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; 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; 45 46 VisitedBBInfo() : HasReturn(false), Cycles(0) {} 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 , Threshold(4) {} 55 56 bool runOnMachineFunction(MachineFunction &MF) override; 57 58 void getAnalysisUsage(AnalysisUsage &AU) const override { 59 AU.addRequired<ProfileSummaryInfoWrapperPass>(); 60 AU.addRequired<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; 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 (DenseMap<MachineBasicBlock*, unsigned int>::iterator I = ReturnBBs.begin(); 132 I != ReturnBBs.end(); ++I) { 133 MachineBasicBlock *MBB = I->first; 134 unsigned Cycles = I->second; 135 136 // Function::hasOptSize is already checked above. 137 bool OptForSize = llvm::shouldOptimizeForSize(MBB, PSI, MBFI); 138 if (OptForSize) 139 continue; 140 141 if (Cycles < Threshold) { 142 // BB ends in a return. Skip over any DBG_VALUE instructions 143 // trailing the terminator. 144 assert(MBB->size() > 0 && 145 "Basic block should contain at least a RET but is empty"); 146 MachineBasicBlock::iterator ReturnLoc = --MBB->end(); 147 148 while (ReturnLoc->isDebugInstr()) 149 --ReturnLoc; 150 assert(ReturnLoc->isReturn() && !ReturnLoc->isCall() && 151 "Basic block does not end with RET"); 152 153 addPadding(MBB, ReturnLoc, Threshold - Cycles); 154 NumBBsPadded++; 155 MadeChange = true; 156 } 157 } 158 159 return MadeChange; 160 } 161 162 /// findReturn - Starting at MBB, follow control flow and add all 163 /// basic blocks that contain a return to ReturnBBs. 164 void PadShortFunc::findReturns(MachineBasicBlock *MBB, unsigned int Cycles) { 165 // If this BB has a return, note how many cycles it takes to get there. 166 bool hasReturn = cyclesUntilReturn(MBB, Cycles); 167 if (Cycles >= Threshold) 168 return; 169 170 if (hasReturn) { 171 ReturnBBs[MBB] = std::max(ReturnBBs[MBB], Cycles); 172 return; 173 } 174 175 // Follow branches in BB and look for returns 176 for (MachineBasicBlock::succ_iterator I = MBB->succ_begin(); 177 I != MBB->succ_end(); ++I) { 178 if (*I == MBB) 179 continue; 180 findReturns(*I, Cycles); 181 } 182 } 183 184 /// cyclesUntilReturn - return true if the MBB has a return instruction, 185 /// and return false otherwise. 186 /// Cycles will be incremented by the number of cycles taken to reach the 187 /// return or the end of the BB, whichever occurs first. 188 bool PadShortFunc::cyclesUntilReturn(MachineBasicBlock *MBB, 189 unsigned int &Cycles) { 190 // Return cached result if BB was previously visited 191 DenseMap<MachineBasicBlock*, VisitedBBInfo>::iterator it 192 = VisitedBBs.find(MBB); 193 if (it != VisitedBBs.end()) { 194 VisitedBBInfo BBInfo = it->second; 195 Cycles += BBInfo.Cycles; 196 return BBInfo.HasReturn; 197 } 198 199 unsigned int CyclesToEnd = 0; 200 201 for (MachineInstr &MI : *MBB) { 202 // Mark basic blocks with a return instruction. Calls to other 203 // functions do not count because the called function will be padded, 204 // if necessary. 205 if (MI.isReturn() && !MI.isCall()) { 206 VisitedBBs[MBB] = VisitedBBInfo(true, CyclesToEnd); 207 Cycles += CyclesToEnd; 208 return true; 209 } 210 211 CyclesToEnd += TSM.computeInstrLatency(&MI); 212 } 213 214 VisitedBBs[MBB] = VisitedBBInfo(false, CyclesToEnd); 215 Cycles += CyclesToEnd; 216 return false; 217 } 218 219 /// addPadding - Add the given number of NOOP instructions to the function 220 /// just prior to the return at MBBI 221 void PadShortFunc::addPadding(MachineBasicBlock *MBB, 222 MachineBasicBlock::iterator &MBBI, 223 unsigned int NOOPsToAdd) { 224 DebugLoc DL = MBBI->getDebugLoc(); 225 unsigned IssueWidth = TSM.getIssueWidth(); 226 227 for (unsigned i = 0, e = IssueWidth * NOOPsToAdd; i != e; ++i) 228 BuildMI(*MBB, MBBI, DL, TSM.getInstrInfo()->get(X86::NOOP)); 229 } 230