1 //===-------- X86PadShortFunction.cpp - pad short functions -----------===// 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 defines the pass which will pad short functions to prevent 10 // a stall if a function returns before the return address is ready. This 11 // is needed for some Intel Atom processors. 12 // 13 //===----------------------------------------------------------------------===// 14 15 16 #include "X86.h" 17 #include "X86InstrInfo.h" 18 #include "X86Subtarget.h" 19 #include "llvm/ADT/Statistic.h" 20 #include "llvm/Analysis/ProfileSummaryInfo.h" 21 #include "llvm/CodeGen/LazyMachineBlockFrequencyInfo.h" 22 #include "llvm/CodeGen/MachineFunctionPass.h" 23 #include "llvm/CodeGen/MachineInstrBuilder.h" 24 #include "llvm/CodeGen/MachineSizeOpts.h" 25 #include "llvm/CodeGen/Passes.h" 26 #include "llvm/CodeGen/TargetSchedule.h" 27 #include "llvm/IR/Function.h" 28 #include "llvm/Support/Debug.h" 29 #include "llvm/Support/raw_ostream.h" 30 31 using namespace llvm; 32 33 #define DEBUG_TYPE "x86-pad-short-functions" 34 35 STATISTIC(NumBBsPadded, "Number of basic blocks padded"); 36 37 namespace { 38 struct VisitedBBInfo { 39 // HasReturn - Whether the BB contains a return instruction 40 bool HasReturn = false; 41 42 // Cycles - Number of cycles until return if HasReturn is true, otherwise 43 // number of cycles until end of the BB 44 unsigned int Cycles = 0; 45 46 VisitedBBInfo() = default; 47 VisitedBBInfo(bool HasReturn, unsigned int Cycles) 48 : HasReturn(HasReturn), Cycles(Cycles) {} 49 }; 50 51 struct PadShortFunc : public MachineFunctionPass { 52 static char ID; 53 PadShortFunc() : MachineFunctionPass(ID) {} 54 55 bool runOnMachineFunction(MachineFunction &MF) override; 56 57 void getAnalysisUsage(AnalysisUsage &AU) const override { 58 AU.addRequired<ProfileSummaryInfoWrapperPass>(); 59 AU.addRequired<LazyMachineBlockFrequencyInfoPass>(); 60 AU.addPreserved<LazyMachineBlockFrequencyInfoPass>(); 61 MachineFunctionPass::getAnalysisUsage(AU); 62 } 63 64 MachineFunctionProperties getRequiredProperties() const override { 65 return MachineFunctionProperties().set( 66 MachineFunctionProperties::Property::NoVRegs); 67 } 68 69 StringRef getPassName() const override { 70 return "X86 Atom pad short functions"; 71 } 72 73 private: 74 void findReturns(MachineBasicBlock *MBB, 75 unsigned int Cycles = 0); 76 77 bool cyclesUntilReturn(MachineBasicBlock *MBB, 78 unsigned int &Cycles); 79 80 void addPadding(MachineBasicBlock *MBB, 81 MachineBasicBlock::iterator &MBBI, 82 unsigned int NOOPsToAdd); 83 84 const unsigned int Threshold = 4; 85 86 // ReturnBBs - Maps basic blocks that return to the minimum number of 87 // cycles until the return, starting from the entry block. 88 DenseMap<MachineBasicBlock*, unsigned int> ReturnBBs; 89 90 // VisitedBBs - Cache of previously visited BBs. 91 DenseMap<MachineBasicBlock*, VisitedBBInfo> VisitedBBs; 92 93 TargetSchedModel TSM; 94 }; 95 96 char PadShortFunc::ID = 0; 97 } 98 99 FunctionPass *llvm::createX86PadShortFunctions() { 100 return new PadShortFunc(); 101 } 102 103 /// runOnMachineFunction - Loop over all of the basic blocks, inserting 104 /// NOOP instructions before early exits. 105 bool PadShortFunc::runOnMachineFunction(MachineFunction &MF) { 106 if (skipFunction(MF.getFunction())) 107 return false; 108 109 if (MF.getFunction().hasOptSize()) 110 return false; 111 112 if (!MF.getSubtarget<X86Subtarget>().padShortFunctions()) 113 return false; 114 115 TSM.init(&MF.getSubtarget()); 116 117 auto *PSI = 118 &getAnalysis<ProfileSummaryInfoWrapperPass>().getPSI(); 119 auto *MBFI = (PSI && PSI->hasProfileSummary()) ? 120 &getAnalysis<LazyMachineBlockFrequencyInfoPass>().getBFI() : 121 nullptr; 122 123 // Search through basic blocks and mark the ones that have early returns 124 ReturnBBs.clear(); 125 VisitedBBs.clear(); 126 findReturns(&MF.front()); 127 128 bool MadeChange = false; 129 130 // Pad the identified basic blocks with NOOPs 131 for (const auto &ReturnBB : ReturnBBs) { 132 MachineBasicBlock *MBB = ReturnBB.first; 133 unsigned Cycles = ReturnBB.second; 134 135 // Function::hasOptSize is already checked above. 136 bool OptForSize = llvm::shouldOptimizeForSize(MBB, PSI, MBFI); 137 if (OptForSize) 138 continue; 139 140 if (Cycles < Threshold) { 141 // BB ends in a return. Skip over any DBG_VALUE instructions 142 // trailing the terminator. 143 assert(MBB->size() > 0 && 144 "Basic block should contain at least a RET but is empty"); 145 MachineBasicBlock::iterator ReturnLoc = --MBB->end(); 146 147 while (ReturnLoc->isDebugInstr()) 148 --ReturnLoc; 149 assert(ReturnLoc->isReturn() && !ReturnLoc->isCall() && 150 "Basic block does not end with RET"); 151 152 addPadding(MBB, ReturnLoc, Threshold - Cycles); 153 NumBBsPadded++; 154 MadeChange = true; 155 } 156 } 157 158 return MadeChange; 159 } 160 161 /// findReturn - Starting at MBB, follow control flow and add all 162 /// basic blocks that contain a return to ReturnBBs. 163 void PadShortFunc::findReturns(MachineBasicBlock *MBB, unsigned int Cycles) { 164 // If this BB has a return, note how many cycles it takes to get there. 165 bool hasReturn = cyclesUntilReturn(MBB, Cycles); 166 if (Cycles >= Threshold) 167 return; 168 169 if (hasReturn) { 170 ReturnBBs[MBB] = std::max(ReturnBBs[MBB], Cycles); 171 return; 172 } 173 174 // Follow branches in BB and look for returns 175 for (MachineBasicBlock *Succ : MBB->successors()) 176 if (Succ != MBB) 177 findReturns(Succ, Cycles); 178 } 179 180 /// cyclesUntilReturn - return true if the MBB has a return instruction, 181 /// and return false otherwise. 182 /// Cycles will be incremented by the number of cycles taken to reach the 183 /// return or the end of the BB, whichever occurs first. 184 bool PadShortFunc::cyclesUntilReturn(MachineBasicBlock *MBB, 185 unsigned int &Cycles) { 186 // Return cached result if BB was previously visited 187 DenseMap<MachineBasicBlock*, VisitedBBInfo>::iterator it 188 = VisitedBBs.find(MBB); 189 if (it != VisitedBBs.end()) { 190 VisitedBBInfo BBInfo = it->second; 191 Cycles += BBInfo.Cycles; 192 return BBInfo.HasReturn; 193 } 194 195 unsigned int CyclesToEnd = 0; 196 197 for (MachineInstr &MI : *MBB) { 198 // Mark basic blocks with a return instruction. Calls to other 199 // functions do not count because the called function will be padded, 200 // if necessary. 201 if (MI.isReturn() && !MI.isCall()) { 202 VisitedBBs[MBB] = VisitedBBInfo(true, CyclesToEnd); 203 Cycles += CyclesToEnd; 204 return true; 205 } 206 207 CyclesToEnd += TSM.computeInstrLatency(&MI); 208 } 209 210 VisitedBBs[MBB] = VisitedBBInfo(false, CyclesToEnd); 211 Cycles += CyclesToEnd; 212 return false; 213 } 214 215 /// addPadding - Add the given number of NOOP instructions to the function 216 /// just prior to the return at MBBI 217 void PadShortFunc::addPadding(MachineBasicBlock *MBB, 218 MachineBasicBlock::iterator &MBBI, 219 unsigned int NOOPsToAdd) { 220 const DebugLoc &DL = MBBI->getDebugLoc(); 221 unsigned IssueWidth = TSM.getIssueWidth(); 222 223 for (unsigned i = 0, e = IssueWidth * NOOPsToAdd; i != e; ++i) 224 BuildMI(*MBB, MBBI, DL, TSM.getInstrInfo()->get(X86::NOOP)); 225 } 226