1 //===-------- X86PadShortFunction.cpp - pad short functions -----------===// 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 // This file defines the pass which will pad short functions to prevent 11 // a stall if a function returns before the return address is ready. This 12 // is needed for some Intel Atom processors. 13 // 14 //===----------------------------------------------------------------------===// 15 16 #include <algorithm> 17 18 #include "X86.h" 19 #include "X86InstrInfo.h" 20 #include "X86Subtarget.h" 21 #include "llvm/ADT/Statistic.h" 22 #include "llvm/CodeGen/MachineFunctionPass.h" 23 #include "llvm/CodeGen/MachineInstrBuilder.h" 24 #include "llvm/CodeGen/MachineRegisterInfo.h" 25 #include "llvm/CodeGen/Passes.h" 26 #include "llvm/IR/Function.h" 27 #include "llvm/Support/Debug.h" 28 #include "llvm/Support/raw_ostream.h" 29 #include "llvm/Target/TargetInstrInfo.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), TM(nullptr), TII(nullptr) {} 55 56 bool runOnMachineFunction(MachineFunction &MF) override; 57 58 const char *getPassName() const override { 59 return "X86 Atom pad short functions"; 60 } 61 62 private: 63 void findReturns(MachineBasicBlock *MBB, 64 unsigned int Cycles = 0); 65 66 bool cyclesUntilReturn(MachineBasicBlock *MBB, 67 unsigned int &Cycles); 68 69 void addPadding(MachineBasicBlock *MBB, 70 MachineBasicBlock::iterator &MBBI, 71 unsigned int NOOPsToAdd); 72 73 const unsigned int Threshold; 74 75 // ReturnBBs - Maps basic blocks that return to the minimum number of 76 // cycles until the return, starting from the entry block. 77 DenseMap<MachineBasicBlock*, unsigned int> ReturnBBs; 78 79 // VisitedBBs - Cache of previously visited BBs. 80 DenseMap<MachineBasicBlock*, VisitedBBInfo> VisitedBBs; 81 82 const TargetMachine *TM; 83 const TargetInstrInfo *TII; 84 }; 85 86 char PadShortFunc::ID = 0; 87 } 88 89 FunctionPass *llvm::createX86PadShortFunctions() { 90 return new PadShortFunc(); 91 } 92 93 /// runOnMachineFunction - Loop over all of the basic blocks, inserting 94 /// NOOP instructions before early exits. 95 bool PadShortFunc::runOnMachineFunction(MachineFunction &MF) { 96 const AttributeSet &FnAttrs = MF.getFunction()->getAttributes(); 97 if (FnAttrs.hasAttribute(AttributeSet::FunctionIndex, 98 Attribute::OptimizeForSize) || 99 FnAttrs.hasAttribute(AttributeSet::FunctionIndex, 100 Attribute::MinSize)) { 101 return false; 102 } 103 104 TM = &MF.getTarget(); 105 if (!TM->getSubtarget<X86Subtarget>().padShortFunctions()) 106 return false; 107 108 TII = TM->getInstrInfo(); 109 110 // Search through basic blocks and mark the ones that have early returns 111 ReturnBBs.clear(); 112 VisitedBBs.clear(); 113 findReturns(MF.begin()); 114 115 bool MadeChange = false; 116 117 MachineBasicBlock *MBB; 118 unsigned int Cycles = 0; 119 120 // Pad the identified basic blocks with NOOPs 121 for (DenseMap<MachineBasicBlock*, unsigned int>::iterator I = ReturnBBs.begin(); 122 I != ReturnBBs.end(); ++I) { 123 MBB = I->first; 124 Cycles = I->second; 125 126 if (Cycles < Threshold) { 127 // BB ends in a return. Skip over any DBG_VALUE instructions 128 // trailing the terminator. 129 assert(MBB->size() > 0 && 130 "Basic block should contain at least a RET but is empty"); 131 MachineBasicBlock::iterator ReturnLoc = --MBB->end(); 132 133 while (ReturnLoc->isDebugValue()) 134 --ReturnLoc; 135 assert(ReturnLoc->isReturn() && !ReturnLoc->isCall() && 136 "Basic block does not end with RET"); 137 138 addPadding(MBB, ReturnLoc, Threshold - Cycles); 139 NumBBsPadded++; 140 MadeChange = true; 141 } 142 } 143 144 return MadeChange; 145 } 146 147 /// findReturn - Starting at MBB, follow control flow and add all 148 /// basic blocks that contain a return to ReturnBBs. 149 void PadShortFunc::findReturns(MachineBasicBlock *MBB, unsigned int Cycles) { 150 // If this BB has a return, note how many cycles it takes to get there. 151 bool hasReturn = cyclesUntilReturn(MBB, Cycles); 152 if (Cycles >= Threshold) 153 return; 154 155 if (hasReturn) { 156 ReturnBBs[MBB] = std::max(ReturnBBs[MBB], Cycles); 157 return; 158 } 159 160 // Follow branches in BB and look for returns 161 for (MachineBasicBlock::succ_iterator I = MBB->succ_begin(); 162 I != MBB->succ_end(); ++I) { 163 if (*I == MBB) 164 continue; 165 findReturns(*I, Cycles); 166 } 167 } 168 169 /// cyclesUntilReturn - return true if the MBB has a return instruction, 170 /// and return false otherwise. 171 /// Cycles will be incremented by the number of cycles taken to reach the 172 /// return or the end of the BB, whichever occurs first. 173 bool PadShortFunc::cyclesUntilReturn(MachineBasicBlock *MBB, 174 unsigned int &Cycles) { 175 // Return cached result if BB was previously visited 176 DenseMap<MachineBasicBlock*, VisitedBBInfo>::iterator it 177 = VisitedBBs.find(MBB); 178 if (it != VisitedBBs.end()) { 179 VisitedBBInfo BBInfo = it->second; 180 Cycles += BBInfo.Cycles; 181 return BBInfo.HasReturn; 182 } 183 184 unsigned int CyclesToEnd = 0; 185 186 for (MachineBasicBlock::iterator MBBI = MBB->begin(); 187 MBBI != MBB->end(); ++MBBI) { 188 MachineInstr *MI = MBBI; 189 // Mark basic blocks with a return instruction. Calls to other 190 // functions do not count because the called function will be padded, 191 // if necessary. 192 if (MI->isReturn() && !MI->isCall()) { 193 VisitedBBs[MBB] = VisitedBBInfo(true, CyclesToEnd); 194 Cycles += CyclesToEnd; 195 return true; 196 } 197 198 CyclesToEnd += TII->getInstrLatency(TM->getInstrItineraryData(), MI); 199 } 200 201 VisitedBBs[MBB] = VisitedBBInfo(false, CyclesToEnd); 202 Cycles += CyclesToEnd; 203 return false; 204 } 205 206 /// addPadding - Add the given number of NOOP instructions to the function 207 /// just prior to the return at MBBI 208 void PadShortFunc::addPadding(MachineBasicBlock *MBB, 209 MachineBasicBlock::iterator &MBBI, 210 unsigned int NOOPsToAdd) { 211 DebugLoc DL = MBBI->getDebugLoc(); 212 213 while (NOOPsToAdd-- > 0) { 214 BuildMI(*MBB, MBBI, DL, TII->get(X86::NOOP)); 215 BuildMI(*MBB, MBBI, DL, TII->get(X86::NOOP)); 216 } 217 } 218