llvm/lib/CodeGen/MacroFusion.cpp

   1 //===- MacroFusion.cpp - Macro Fusion -------------------------------------===//
   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 /// \file This file contains the implementation of the DAG scheduling mutation
  10 /// to pair instructions back to back.
  11 //
  12 //===----------------------------------------------------------------------===//
  13
  14 #include "llvm/CodeGen/MacroFusion.h"
  15 #include "llvm/ADT/Statistic.h"
  16 #include "llvm/CodeGen/MachineInstr.h"
  17 #include "llvm/CodeGen/ScheduleDAG.h"
  18 #include "llvm/CodeGen/ScheduleDAGInstrs.h"
  19 #include "llvm/CodeGen/ScheduleDAGMutation.h"
  20 #include "llvm/CodeGen/TargetInstrInfo.h"
  21 #include "llvm/Support/CommandLine.h"
  22 #include "llvm/Support/Debug.h"
  23 #include "llvm/Support/raw_ostream.h"
  24
  25 #define DEBUG_TYPE "machine-scheduler"
  26
  27 STATISTIC(NumFused, "Number of instr pairs fused");
  28
  29 using namespace llvm;
  30
  31 static cl::opt<bool> EnableMacroFusion("misched-fusion", cl::Hidden,
  32   cl::desc("Enable scheduling for macro fusion."), cl::init(true));
  33
  34 static bool isHazard(const SDep &Dep) {
  35   return Dep.getKind() == SDep::Anti || Dep.getKind() == SDep::Output;
  36 }
  37
  38 static SUnit *getPredClusterSU(const SUnit &SU) {
  39   for (const SDep &SI : SU.Preds)
  40     if (SI.isCluster())
  41       return SI.getSUnit();
  42
  43   return nullptr;
  44 }
  45
  46 bool llvm::hasLessThanNumFused(const SUnit &SU, unsigned FuseLimit) {
  47   unsigned Num = 1;
  48   const SUnit *CurrentSU = &SU;
  49   while ((CurrentSU = getPredClusterSU(*CurrentSU)) && Num < FuseLimit) Num ++;
  50   return Num < FuseLimit;
  51 }
  52
  53 bool llvm::fuseInstructionPair(ScheduleDAGInstrs &DAG, SUnit &FirstSU,
  54                                SUnit &SecondSU) {
  55   // Check that neither instr is already paired with another along the edge
  56   // between them.
  57   for (SDep &SI : FirstSU.Succs)
  58     if (SI.isCluster())
  59       return false;
  60
  61   for (SDep &SI : SecondSU.Preds)
  62     if (SI.isCluster())
  63       return false;
  64   // Though the reachability checks above could be made more generic,
  65   // perhaps as part of ScheduleDAGInstrs::addEdge(), since such edges are valid,
  66   // the extra computation cost makes it less interesting in general cases.
  67
  68   // Create a single weak edge between the adjacent instrs. The only effect is
  69   // to cause bottom-up scheduling to heavily prioritize the clustered instrs.
  70   if (!DAG.addEdge(&SecondSU, SDep(&FirstSU, SDep::Cluster)))
  71     return false;
  72
  73   // TODO - If we want to chain more than two instructions, we need to create
  74   // artifical edges to make dependencies from the FirstSU also dependent
  75   // on other chained instructions, and other chained instructions also
  76   // dependent on the dependencies of the SecondSU, to prevent them from being
  77   // scheduled into these chained instructions.
  78   assert(hasLessThanNumFused(FirstSU, 2) &&
  79          "Currently we only support chaining together two instructions");
  80
  81   // Adjust the latency between both instrs.
  82   for (SDep &SI : FirstSU.Succs)
  83     if (SI.getSUnit() == &SecondSU)
  84       SI.setLatency(0);
  85
  86   for (SDep &SI : SecondSU.Preds)
  87     if (SI.getSUnit() == &FirstSU)
  88       SI.setLatency(0);
  89
  90   LLVM_DEBUG(
  91       dbgs() << "Macro fuse: "; DAG.dumpNodeName(FirstSU); dbgs() << " - ";
  92       DAG.dumpNodeName(SecondSU); dbgs() << " /  ";
  93       dbgs() << DAG.TII->getName(FirstSU.getInstr()->getOpcode()) << " - "
  94              << DAG.TII->getName(SecondSU.getInstr()->getOpcode()) << '\n';);
  95
  96   // Make data dependencies from the FirstSU also dependent on the SecondSU to
  97   // prevent them from being scheduled between the FirstSU and the SecondSU.
  98   if (&SecondSU != &DAG.ExitSU)
  99     for (const SDep &SI : FirstSU.Succs) {
 100       SUnit *SU = SI.getSUnit();
 101       if (SI.isWeak() || isHazard(SI) ||
 102           SU == &DAG.ExitSU || SU == &SecondSU || SU->isPred(&SecondSU))
 103         continue;
 104       LLVM_DEBUG(dbgs() << "  Bind "; DAG.dumpNodeName(SecondSU);
 105                  dbgs() << " - "; DAG.dumpNodeName(*SU); dbgs() << '\n';);
 106       DAG.addEdge(SU, SDep(&SecondSU, SDep::Artificial));
 107     }
 108
 109   // Make the FirstSU also dependent on the dependencies of the SecondSU to
 110   // prevent them from being scheduled between the FirstSU and the SecondSU.
 111   if (&FirstSU != &DAG.EntrySU) {
 112     for (const SDep &SI : SecondSU.Preds) {
 113       SUnit *SU = SI.getSUnit();
 114       if (SI.isWeak() || isHazard(SI) || &FirstSU == SU || FirstSU.isSucc(SU))
 115         continue;
 116       LLVM_DEBUG(dbgs() << "  Bind "; DAG.dumpNodeName(*SU); dbgs() << " - ";
 117                  DAG.dumpNodeName(FirstSU); dbgs() << '\n';);
 118       DAG.addEdge(&FirstSU, SDep(SU, SDep::Artificial));
 119     }
 120     // ExitSU comes last by design, which acts like an implicit dependency
 121     // between ExitSU and any bottom root in the graph. We should transfer
 122     // this to FirstSU as well.
 123     if (&SecondSU == &DAG.ExitSU) {
 124       for (SUnit &SU : DAG.SUnits) {
 125         if (SU.Succs.empty())
 126           DAG.addEdge(&FirstSU, SDep(&SU, SDep::Artificial));
 127       }
 128     }
 129   }
 130
 131   ++NumFused;
 132   return true;
 133 }
 134
 135 namespace {
 136
 137 /// Post-process the DAG to create cluster edges between instrs that may
 138 /// be fused by the processor into a single operation.
 139 class MacroFusion : public ScheduleDAGMutation {
 140   std::vector<MacroFusionPredTy> Predicates;
 141   bool FuseBlock;
 142   bool scheduleAdjacentImpl(ScheduleDAGInstrs &DAG, SUnit &AnchorSU);
 143
 144 public:
 145   MacroFusion(ArrayRef<MacroFusionPredTy> Predicates, bool FuseBlock)
 146       : Predicates(Predicates.begin(), Predicates.end()), FuseBlock(FuseBlock) {
 147   }
 148
 149   void apply(ScheduleDAGInstrs *DAGInstrs) override;
 150
 151   bool shouldScheduleAdjacent(const TargetInstrInfo &TII,
 152                               const TargetSubtargetInfo &STI,
 153                               const MachineInstr *FirstMI,
 154                               const MachineInstr &SecondMI);
 155 };
 156
 157 } // end anonymous namespace
 158
 159 bool MacroFusion::shouldScheduleAdjacent(const TargetInstrInfo &TII,
 160                                          const TargetSubtargetInfo &STI,
 161                                          const MachineInstr *FirstMI,
 162                                          const MachineInstr &SecondMI) {
 163   return llvm::any_of(Predicates, [&](MacroFusionPredTy Predicate) {
 164     return Predicate(TII, STI, FirstMI, SecondMI);
 165   });
 166 }
 167
 168 void MacroFusion::apply(ScheduleDAGInstrs *DAG) {
 169   if (FuseBlock)
 170     // For each of the SUnits in the scheduling block, try to fuse the instr in
 171     // it with one in its predecessors.
 172     for (SUnit &ISU : DAG->SUnits)
 173         scheduleAdjacentImpl(*DAG, ISU);
 174
 175   if (DAG->ExitSU.getInstr())
 176     // Try to fuse the instr in the ExitSU with one in its predecessors.
 177     scheduleAdjacentImpl(*DAG, DAG->ExitSU);
 178 }
 179
 180 /// Implement the fusion of instr pairs in the scheduling DAG,
 181 /// anchored at the instr in AnchorSU..
 182 bool MacroFusion::scheduleAdjacentImpl(ScheduleDAGInstrs &DAG, SUnit &AnchorSU) {
 183   const MachineInstr &AnchorMI = *AnchorSU.getInstr();
 184   const TargetInstrInfo &TII = *DAG.TII;
 185   const TargetSubtargetInfo &ST = DAG.MF.getSubtarget();
 186
 187   // Check if the anchor instr may be fused.
 188   if (!shouldScheduleAdjacent(TII, ST, nullptr, AnchorMI))
 189     return false;
 190
 191   // Explorer for fusion candidates among the dependencies of the anchor instr.
 192   for (SDep &Dep : AnchorSU.Preds) {
 193     // Ignore dependencies other than data or strong ordering.
 194     if (Dep.isWeak() || isHazard(Dep))
 195       continue;
 196
 197     SUnit &DepSU = *Dep.getSUnit();
 198     if (DepSU.isBoundaryNode())
 199       continue;
 200
 201     // Only chain two instructions together at most.
 202     const MachineInstr *DepMI = DepSU.getInstr();
 203     if (!hasLessThanNumFused(DepSU, 2) ||
 204         !shouldScheduleAdjacent(TII, ST, DepMI, AnchorMI))
 205       continue;
 206
 207     if (fuseInstructionPair(DAG, DepSU, AnchorSU))
 208       return true;
 209   }
 210
 211   return false;
 212 }
 213
 214 std::unique_ptr<ScheduleDAGMutation>
 215 llvm::createMacroFusionDAGMutation(ArrayRef<MacroFusionPredTy> Predicates,
 216                                    bool BranchOnly) {
 217   if (EnableMacroFusion)
 218     return std::make_unique<MacroFusion>(Predicates, !BranchOnly);
 219   return nullptr;
 220 }