lib/Target/X86/X86InsertPrefetch.cpp

   1 //===------- X86InsertPrefetch.cpp - Insert cache prefetch hints ----------===//
   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 pass applies cache prefetch instructions based on a profile. The pass
  10 // assumes DiscriminateMemOps ran immediately before, to ensure debug info
  11 // matches the one used at profile generation time. The profile is encoded in
  12 // afdo format (text or binary). It contains prefetch hints recommendations.
  13 // Each recommendation is made in terms of debug info locations, a type (i.e.
  14 // nta, t{0|1|2}) and a delta. The debug info identifies an instruction with a
  15 // memory operand (see X86DiscriminateMemOps). The prefetch will be made for
  16 // a location at that memory operand + the delta specified in the
  17 // recommendation.
  18 //
  19 //===----------------------------------------------------------------------===//
  20
  21 #include "X86.h"
  22 #include "X86InstrBuilder.h"
  23 #include "X86InstrInfo.h"
  24 #include "X86MachineFunctionInfo.h"
  25 #include "X86Subtarget.h"
  26 #include "llvm/CodeGen/MachineModuleInfo.h"
  27 #include "llvm/IR/DebugInfoMetadata.h"
  28 #include "llvm/ProfileData/SampleProf.h"
  29 #include "llvm/ProfileData/SampleProfReader.h"
  30 #include "llvm/Transforms/IPO/SampleProfile.h"
  31 using namespace llvm;
  32 using namespace sampleprof;
  33
  34 static cl::opt<std::string>
  35     PrefetchHintsFile("prefetch-hints-file",
  36                       cl::desc("Path to the prefetch hints profile. See also "
  37                                "-x86-discriminate-memops"),
  38                       cl::Hidden);
  39 namespace {
  40
  41 class X86InsertPrefetch : public MachineFunctionPass {
  42   void getAnalysisUsage(AnalysisUsage &AU) const override;
  43   bool doInitialization(Module &) override;
  44
  45   bool runOnMachineFunction(MachineFunction &MF) override;
  46   struct PrefetchInfo {
  47     unsigned InstructionID;
  48     int64_t Delta;
  49   };
  50   typedef SmallVectorImpl<PrefetchInfo> Prefetches;
  51   bool findPrefetchInfo(const FunctionSamples *Samples, const MachineInstr &MI,
  52                         Prefetches &prefetches) const;
  53
  54 public:
  55   static char ID;
  56   X86InsertPrefetch(const std::string &PrefetchHintsFilename);
  57   StringRef getPassName() const override {
  58     return "X86 Insert Cache Prefetches";
  59   }
  60
  61 private:
  62   std::string Filename;
  63   std::unique_ptr<SampleProfileReader> Reader;
  64 };
  65
  66 using PrefetchHints = SampleRecord::CallTargetMap;
  67
  68 // Return any prefetching hints for the specified MachineInstruction. The hints
  69 // are returned as pairs (name, delta).
  70 ErrorOr<PrefetchHints> getPrefetchHints(const FunctionSamples *TopSamples,
  71                                         const MachineInstr &MI) {
  72   if (const auto &Loc = MI.getDebugLoc())
  73     if (const auto *Samples = TopSamples->findFunctionSamples(Loc))
  74       return Samples->findCallTargetMapAt(FunctionSamples::getOffset(Loc),
  75                                           Loc->getBaseDiscriminator());
  76   return std::error_code();
  77 }
  78
  79 // The prefetch instruction can't take memory operands involving vector
  80 // registers.
  81 bool IsMemOpCompatibleWithPrefetch(const MachineInstr &MI, int Op) {
  82   Register BaseReg = MI.getOperand(Op + X86::AddrBaseReg).getReg();
  83   Register IndexReg = MI.getOperand(Op + X86::AddrIndexReg).getReg();
  84   return (BaseReg == 0 ||
  85           X86MCRegisterClasses[X86::GR64RegClassID].contains(BaseReg) ||
  86           X86MCRegisterClasses[X86::GR32RegClassID].contains(BaseReg)) &&
  87          (IndexReg == 0 ||
  88           X86MCRegisterClasses[X86::GR64RegClassID].contains(IndexReg) ||
  89           X86MCRegisterClasses[X86::GR32RegClassID].contains(IndexReg));
  90 }
  91
  92 } // end anonymous namespace
  93
  94 //===----------------------------------------------------------------------===//
  95 //            Implementation
  96 //===----------------------------------------------------------------------===//
  97
  98 char X86InsertPrefetch::ID = 0;
  99
 100 X86InsertPrefetch::X86InsertPrefetch(const std::string &PrefetchHintsFilename)
 101     : MachineFunctionPass(ID), Filename(PrefetchHintsFilename) {}
 102
 103 /// Return true if the provided MachineInstruction has cache prefetch hints. In
 104 /// that case, the prefetch hints are stored, in order, in the Prefetches
 105 /// vector.
 106 bool X86InsertPrefetch::findPrefetchInfo(const FunctionSamples *TopSamples,
 107                                          const MachineInstr &MI,
 108                                          Prefetches &Prefetches) const {
 109   assert(Prefetches.empty() &&
 110          "Expected caller passed empty PrefetchInfo vector.");
 111   static constexpr std::pair<StringLiteral, unsigned> HintTypes[] = {
 112       {"_nta_", X86::PREFETCHNTA},
 113       {"_t0_", X86::PREFETCHT0},
 114       {"_t1_", X86::PREFETCHT1},
 115       {"_t2_", X86::PREFETCHT2},
 116   };
 117   static const char *SerializedPrefetchPrefix = "__prefetch";
 118
 119   const ErrorOr<PrefetchHints> T = getPrefetchHints(TopSamples, MI);
 120   if (!T)
 121     return false;
 122   int16_t max_index = -1;
 123   // Convert serialized prefetch hints into PrefetchInfo objects, and populate
 124   // the Prefetches vector.
 125   for (const auto &S_V : *T) {
 126     StringRef Name = S_V.getKey();
 127     if (Name.consume_front(SerializedPrefetchPrefix)) {
 128       int64_t D = static_cast<int64_t>(S_V.second);
 129       unsigned IID = 0;
 130       for (const auto &HintType : HintTypes) {
 131         if (Name.startswith(HintType.first)) {
 132           Name = Name.drop_front(HintType.first.size());
 133           IID = HintType.second;
 134           break;
 135         }
 136       }
 137       if (IID == 0)
 138         return false;
 139       uint8_t index = 0;
 140       Name.consumeInteger(10, index);
 141
 142       if (index >= Prefetches.size())
 143         Prefetches.resize(index + 1);
 144       Prefetches[index] = {IID, D};
 145       max_index = std::max(max_index, static_cast<int16_t>(index));
 146     }
 147   }
 148   assert(max_index + 1 >= 0 &&
 149          "Possible overflow: max_index + 1 should be positive.");
 150   assert(static_cast<size_t>(max_index + 1) == Prefetches.size() &&
 151          "The number of prefetch hints received should match the number of "
 152          "PrefetchInfo objects returned");
 153   return !Prefetches.empty();
 154 }
 155
 156 bool X86InsertPrefetch::doInitialization(Module &M) {
 157   if (Filename.empty())
 158     return false;
 159
 160   LLVMContext &Ctx = M.getContext();
 161   ErrorOr<std::unique_ptr<SampleProfileReader>> ReaderOrErr =
 162       SampleProfileReader::create(Filename, Ctx);
 163   if (std::error_code EC = ReaderOrErr.getError()) {
 164     std::string Msg = "Could not open profile: " + EC.message();
 165     Ctx.diagnose(DiagnosticInfoSampleProfile(Filename, Msg,
 166                                              DiagnosticSeverity::DS_Warning));
 167     return false;
 168   }
 169   Reader = std::move(ReaderOrErr.get());
 170   Reader->read();
 171   return true;
 172 }
 173
 174 void X86InsertPrefetch::getAnalysisUsage(AnalysisUsage &AU) const {
 175   AU.setPreservesAll();
 176   AU.addRequired<MachineModuleInfo>();
 177 }
 178
 179 bool X86InsertPrefetch::runOnMachineFunction(MachineFunction &MF) {
 180   if (!Reader)
 181     return false;
 182   const FunctionSamples *Samples = Reader->getSamplesFor(MF.getFunction());
 183   if (!Samples)
 184     return false;
 185
 186   bool Changed = false;
 187
 188   const TargetInstrInfo *TII = MF.getSubtarget().getInstrInfo();
 189   SmallVector<PrefetchInfo, 4> Prefetches;
 190   for (auto &MBB : MF) {
 191     for (auto MI = MBB.instr_begin(); MI != MBB.instr_end();) {
 192       auto Current = MI;
 193       ++MI;
 194
 195       int Offset = X86II::getMemoryOperandNo(Current->getDesc().TSFlags);
 196       if (Offset < 0)
 197         continue;
 198       unsigned Bias = X86II::getOperandBias(Current->getDesc());
 199       int MemOpOffset = Offset + Bias;
 200       // FIXME(mtrofin): ORE message when the recommendation cannot be taken.
 201       if (!IsMemOpCompatibleWithPrefetch(*Current, MemOpOffset))
 202         continue;
 203       Prefetches.clear();
 204       if (!findPrefetchInfo(Samples, *Current, Prefetches))
 205         continue;
 206       assert(!Prefetches.empty() &&
 207              "The Prefetches vector should contain at least a value if "
 208              "findPrefetchInfo returned true.");
 209       for (auto &PrefInfo : Prefetches) {
 210         unsigned PFetchInstrID = PrefInfo.InstructionID;
 211         int64_t Delta = PrefInfo.Delta;
 212         const MCInstrDesc &Desc = TII->get(PFetchInstrID);
 213         MachineInstr *PFetch =
 214             MF.CreateMachineInstr(Desc, Current->getDebugLoc(), true);
 215         MachineInstrBuilder MIB(MF, PFetch);
 216
 217         assert(X86::AddrBaseReg == 0 && X86::AddrScaleAmt == 1 &&
 218                X86::AddrIndexReg == 2 && X86::AddrDisp == 3 &&
 219                X86::AddrSegmentReg == 4 &&
 220                "Unexpected change in X86 operand offset order.");
 221
 222         // This assumes X86::AddBaseReg = 0, {...}ScaleAmt = 1, etc.
 223         // FIXME(mtrofin): consider adding a:
 224         //     MachineInstrBuilder::set(unsigned offset, op).
 225         MIB.addReg(Current->getOperand(MemOpOffset + X86::AddrBaseReg).getReg())
 226             .addImm(
 227                 Current->getOperand(MemOpOffset + X86::AddrScaleAmt).getImm())
 228             .addReg(
 229                 Current->getOperand(MemOpOffset + X86::AddrIndexReg).getReg())
 230             .addImm(Current->getOperand(MemOpOffset + X86::AddrDisp).getImm() +
 231                     Delta)
 232             .addReg(Current->getOperand(MemOpOffset + X86::AddrSegmentReg)
 233                         .getReg());
 234
 235         if (!Current->memoperands_empty()) {
 236           MachineMemOperand *CurrentOp = *(Current->memoperands_begin());
 237           MIB.addMemOperand(MF.getMachineMemOperand(
 238               CurrentOp, CurrentOp->getOffset() + Delta, CurrentOp->getSize()));
 239         }
 240
 241         // Insert before Current. This is because Current may clobber some of
 242         // the registers used to describe the input memory operand.
 243         MBB.insert(Current, PFetch);
 244         Changed = true;
 245       }
 246     }
 247   }
 248   return Changed;
 249 }
 250
 251 FunctionPass *llvm::createX86InsertPrefetchPass() {
 252   return new X86InsertPrefetch(PrefetchHintsFile);
 253 }