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/MachineFunctionPass.h"
27 #include "llvm/CodeGen/MachineModuleInfo.h"
28 #include "llvm/IR/DebugInfoMetadata.h"
29 #include "llvm/ProfileData/SampleProf.h"
30 #include "llvm/ProfileData/SampleProfReader.h"
31 #include "llvm/Transforms/IPO/SampleProfile.h"
32 using namespace llvm;
33 using namespace sampleprof;
34 
35 static cl::opt<std::string>
36     PrefetchHintsFile("prefetch-hints-file",
37                       cl::desc("Path to the prefetch hints profile. See also "
38                                "-x86-discriminate-memops"),
39                       cl::Hidden);
40 namespace {
41 
42 class X86InsertPrefetch : public MachineFunctionPass {
43   void getAnalysisUsage(AnalysisUsage &AU) const override;
44   bool doInitialization(Module &) override;
45 
46   bool runOnMachineFunction(MachineFunction &MF) override;
47   struct PrefetchInfo {
48     unsigned InstructionID;
49     int64_t Delta;
50   };
51   typedef SmallVectorImpl<PrefetchInfo> Prefetches;
52   bool findPrefetchInfo(const FunctionSamples *Samples, const MachineInstr &MI,
53                         Prefetches &prefetches) const;
54 
55 public:
56   static char ID;
57   X86InsertPrefetch(const std::string &PrefetchHintsFilename);
58   StringRef getPassName() const override {
59     return "X86 Insert Cache Prefetches";
60   }
61 
62 private:
63   std::string Filename;
64   std::unique_ptr<SampleProfileReader> Reader;
65 };
66 
67 using PrefetchHints = SampleRecord::CallTargetMap;
68 
69 // Return any prefetching hints for the specified MachineInstruction. The hints
70 // are returned as pairs (name, delta).
71 ErrorOr<PrefetchHints> getPrefetchHints(const FunctionSamples *TopSamples,
72                                         const MachineInstr &MI) {
73   if (const auto &Loc = MI.getDebugLoc())
74     if (const auto *Samples = TopSamples->findFunctionSamples(Loc))
75       return Samples->findCallTargetMapAt(FunctionSamples::getOffset(Loc),
76                                           Loc->getBaseDiscriminator());
77   return std::error_code();
78 }
79 
80 // The prefetch instruction can't take memory operands involving vector
81 // registers.
82 bool IsMemOpCompatibleWithPrefetch(const MachineInstr &MI, int Op) {
83   Register BaseReg = MI.getOperand(Op + X86::AddrBaseReg).getReg();
84   Register IndexReg = MI.getOperand(Op + X86::AddrIndexReg).getReg();
85   return (BaseReg == 0 ||
86           X86MCRegisterClasses[X86::GR64RegClassID].contains(BaseReg) ||
87           X86MCRegisterClasses[X86::GR32RegClassID].contains(BaseReg)) &&
88          (IndexReg == 0 ||
89           X86MCRegisterClasses[X86::GR64RegClassID].contains(IndexReg) ||
90           X86MCRegisterClasses[X86::GR32RegClassID].contains(IndexReg));
91 }
92 
93 } // end anonymous namespace
94 
95 //===----------------------------------------------------------------------===//
96 //            Implementation
97 //===----------------------------------------------------------------------===//
98 
99 char X86InsertPrefetch::ID = 0;
100 
101 X86InsertPrefetch::X86InsertPrefetch(const std::string &PrefetchHintsFilename)
102     : MachineFunctionPass(ID), Filename(PrefetchHintsFilename) {}
103 
104 /// Return true if the provided MachineInstruction has cache prefetch hints. In
105 /// that case, the prefetch hints are stored, in order, in the Prefetches
106 /// vector.
107 bool X86InsertPrefetch::findPrefetchInfo(const FunctionSamples *TopSamples,
108                                          const MachineInstr &MI,
109                                          Prefetches &Prefetches) const {
110   assert(Prefetches.empty() &&
111          "Expected caller passed empty PrefetchInfo vector.");
112   static constexpr std::pair<StringLiteral, unsigned> HintTypes[] = {
113       {"_nta_", X86::PREFETCHNTA},
114       {"_t0_", X86::PREFETCHT0},
115       {"_t1_", X86::PREFETCHT1},
116       {"_t2_", X86::PREFETCHT2},
117   };
118   static const char *SerializedPrefetchPrefix = "__prefetch";
119 
120   const ErrorOr<PrefetchHints> T = getPrefetchHints(TopSamples, MI);
121   if (!T)
122     return false;
123   int16_t max_index = -1;
124   // Convert serialized prefetch hints into PrefetchInfo objects, and populate
125   // the Prefetches vector.
126   for (const auto &S_V : *T) {
127     StringRef Name = S_V.getKey();
128     if (Name.consume_front(SerializedPrefetchPrefix)) {
129       int64_t D = static_cast<int64_t>(S_V.second);
130       unsigned IID = 0;
131       for (const auto &HintType : HintTypes) {
132         if (Name.startswith(HintType.first)) {
133           Name = Name.drop_front(HintType.first.size());
134           IID = HintType.second;
135           break;
136         }
137       }
138       if (IID == 0)
139         return false;
140       uint8_t index = 0;
141       Name.consumeInteger(10, index);
142 
143       if (index >= Prefetches.size())
144         Prefetches.resize(index + 1);
145       Prefetches[index] = {IID, D};
146       max_index = std::max(max_index, static_cast<int16_t>(index));
147     }
148   }
149   assert(max_index + 1 >= 0 &&
150          "Possible overflow: max_index + 1 should be positive.");
151   assert(static_cast<size_t>(max_index + 1) == Prefetches.size() &&
152          "The number of prefetch hints received should match the number of "
153          "PrefetchInfo objects returned");
154   return !Prefetches.empty();
155 }
156 
157 bool X86InsertPrefetch::doInitialization(Module &M) {
158   if (Filename.empty())
159     return false;
160 
161   LLVMContext &Ctx = M.getContext();
162   ErrorOr<std::unique_ptr<SampleProfileReader>> ReaderOrErr =
163       SampleProfileReader::create(Filename, Ctx);
164   if (std::error_code EC = ReaderOrErr.getError()) {
165     std::string Msg = "Could not open profile: " + EC.message();
166     Ctx.diagnose(DiagnosticInfoSampleProfile(Filename, Msg,
167                                              DiagnosticSeverity::DS_Warning));
168     return false;
169   }
170   Reader = std::move(ReaderOrErr.get());
171   Reader->read();
172   return true;
173 }
174 
175 void X86InsertPrefetch::getAnalysisUsage(AnalysisUsage &AU) const {
176   AU.setPreservesAll();
177   MachineFunctionPass::getAnalysisUsage(AU);
178 }
179 
180 bool X86InsertPrefetch::runOnMachineFunction(MachineFunction &MF) {
181   if (!Reader)
182     return false;
183   const FunctionSamples *Samples = Reader->getSamplesFor(MF.getFunction());
184   if (!Samples)
185     return false;
186 
187   bool Changed = false;
188 
189   const TargetInstrInfo *TII = MF.getSubtarget().getInstrInfo();
190   SmallVector<PrefetchInfo, 4> Prefetches;
191   for (auto &MBB : MF) {
192     for (auto MI = MBB.instr_begin(); MI != MBB.instr_end();) {
193       auto Current = MI;
194       ++MI;
195 
196       int Offset = X86II::getMemoryOperandNo(Current->getDesc().TSFlags);
197       if (Offset < 0)
198         continue;
199       unsigned Bias = X86II::getOperandBias(Current->getDesc());
200       int MemOpOffset = Offset + Bias;
201       // FIXME(mtrofin): ORE message when the recommendation cannot be taken.
202       if (!IsMemOpCompatibleWithPrefetch(*Current, MemOpOffset))
203         continue;
204       Prefetches.clear();
205       if (!findPrefetchInfo(Samples, *Current, Prefetches))
206         continue;
207       assert(!Prefetches.empty() &&
208              "The Prefetches vector should contain at least a value if "
209              "findPrefetchInfo returned true.");
210       for (auto &PrefInfo : Prefetches) {
211         unsigned PFetchInstrID = PrefInfo.InstructionID;
212         int64_t Delta = PrefInfo.Delta;
213         const MCInstrDesc &Desc = TII->get(PFetchInstrID);
214         MachineInstr *PFetch =
215             MF.CreateMachineInstr(Desc, Current->getDebugLoc(), true);
216         MachineInstrBuilder MIB(MF, PFetch);
217 
218         static_assert(X86::AddrBaseReg == 0 && X86::AddrScaleAmt == 1 &&
219                           X86::AddrIndexReg == 2 && X86::AddrDisp == 3 &&
220                           X86::AddrSegmentReg == 4,
221                       "Unexpected change in X86 operand offset order.");
222 
223         // This assumes X86::AddBaseReg = 0, {...}ScaleAmt = 1, etc.
224         // FIXME(mtrofin): consider adding a:
225         //     MachineInstrBuilder::set(unsigned offset, op).
226         MIB.addReg(Current->getOperand(MemOpOffset + X86::AddrBaseReg).getReg())
227             .addImm(
228                 Current->getOperand(MemOpOffset + X86::AddrScaleAmt).getImm())
229             .addReg(
230                 Current->getOperand(MemOpOffset + X86::AddrIndexReg).getReg())
231             .addImm(Current->getOperand(MemOpOffset + X86::AddrDisp).getImm() +
232                     Delta)
233             .addReg(Current->getOperand(MemOpOffset + X86::AddrSegmentReg)
234                         .getReg());
235 
236         if (!Current->memoperands_empty()) {
237           MachineMemOperand *CurrentOp = *(Current->memoperands_begin());
238           MIB.addMemOperand(MF.getMachineMemOperand(
239               CurrentOp, CurrentOp->getOffset() + Delta, CurrentOp->getSize()));
240         }
241 
242         // Insert before Current. This is because Current may clobber some of
243         // the registers used to describe the input memory operand.
244         MBB.insert(Current, PFetch);
245         Changed = true;
246       }
247     }
248   }
249   return Changed;
250 }
251 
252 FunctionPass *llvm::createX86InsertPrefetchPass() {
253   return new X86InsertPrefetch(PrefetchHintsFile);
254 }
255