llvm/lib/CodeGen/SelectionDAG/ScheduleDAGVLIW.cpp

   1 //===- ScheduleDAGVLIW.cpp - SelectionDAG list scheduler for VLIW -*- C++ -*-=//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // This implements a top-down list scheduler, using standard algorithms.
  10 // The basic approach uses a priority queue of available nodes to schedule.
  11 // One at a time, nodes are taken from the priority queue (thus in priority
  12 // order), checked for legality to schedule, and emitted if legal.
  13 //
  14 // Nodes may not be legal to schedule either due to structural hazards (e.g.
  15 // pipeline or resource constraints) or because an input to the instruction has
  16 // not completed execution.
  17 //
  18 //===----------------------------------------------------------------------===//
  19
  20 #include "ScheduleDAGSDNodes.h"
  21 #include "llvm/ADT/Statistic.h"
  22 #include "llvm/CodeGen/ResourcePriorityQueue.h"
  23 #include "llvm/CodeGen/ScheduleHazardRecognizer.h"
  24 #include "llvm/CodeGen/SchedulerRegistry.h"
  25 #include "llvm/CodeGen/SelectionDAGISel.h"
  26 #include "llvm/CodeGen/TargetInstrInfo.h"
  27 #include "llvm/CodeGen/TargetSubtargetInfo.h"
  28 #include "llvm/Support/Debug.h"
  29 #include "llvm/Support/ErrorHandling.h"
  30 #include "llvm/Support/raw_ostream.h"
  31 using namespace llvm;
  32
  33 #define DEBUG_TYPE "pre-RA-sched"
  34
  35 STATISTIC(NumNoops , "Number of noops inserted");
  36 STATISTIC(NumStalls, "Number of pipeline stalls");
  37
  38 static RegisterScheduler
  39   VLIWScheduler("vliw-td", "VLIW scheduler",
  40                 createVLIWDAGScheduler);
  41
  42 namespace {
  43 //===----------------------------------------------------------------------===//
  44 /// ScheduleDAGVLIW - The actual DFA list scheduler implementation.  This
  45 /// supports / top-down scheduling.
  46 ///
  47 class ScheduleDAGVLIW : public ScheduleDAGSDNodes {
  48 private:
  49   /// AvailableQueue - The priority queue to use for the available SUnits.
  50   ///
  51   SchedulingPriorityQueue *AvailableQueue;
  52
  53   /// PendingQueue - This contains all of the instructions whose operands have
  54   /// been issued, but their results are not ready yet (due to the latency of
  55   /// the operation).  Once the operands become available, the instruction is
  56   /// added to the AvailableQueue.
  57   std::vector<SUnit*> PendingQueue;
  58
  59   /// HazardRec - The hazard recognizer to use.
  60   ScheduleHazardRecognizer *HazardRec;
  61
  62   /// AA - AAResults for making memory reference queries.
  63   AAResults *AA;
  64
  65 public:
  66   ScheduleDAGVLIW(MachineFunction &mf, AAResults *aa,
  67                   SchedulingPriorityQueue *availqueue)
  68       : ScheduleDAGSDNodes(mf), AvailableQueue(availqueue), AA(aa) {
  69     const TargetSubtargetInfo &STI = mf.getSubtarget();
  70     HazardRec = STI.getInstrInfo()->CreateTargetHazardRecognizer(&STI, this);
  71   }
  72
  73   ~ScheduleDAGVLIW() override {
  74     delete HazardRec;
  75     delete AvailableQueue;
  76   }
  77
  78   void Schedule() override;
  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 ScheduleDAGVLIW::Schedule() {
  90   LLVM_DEBUG(dbgs() << "********** List Scheduling " << printMBBReference(*BB)
  91                     << " '" << BB->getName() << "' **********\n");
  92
  93   // Build the scheduling graph.
  94   BuildSchedGraph(AA);
  95
  96   AvailableQueue->initNodes(SUnits);
  97
  98   listScheduleTopDown();
  99
 100   AvailableQueue->releaseState();
 101 }
 102
 103 //===----------------------------------------------------------------------===//
 104 //  Top-Down Scheduling
 105 //===----------------------------------------------------------------------===//
 106
 107 /// releaseSucc - Decrement the NumPredsLeft count of a successor. Add it to
 108 /// the PendingQueue if the count reaches zero. Also update its cycle bound.
 109 void ScheduleDAGVLIW::releaseSucc(SUnit *SU, const SDep &D) {
 110   SUnit *SuccSU = D.getSUnit();
 111
 112 #ifndef NDEBUG
 113   if (SuccSU->NumPredsLeft == 0) {
 114     dbgs() << "*** Scheduling failed! ***\n";
 115     dumpNode(*SuccSU);
 116     dbgs() << " has been released too many times!\n";
 117     llvm_unreachable(nullptr);
 118   }
 119 #endif
 120   assert(!D.isWeak() && "unexpected artificial DAG edge");
 121
 122   --SuccSU->NumPredsLeft;
 123
 124   SuccSU->setDepthToAtLeast(SU->getDepth() + D.getLatency());
 125
 126   // If all the node's predecessors are scheduled, this node is ready
 127   // to be scheduled. Ignore the special ExitSU node.
 128   if (SuccSU->NumPredsLeft == 0 && SuccSU != &ExitSU) {
 129     PendingQueue.push_back(SuccSU);
 130   }
 131 }
 132
 133 void ScheduleDAGVLIW::releaseSuccessors(SUnit *SU) {
 134   // Top down: release successors.
 135   for (SDep &Succ : SU->Succs) {
 136     assert(!Succ.isAssignedRegDep() &&
 137            "The list-td scheduler doesn't yet support physreg dependencies!");
 138
 139     releaseSucc(SU, Succ);
 140   }
 141 }
 142
 143 /// scheduleNodeTopDown - Add the node to the schedule. Decrement the pending
 144 /// count of its successors. If a successor pending count is zero, add it to
 145 /// the Available queue.
 146 void ScheduleDAGVLIW::scheduleNodeTopDown(SUnit *SU, unsigned CurCycle) {
 147   LLVM_DEBUG(dbgs() << "*** Scheduling [" << CurCycle << "]: ");
 148   LLVM_DEBUG(dumpNode(*SU));
 149
 150   Sequence.push_back(SU);
 151   assert(CurCycle >= SU->getDepth() && "Node scheduled above its depth!");
 152   SU->setDepthToAtLeast(CurCycle);
 153
 154   releaseSuccessors(SU);
 155   SU->isScheduled = true;
 156   AvailableQueue->scheduledNode(SU);
 157 }
 158
 159 /// listScheduleTopDown - The main loop of list scheduling for top-down
 160 /// schedulers.
 161 void ScheduleDAGVLIW::listScheduleTopDown() {
 162   unsigned CurCycle = 0;
 163
 164   // Release any successors of the special Entry node.
 165   releaseSuccessors(&EntrySU);
 166
 167   // All leaves to AvailableQueue.
 168   for (SUnit &SU : SUnits) {
 169     // It is available if it has no predecessors.
 170     if (SU.Preds.empty()) {
 171       AvailableQueue->push(&SU);
 172       SU.isAvailable = true;
 173     }
 174   }
 175
 176   // While AvailableQueue is not empty, grab the node with the highest
 177   // priority. If it is not ready put it back.  Schedule the node.
 178   std::vector<SUnit*> NotReady;
 179   Sequence.reserve(SUnits.size());
 180   while (!AvailableQueue->empty() || !PendingQueue.empty()) {
 181     // Check to see if any of the pending instructions are ready to issue.  If
 182     // so, add them to the available queue.
 183     for (unsigned i = 0, e = PendingQueue.size(); i != e; ++i) {
 184       if (PendingQueue[i]->getDepth() == CurCycle) {
 185         AvailableQueue->push(PendingQueue[i]);
 186         PendingQueue[i]->isAvailable = true;
 187         PendingQueue[i] = PendingQueue.back();
 188         PendingQueue.pop_back();
 189         --i; --e;
 190       }
 191       else {
 192         assert(PendingQueue[i]->getDepth() > CurCycle && "Negative latency?");
 193       }
 194     }
 195
 196     // If there are no instructions available, don't try to issue anything, and
 197     // don't advance the hazard recognizer.
 198     if (AvailableQueue->empty()) {
 199       // Reset DFA state.
 200       AvailableQueue->scheduledNode(nullptr);
 201       ++CurCycle;
 202       continue;
 203     }
 204
 205     SUnit *FoundSUnit = nullptr;
 206
 207     bool HasNoopHazards = false;
 208     while (!AvailableQueue->empty()) {
 209       SUnit *CurSUnit = AvailableQueue->pop();
 210
 211       ScheduleHazardRecognizer::HazardType HT =
 212         HazardRec->getHazardType(CurSUnit, 0/*no stalls*/);
 213       if (HT == ScheduleHazardRecognizer::NoHazard) {
 214         FoundSUnit = CurSUnit;
 215         break;
 216       }
 217
 218       // Remember if this is a noop hazard.
 219       HasNoopHazards |= HT == ScheduleHazardRecognizer::NoopHazard;
 220
 221       NotReady.push_back(CurSUnit);
 222     }
 223
 224     // Add the nodes that aren't ready back onto the available list.
 225     if (!NotReady.empty()) {
 226       AvailableQueue->push_all(NotReady);
 227       NotReady.clear();
 228     }
 229
 230     // If we found a node to schedule, do it now.
 231     if (FoundSUnit) {
 232       scheduleNodeTopDown(FoundSUnit, CurCycle);
 233       HazardRec->EmitInstruction(FoundSUnit);
 234
 235       // If this is a pseudo-op node, we don't want to increment the current
 236       // cycle.
 237       if (FoundSUnit->Latency)  // Don't increment CurCycle for pseudo-ops!
 238         ++CurCycle;
 239     } else if (!HasNoopHazards) {
 240       // Otherwise, we have a pipeline stall, but no other problem, just advance
 241       // the current cycle and try again.
 242       LLVM_DEBUG(dbgs() << "*** Advancing cycle, no work to do\n");
 243       HazardRec->AdvanceCycle();
 244       ++NumStalls;
 245       ++CurCycle;
 246     } else {
 247       // Otherwise, we have no instructions to issue and we have instructions
 248       // that will fault if we don't do this right.  This is the case for
 249       // processors without pipeline interlocks and other cases.
 250       LLVM_DEBUG(dbgs() << "*** Emitting noop\n");
 251       HazardRec->EmitNoop();
 252       Sequence.push_back(nullptr);   // NULL here means noop
 253       ++NumNoops;
 254       ++CurCycle;
 255     }
 256   }
 257
 258 #ifndef NDEBUG
 259   VerifyScheduledSequence(/*isBottomUp=*/false);
 260 #endif
 261 }
 262
 263 //===----------------------------------------------------------------------===//
 264 //                         Public Constructor Functions
 265 //===----------------------------------------------------------------------===//
 266
 267 /// createVLIWDAGScheduler - This creates a top-down list scheduler.
 268 ScheduleDAGSDNodes *llvm::createVLIWDAGScheduler(SelectionDAGISel *IS,
 269                                                  CodeGenOptLevel) {
 270   return new ScheduleDAGVLIW(*IS->MF, IS->AA, new ResourcePriorityQueue(IS));
 271 }