llvm/lib/CodeGen/ScoreboardHazardRecognizer.cpp

   1 //===- ScoreboardHazardRecognizer.cpp - Scheduler Support -----------------===//
   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 file implements the ScoreboardHazardRecognizer class, which
  10 // encapsultes hazard-avoidance heuristics for scheduling, based on the
  11 // scheduling itineraries specified for the target.
  12 //
  13 //===----------------------------------------------------------------------===//
  14
  15 #include "llvm/CodeGen/ScoreboardHazardRecognizer.h"
  16 #include "llvm/CodeGen/ScheduleDAG.h"
  17 #include "llvm/CodeGen/TargetInstrInfo.h"
  18 #include "llvm/Config/llvm-config.h"
  19 #include "llvm/MC/MCInstrDesc.h"
  20 #include "llvm/MC/MCInstrItineraries.h"
  21 #include "llvm/Support/Compiler.h"
  22 #include "llvm/Support/Debug.h"
  23 #include "llvm/Support/raw_ostream.h"
  24 #include <cassert>
  25
  26 using namespace llvm;
  27
  28 #define DEBUG_TYPE DebugType
  29
  30 ScoreboardHazardRecognizer::ScoreboardHazardRecognizer(
  31     const InstrItineraryData *II, const ScheduleDAG *SchedDAG,
  32     const char *ParentDebugType)
  33     : DebugType(ParentDebugType), ItinData(II), DAG(SchedDAG) {
  34   (void)DebugType;
  35   // Determine the maximum depth of any itinerary. This determines the depth of
  36   // the scoreboard. We always make the scoreboard at least 1 cycle deep to
  37   // avoid dealing with the boundary condition.
  38   unsigned ScoreboardDepth = 1;
  39   if (ItinData && !ItinData->isEmpty()) {
  40     for (unsigned idx = 0; ; ++idx) {
  41       if (ItinData->isEndMarker(idx))
  42         break;
  43
  44       const InstrStage *IS = ItinData->beginStage(idx);
  45       const InstrStage *E = ItinData->endStage(idx);
  46       unsigned CurCycle = 0;
  47       unsigned ItinDepth = 0;
  48       for (; IS != E; ++IS) {
  49         unsigned StageDepth = CurCycle + IS->getCycles();
  50         if (ItinDepth < StageDepth) ItinDepth = StageDepth;
  51         CurCycle += IS->getNextCycles();
  52       }
  53
  54       // Find the next power-of-2 >= ItinDepth
  55       while (ItinDepth > ScoreboardDepth) {
  56         ScoreboardDepth *= 2;
  57         // Don't set MaxLookAhead until we find at least one nonzero stage.
  58         // This way, an itinerary with no stages has MaxLookAhead==0, which
  59         // completely bypasses the scoreboard hazard logic.
  60         MaxLookAhead = ScoreboardDepth;
  61       }
  62     }
  63   }
  64
  65   ReservedScoreboard.reset(ScoreboardDepth);
  66   RequiredScoreboard.reset(ScoreboardDepth);
  67
  68   // If MaxLookAhead is not set above, then we are not enabled.
  69   if (!isEnabled())
  70     LLVM_DEBUG(dbgs() << "Disabled scoreboard hazard recognizer\n");
  71   else {
  72     // A nonempty itinerary must have a SchedModel.
  73     IssueWidth = ItinData->SchedModel.IssueWidth;
  74     LLVM_DEBUG(dbgs() << "Using scoreboard hazard recognizer: Depth = "
  75                       << ScoreboardDepth << '\n');
  76   }
  77 }
  78
  79 void ScoreboardHazardRecognizer::Reset() {
  80   IssueCount = 0;
  81   RequiredScoreboard.reset();
  82   ReservedScoreboard.reset();
  83 }
  84
  85 #if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
  86 LLVM_DUMP_METHOD void ScoreboardHazardRecognizer::Scoreboard::dump() const {
  87   dbgs() << "Scoreboard:\n";
  88
  89   unsigned last = Depth - 1;
  90   while ((last > 0) && ((*this)[last] == 0))
  91     last--;
  92
  93   for (unsigned i = 0; i <= last; i++) {
  94     InstrStage::FuncUnits FUs = (*this)[i];
  95     dbgs() << "\t";
  96     for (int j = std::numeric_limits<InstrStage::FuncUnits>::digits - 1;
  97          j >= 0; j--)
  98       dbgs() << ((FUs & (1ULL << j)) ? '1' : '0');
  99     dbgs() << '\n';
 100   }
 101 }
 102 #endif
 103
 104 bool ScoreboardHazardRecognizer::atIssueLimit() const {
 105   if (IssueWidth == 0)
 106     return false;
 107
 108   return IssueCount == IssueWidth;
 109 }
 110
 111 ScheduleHazardRecognizer::HazardType
 112 ScoreboardHazardRecognizer::getHazardType(SUnit *SU, int Stalls) {
 113   if (!ItinData || ItinData->isEmpty())
 114     return NoHazard;
 115
 116   // Note that stalls will be negative for bottom-up scheduling.
 117   int cycle = Stalls;
 118
 119   // Use the itinerary for the underlying instruction to check for
 120   // free FU's in the scoreboard at the appropriate future cycles.
 121
 122   const MCInstrDesc *MCID = DAG->getInstrDesc(SU);
 123   if (!MCID) {
 124     // Don't check hazards for non-machineinstr Nodes.
 125     return NoHazard;
 126   }
 127   unsigned idx = MCID->getSchedClass();
 128   for (const InstrStage *IS = ItinData->beginStage(idx),
 129          *E = ItinData->endStage(idx); IS != E; ++IS) {
 130     // We must find one of the stage's units free for every cycle the
 131     // stage is occupied. FIXME it would be more accurate to find the
 132     // same unit free in all the cycles.
 133     for (unsigned int i = 0; i < IS->getCycles(); ++i) {
 134       int StageCycle = cycle + (int)i;
 135       if (StageCycle < 0)
 136         continue;
 137
 138       if (StageCycle >= (int)RequiredScoreboard.getDepth()) {
 139         assert((StageCycle - Stalls) < (int)RequiredScoreboard.getDepth() &&
 140                "Scoreboard depth exceeded!");
 141         // This stage was stalled beyond pipeline depth, so cannot conflict.
 142         break;
 143       }
 144
 145       InstrStage::FuncUnits freeUnits = IS->getUnits();
 146       switch (IS->getReservationKind()) {
 147       case InstrStage::Required:
 148         // Required FUs conflict with both reserved and required ones
 149         freeUnits &= ~ReservedScoreboard[StageCycle];
 150         [[fallthrough]];
 151       case InstrStage::Reserved:
 152         // Reserved FUs can conflict only with required ones.
 153         freeUnits &= ~RequiredScoreboard[StageCycle];
 154         break;
 155       }
 156
 157       if (!freeUnits) {
 158         LLVM_DEBUG(dbgs() << "*** Hazard in cycle +" << StageCycle << ", ");
 159         LLVM_DEBUG(DAG->dumpNode(*SU));
 160         return Hazard;
 161       }
 162     }
 163
 164     // Advance the cycle to the next stage.
 165     cycle += IS->getNextCycles();
 166   }
 167
 168   return NoHazard;
 169 }
 170
 171 void ScoreboardHazardRecognizer::EmitInstruction(SUnit *SU) {
 172   if (!ItinData || ItinData->isEmpty())
 173     return;
 174
 175   // Use the itinerary for the underlying instruction to reserve FU's
 176   // in the scoreboard at the appropriate future cycles.
 177   const MCInstrDesc *MCID = DAG->getInstrDesc(SU);
 178   assert(MCID && "The scheduler must filter non-machineinstrs");
 179   if (DAG->TII->isZeroCost(MCID->Opcode))
 180     return;
 181
 182   ++IssueCount;
 183
 184   unsigned cycle = 0;
 185
 186   unsigned idx = MCID->getSchedClass();
 187   for (const InstrStage *IS = ItinData->beginStage(idx),
 188          *E = ItinData->endStage(idx); IS != E; ++IS) {
 189     // We must reserve one of the stage's units for every cycle the
 190     // stage is occupied. FIXME it would be more accurate to reserve
 191     // the same unit free in all the cycles.
 192     for (unsigned int i = 0; i < IS->getCycles(); ++i) {
 193       assert(((cycle + i) < RequiredScoreboard.getDepth()) &&
 194              "Scoreboard depth exceeded!");
 195
 196       InstrStage::FuncUnits freeUnits = IS->getUnits();
 197       switch (IS->getReservationKind()) {
 198       case InstrStage::Required:
 199         // Required FUs conflict with both reserved and required ones
 200         freeUnits &= ~ReservedScoreboard[cycle + i];
 201         [[fallthrough]];
 202       case InstrStage::Reserved:
 203         // Reserved FUs can conflict only with required ones.
 204         freeUnits &= ~RequiredScoreboard[cycle + i];
 205         break;
 206       }
 207
 208       // reduce to a single unit
 209       InstrStage::FuncUnits freeUnit = 0;
 210       do {
 211         freeUnit = freeUnits;
 212         freeUnits = freeUnit & (freeUnit - 1);
 213       } while (freeUnits);
 214
 215       if (IS->getReservationKind() == InstrStage::Required)
 216         RequiredScoreboard[cycle + i] |= freeUnit;
 217       else
 218         ReservedScoreboard[cycle + i] |= freeUnit;
 219     }
 220
 221     // Advance the cycle to the next stage.
 222     cycle += IS->getNextCycles();
 223   }
 224
 225   LLVM_DEBUG(ReservedScoreboard.dump());
 226   LLVM_DEBUG(RequiredScoreboard.dump());
 227 }
 228
 229 void ScoreboardHazardRecognizer::AdvanceCycle() {
 230   IssueCount = 0;
 231   ReservedScoreboard[0] = 0; ReservedScoreboard.advance();
 232   RequiredScoreboard[0] = 0; RequiredScoreboard.advance();
 233 }
 234
 235 void ScoreboardHazardRecognizer::RecedeCycle() {
 236   IssueCount = 0;
 237   ReservedScoreboard[ReservedScoreboard.getDepth()-1] = 0;
 238   ReservedScoreboard.recede();
 239   RequiredScoreboard[RequiredScoreboard.getDepth()-1] = 0;
 240   RequiredScoreboard.recede();
 241 }