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      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/ErrorHandling.h"
     32 #include "llvm/Support/raw_ostream.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 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     : ScheduleDAGSDNodes(mf), AvailableQueue(availqueue) {
     68 
     69     const TargetMachine &tm = mf.getTarget();
     70     HazardRec = tm.getInstrInfo()->CreateTargetHazardRecognizer(&tm, this);
     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   DEBUG(dbgs() << "********** List Scheduling **********\n");
     91 
     92   // Build the scheduling graph.
     93   BuildSchedGraph(NULL);
     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 
    111 #ifndef NDEBUG
    112   if (SuccSU->NumPredsLeft == 0) {
    113     dbgs() << "*** Scheduling failed! ***\n";
    114     SuccSU->dump(this);
    115     dbgs() << " has been released too many times!\n";
    116     llvm_unreachable(0);
    117   }
    118 #endif
    119   --SuccSU->NumPredsLeft;
    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   DEBUG(dbgs() << "*** 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, 0/*no stalls*/);
    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       DEBUG(dbgs() << "*** 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       DEBUG(dbgs() << "*** 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.
    262 ScheduleDAGSDNodes *
    263 llvm::createTDListDAGScheduler(SelectionDAGISel *IS, CodeGenOpt::Level) {
    264   return new ScheduleDAGList(*IS->MF, new LatencyPriorityQueue());
    265 }
    266