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annotate lib/Target/X86/X86PadShortFunction.cpp @ 128:c347d3398279 default tip

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fix
author mir3636
date Wed, 06 Dec 2017 14:37:17 +0900
parents 1172e4bd9c6f
children
Ignore whitespace changes - Everywhere: Within whitespace: At end of lines:
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1 //===-------- X86PadShortFunction.cpp - pad short functions -----------===//
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2 //
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3 // The LLVM Compiler Infrastructure
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4 //
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5 // This file is distributed under the University of Illinois Open Source
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6 // License. See LICENSE.TXT for details.
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7 //
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8 //===----------------------------------------------------------------------===//
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9 //
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10 // This file defines the pass which will pad short functions to prevent
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11 // a stall if a function returns before the return address is ready. This
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12 // is needed for some Intel Atom processors.
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13 //
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14 //===----------------------------------------------------------------------===//
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15
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16 #include <algorithm>
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17
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18 #include "X86.h"
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19 #include "X86InstrInfo.h"
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20 #include "X86Subtarget.h"
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21 #include "llvm/ADT/Statistic.h"
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22 #include "llvm/CodeGen/MachineFunctionPass.h"
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23 #include "llvm/CodeGen/MachineInstrBuilder.h"
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24 #include "llvm/CodeGen/MachineRegisterInfo.h"
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25 #include "llvm/CodeGen/Passes.h"
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26 #include "llvm/IR/Function.h"
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27 #include "llvm/Support/Debug.h"
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28 #include "llvm/Support/raw_ostream.h"
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29 #include "llvm/Target/TargetInstrInfo.h"
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30
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31 using namespace llvm;
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32
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33 #define DEBUG_TYPE "x86-pad-short-functions"
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34
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35 STATISTIC(NumBBsPadded, "Number of basic blocks padded");
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36
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37 namespace {
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38 struct VisitedBBInfo {
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39 // HasReturn - Whether the BB contains a return instruction
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40 bool HasReturn;
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41
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42 // Cycles - Number of cycles until return if HasReturn is true, otherwise
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43 // number of cycles until end of the BB
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44 unsigned int Cycles;
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45
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46 VisitedBBInfo() : HasReturn(false), Cycles(0) {}
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47 VisitedBBInfo(bool HasReturn, unsigned int Cycles)
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48 : HasReturn(HasReturn), Cycles(Cycles) {}
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49 };
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50
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51 struct PadShortFunc : public MachineFunctionPass {
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52 static char ID;
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53 PadShortFunc() : MachineFunctionPass(ID)
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54 , Threshold(4), STI(nullptr), TII(nullptr) {}
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55
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56 bool runOnMachineFunction(MachineFunction &MF) override;
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57
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58 MachineFunctionProperties getRequiredProperties() const override {
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59 return MachineFunctionProperties().set(
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60 MachineFunctionProperties::Property::NoVRegs);
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61 }
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62
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63 StringRef getPassName() const override {
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64 return "X86 Atom pad short functions";
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65 }
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66
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67 private:
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68 void findReturns(MachineBasicBlock *MBB,
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69 unsigned int Cycles = 0);
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70
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71 bool cyclesUntilReturn(MachineBasicBlock *MBB,
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72 unsigned int &Cycles);
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73
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74 void addPadding(MachineBasicBlock *MBB,
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75 MachineBasicBlock::iterator &MBBI,
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76 unsigned int NOOPsToAdd);
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77
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78 const unsigned int Threshold;
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79
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80 // ReturnBBs - Maps basic blocks that return to the minimum number of
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81 // cycles until the return, starting from the entry block.
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82 DenseMap<MachineBasicBlock*, unsigned int> ReturnBBs;
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83
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84 // VisitedBBs - Cache of previously visited BBs.
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85 DenseMap<MachineBasicBlock*, VisitedBBInfo> VisitedBBs;
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86
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87 const X86Subtarget *STI;
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88 const TargetInstrInfo *TII;
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89 };
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90
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91 char PadShortFunc::ID = 0;
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92 }
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93
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94 FunctionPass *llvm::createX86PadShortFunctions() {
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95 return new PadShortFunc();
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96 }
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97
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98 /// runOnMachineFunction - Loop over all of the basic blocks, inserting
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99 /// NOOP instructions before early exits.
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100 bool PadShortFunc::runOnMachineFunction(MachineFunction &MF) {
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101 if (skipFunction(*MF.getFunction()))
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102 return false;
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103
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104 if (MF.getFunction()->optForSize()) {
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105 return false;
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106 }
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107
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108 STI = &MF.getSubtarget<X86Subtarget>();
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109 if (!STI->padShortFunctions())
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110 return false;
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111
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112 TII = STI->getInstrInfo();
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113
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114 // Search through basic blocks and mark the ones that have early returns
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115 ReturnBBs.clear();
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116 VisitedBBs.clear();
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117 findReturns(&MF.front());
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118
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119 bool MadeChange = false;
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120
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121 MachineBasicBlock *MBB;
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122 unsigned int Cycles = 0;
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123
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124 // Pad the identified basic blocks with NOOPs
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125 for (DenseMap<MachineBasicBlock*, unsigned int>::iterator I = ReturnBBs.begin();
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126 I != ReturnBBs.end(); ++I) {
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127 MBB = I->first;
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128 Cycles = I->second;
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129
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130 if (Cycles < Threshold) {
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131 // BB ends in a return. Skip over any DBG_VALUE instructions
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132 // trailing the terminator.
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133 assert(MBB->size() > 0 &&
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134 "Basic block should contain at least a RET but is empty");
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135 MachineBasicBlock::iterator ReturnLoc = --MBB->end();
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136
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137 while (ReturnLoc->isDebugValue())
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138 --ReturnLoc;
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139 assert(ReturnLoc->isReturn() && !ReturnLoc->isCall() &&
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140 "Basic block does not end with RET");
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141
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142 addPadding(MBB, ReturnLoc, Threshold - Cycles);
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143 NumBBsPadded++;
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144 MadeChange = true;
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145 }
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146 }
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147
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148 return MadeChange;
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149 }
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150
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151 /// findReturn - Starting at MBB, follow control flow and add all
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152 /// basic blocks that contain a return to ReturnBBs.
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153 void PadShortFunc::findReturns(MachineBasicBlock *MBB, unsigned int Cycles) {
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154 // If this BB has a return, note how many cycles it takes to get there.
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155 bool hasReturn = cyclesUntilReturn(MBB, Cycles);
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156 if (Cycles >= Threshold)
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157 return;
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158
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159 if (hasReturn) {
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160 ReturnBBs[MBB] = std::max(ReturnBBs[MBB], Cycles);
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161 return;
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162 }
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163
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164 // Follow branches in BB and look for returns
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165 for (MachineBasicBlock::succ_iterator I = MBB->succ_begin();
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166 I != MBB->succ_end(); ++I) {
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167 if (*I == MBB)
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168 continue;
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169 findReturns(*I, Cycles);
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170 }
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171 }
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172
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173 /// cyclesUntilReturn - return true if the MBB has a return instruction,
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174 /// and return false otherwise.
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175 /// Cycles will be incremented by the number of cycles taken to reach the
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176 /// return or the end of the BB, whichever occurs first.
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177 bool PadShortFunc::cyclesUntilReturn(MachineBasicBlock *MBB,
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178 unsigned int &Cycles) {
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179 // Return cached result if BB was previously visited
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180 DenseMap<MachineBasicBlock*, VisitedBBInfo>::iterator it
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181 = VisitedBBs.find(MBB);
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182 if (it != VisitedBBs.end()) {
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183 VisitedBBInfo BBInfo = it->second;
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184 Cycles += BBInfo.Cycles;
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185 return BBInfo.HasReturn;
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186 }
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187
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188 unsigned int CyclesToEnd = 0;
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189
120
1172e4bd9c6f update 4.0.0
mir3636
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diff changeset
190 for (MachineInstr &MI : *MBB) {
0
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parents:
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191 // Mark basic blocks with a return instruction. Calls to other
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parents:
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192 // functions do not count because the called function will be padded,
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parents:
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193 // if necessary.
120
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diff changeset
194 if (MI.isReturn() && !MI.isCall()) {
0
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195 VisitedBBs[MBB] = VisitedBBInfo(true, CyclesToEnd);
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196 Cycles += CyclesToEnd;
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197 return true;
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198 }
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199
83
60c9769439b8 LLVM 3.7
Tatsuki IHA <e125716@ie.u-ryukyu.ac.jp>
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diff changeset
200 CyclesToEnd += TII->getInstrLatency(STI->getInstrItineraryData(), MI);
0
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201 }
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diff changeset
202
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203 VisitedBBs[MBB] = VisitedBBInfo(false, CyclesToEnd);
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204 Cycles += CyclesToEnd;
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parents:
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205 return false;
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parents:
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206 }
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parents:
diff changeset
207
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208 /// addPadding - Add the given number of NOOP instructions to the function
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parents:
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209 /// just prior to the return at MBBI
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diff changeset
210 void PadShortFunc::addPadding(MachineBasicBlock *MBB,
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parents:
diff changeset
211 MachineBasicBlock::iterator &MBBI,
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
212 unsigned int NOOPsToAdd) {
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parents:
diff changeset
213 DebugLoc DL = MBBI->getDebugLoc();
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
214
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Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
215 while (NOOPsToAdd-- > 0) {
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
216 BuildMI(*MBB, MBBI, DL, TII->get(X86::NOOP));
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
217 BuildMI(*MBB, MBBI, DL, TII->get(X86::NOOP));
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
218 }
95c75e76d11b LLVM 3.4
Kaito Tokumori <e105711@ie.u-ryukyu.ac.jp>
parents:
diff changeset
219 }