222     }
 223
 224     // If we found a node to schedule, do it now.
 225     if (FoundSUnit) {
 226       ScheduleNodeTopDown(FoundSUnit, CurCycle);
 227       HazardRec->EmitInstruction(FoundSUnit);
 228
 229       // If this is a pseudo-op node, we don't want to increment the current
 230       // cycle.
 231       if (FoundSUnit->Latency)  // Don't increment CurCycle for pseudo-ops!
 232         ++CurCycle;
 233     } else if (!HasNoopHazards) {
 234       // Otherwise, we have a pipeline stall, but no other problem, just advance
 235       // the current cycle and try again.
 236       DOUT << "*** Advancing cycle, no work to do\n";
 237       HazardRec->AdvanceCycle();
 238       ++NumStalls;
 239       ++CurCycle;
 240     } else {
 241       // Otherwise, we have no instructions to issue and we have instructions
 242       // that will fault if we don't do this right.  This is the case for
 243       // processors without pipeline interlocks and other cases.
 244       DOUT << "*** Emitting noop\n";
 245       HazardRec->EmitNoop();
 246       Sequence.push_back(0);   // NULL here means noop
 247       ++NumNoops;
 248       ++CurCycle;
 249     }
 250   }
 251
 252 #ifndef NDEBUG
 253   VerifySchedule(/*isBottomUp=*/false);
 254 #endif
 255 }
 256
 257 //===----------------------------------------------------------------------===//
 258 //                         Public Constructor Functions
 259 //===----------------------------------------------------------------------===//
 260
 261 /// createTDListDAGScheduler - This creates a top-down list scheduler with a
 262 /// new hazard recognizer. This scheduler takes ownership of the hazard
 263 /// recognizer and deletes it when done.
 264 ScheduleDAGSDNodes *
 265 llvm::createTDListDAGScheduler(SelectionDAGISel *IS, CodeGenOpt::Level) {
 266   return new ScheduleDAGList(*IS->MF,
 267                              new LatencyPriorityQueue(),
 268                              IS->CreateTargetHazardRecognizer());
 269 }