lib/CodeGen/MacroFusion.cpp

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