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annotate lib/Transforms/Scalar/SimplifyCFGPass.cpp @ 83:60c9769439b8

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LLVM 3.7
author Tatsuki IHA <e125716@ie.u-ryukyu.ac.jp>
date Wed, 18 Feb 2015 14:55:36 +0900
parents 54457678186b
children afa8332a0e37
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1 //===- SimplifyCFGPass.cpp - CFG Simplification Pass ----------------------===//
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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 implements dead code elimination and basic block merging, along
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11 // with a collection of other peephole control flow optimizations. For example:
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12 //
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13 // * Removes basic blocks with no predecessors.
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14 // * Merges a basic block into its predecessor if there is only one and the
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15 // predecessor only has one successor.
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16 // * Eliminates PHI nodes for basic blocks with a single predecessor.
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17 // * Eliminates a basic block that only contains an unconditional branch.
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18 // * Changes invoke instructions to nounwind functions to be calls.
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19 // * Change things like "if (x) if (y)" into "if (x&y)".
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20 // * etc..
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21 //
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22 //===----------------------------------------------------------------------===//
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23
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24 #include "llvm/Transforms/Scalar/SimplifyCFG.h"
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25 #include "llvm/ADT/SmallPtrSet.h"
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26 #include "llvm/ADT/SmallVector.h"
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27 #include "llvm/ADT/Statistic.h"
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28 #include "llvm/Analysis/AssumptionCache.h"
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29 #include "llvm/Analysis/TargetTransformInfo.h"
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30 #include "llvm/IR/Attributes.h"
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31 #include "llvm/IR/CFG.h"
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32 #include "llvm/IR/Constants.h"
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33 #include "llvm/IR/DataLayout.h"
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34 #include "llvm/IR/Instructions.h"
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35 #include "llvm/IR/IntrinsicInst.h"
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36 #include "llvm/IR/Module.h"
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37 #include "llvm/Pass.h"
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38 #include "llvm/Support/CommandLine.h"
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39 #include "llvm/Transforms/Utils/Local.h"
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40 #include "llvm/Transforms/Scalar.h"
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41 using namespace llvm;
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42
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43 #define DEBUG_TYPE "simplifycfg"
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44
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45 static cl::opt<unsigned>
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46 UserBonusInstThreshold("bonus-inst-threshold", cl::Hidden, cl::init(1),
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47 cl::desc("Control the number of bonus instructions (default = 1)"));
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48
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49 STATISTIC(NumSimpl, "Number of blocks simplified");
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50
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51 /// mergeEmptyReturnBlocks - If we have more than one empty (other than phi
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52 /// node) return blocks, merge them together to promote recursive block merging.
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53 static bool mergeEmptyReturnBlocks(Function &F) {
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54 bool Changed = false;
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55
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56 BasicBlock *RetBlock = nullptr;
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57
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58 // Scan all the blocks in the function, looking for empty return blocks.
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59 for (Function::iterator BBI = F.begin(), E = F.end(); BBI != E; ) {
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60 BasicBlock &BB = *BBI++;
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61
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62 // Only look at return blocks.
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63 ReturnInst *Ret = dyn_cast<ReturnInst>(BB.getTerminator());
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64 if (!Ret) continue;
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65
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66 // Only look at the block if it is empty or the only other thing in it is a
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67 // single PHI node that is the operand to the return.
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68 if (Ret != &BB.front()) {
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69 // Check for something else in the block.
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70 BasicBlock::iterator I = Ret;
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71 --I;
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72 // Skip over debug info.
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73 while (isa<DbgInfoIntrinsic>(I) && I != BB.begin())
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74 --I;
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75 if (!isa<DbgInfoIntrinsic>(I) &&
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76 (!isa<PHINode>(I) || I != BB.begin() ||
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77 Ret->getNumOperands() == 0 ||
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78 Ret->getOperand(0) != I))
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79 continue;
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80 }
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81
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82 // If this is the first returning block, remember it and keep going.
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83 if (!RetBlock) {
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84 RetBlock = &BB;
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85 continue;
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86 }
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87
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88 // Otherwise, we found a duplicate return block. Merge the two.
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89 Changed = true;
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90
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91 // Case when there is no input to the return or when the returned values
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92 // agree is trivial. Note that they can't agree if there are phis in the
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93 // blocks.
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94 if (Ret->getNumOperands() == 0 ||
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95 Ret->getOperand(0) ==
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96 cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0)) {
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97 BB.replaceAllUsesWith(RetBlock);
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98 BB.eraseFromParent();
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99 continue;
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100 }
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101
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102 // If the canonical return block has no PHI node, create one now.
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103 PHINode *RetBlockPHI = dyn_cast<PHINode>(RetBlock->begin());
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104 if (!RetBlockPHI) {
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105 Value *InVal = cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0);
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106 pred_iterator PB = pred_begin(RetBlock), PE = pred_end(RetBlock);
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107 RetBlockPHI = PHINode::Create(Ret->getOperand(0)->getType(),
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108 std::distance(PB, PE), "merge",
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109 &RetBlock->front());
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110
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111 for (pred_iterator PI = PB; PI != PE; ++PI)
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112 RetBlockPHI->addIncoming(InVal, *PI);
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113 RetBlock->getTerminator()->setOperand(0, RetBlockPHI);
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114 }
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115
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116 // Turn BB into a block that just unconditionally branches to the return
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117 // block. This handles the case when the two return blocks have a common
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118 // predecessor but that return different things.
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119 RetBlockPHI->addIncoming(Ret->getOperand(0), &BB);
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120 BB.getTerminator()->eraseFromParent();
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121 BranchInst::Create(RetBlock, &BB);
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122 }
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123
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124 return Changed;
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125 }
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126
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127 /// iterativelySimplifyCFG - Call SimplifyCFG on all the blocks in the function,
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128 /// iterating until no more changes are made.
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129 static bool iterativelySimplifyCFG(Function &F, const TargetTransformInfo &TTI,
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130 const DataLayout *DL, AssumptionCache *AC,
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131 unsigned BonusInstThreshold) {
0
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132 bool Changed = false;
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133 bool LocalChange = true;
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134 while (LocalChange) {
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135 LocalChange = false;
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136
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137 // Loop over all of the basic blocks and remove them if they are unneeded...
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138 //
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139 for (Function::iterator BBIt = F.begin(); BBIt != F.end(); ) {
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140 if (SimplifyCFG(BBIt++, TTI, BonusInstThreshold, DL, AC)) {
0
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141 LocalChange = true;
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142 ++NumSimpl;
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143 }
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144 }
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145 Changed |= LocalChange;
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146 }
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147 return Changed;
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148 }
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149
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150 static bool simplifyFunctionCFG(Function &F, const TargetTransformInfo &TTI,
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151 const DataLayout *DL, AssumptionCache *AC,
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152 int BonusInstThreshold) {
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153 bool EverChanged = removeUnreachableBlocks(F);
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154 EverChanged |= mergeEmptyReturnBlocks(F);
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155 EverChanged |= iterativelySimplifyCFG(F, TTI, DL, AC, BonusInstThreshold);
0
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156
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157 // If neither pass changed anything, we're done.
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158 if (!EverChanged) return false;
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159
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160 // iterativelySimplifyCFG can (rarely) make some loops dead. If this happens,
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161 // removeUnreachableBlocks is needed to nuke them, which means we should
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162 // iterate between the two optimizations. We structure the code like this to
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163 // avoid reruning iterativelySimplifyCFG if the second pass of
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164 // removeUnreachableBlocks doesn't do anything.
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165 if (!removeUnreachableBlocks(F))
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166 return true;
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167
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168 do {
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169 EverChanged = iterativelySimplifyCFG(F, TTI, DL, AC, BonusInstThreshold);
0
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170 EverChanged |= removeUnreachableBlocks(F);
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171 } while (EverChanged);
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172
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173 return true;
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174 }
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175
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176 SimplifyCFGPass::SimplifyCFGPass()
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177 : BonusInstThreshold(UserBonusInstThreshold) {}
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178
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179 SimplifyCFGPass::SimplifyCFGPass(int BonusInstThreshold)
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180 : BonusInstThreshold(BonusInstThreshold) {}
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181
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182 PreservedAnalyses SimplifyCFGPass::run(Function &F,
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183 AnalysisManager<Function> *AM) {
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184 auto *DL = F.getParent()->getDataLayout();
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185 auto &TTI = AM->getResult<TargetIRAnalysis>(F);
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186 auto &AC = AM->getResult<AssumptionAnalysis>(F);
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187
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188 if (!simplifyFunctionCFG(F, TTI, DL, &AC, BonusInstThreshold))
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189 return PreservedAnalyses::none();
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190
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191 return PreservedAnalyses::all();
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192 }
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193
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194 namespace {
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195 struct CFGSimplifyPass : public FunctionPass {
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196 static char ID; // Pass identification, replacement for typeid
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197 unsigned BonusInstThreshold;
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198 CFGSimplifyPass(int T = -1) : FunctionPass(ID) {
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199 BonusInstThreshold = (T == -1) ? UserBonusInstThreshold : unsigned(T);
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200 initializeCFGSimplifyPassPass(*PassRegistry::getPassRegistry());
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201 }
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202 bool runOnFunction(Function &F) override {
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203 if (skipOptnoneFunction(F))
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204 return false;
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205
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206 AssumptionCache *AC =
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207 &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
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208 const TargetTransformInfo &TTI =
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209 getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
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210 DataLayoutPass *DLP = getAnalysisIfAvailable<DataLayoutPass>();
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211 const DataLayout *DL = DLP ? &DLP->getDataLayout() : nullptr;
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212 return simplifyFunctionCFG(F, TTI, DL, AC, BonusInstThreshold);
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213 }
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214
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215 void getAnalysisUsage(AnalysisUsage &AU) const override {
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216 AU.addRequired<AssumptionCacheTracker>();
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217 AU.addRequired<TargetTransformInfoWrapperPass>();
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218 }
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219 };
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220 }
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221
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222 char CFGSimplifyPass::ID = 0;
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223 INITIALIZE_PASS_BEGIN(CFGSimplifyPass, "simplifycfg", "Simplify the CFG", false,
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224 false)
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225 INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
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226 INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
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227 INITIALIZE_PASS_END(CFGSimplifyPass, "simplifycfg", "Simplify the CFG", false,
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228 false)
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229
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230 // Public interface to the CFGSimplification pass
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231 FunctionPass *llvm::createCFGSimplificationPass(int Threshold) {
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232 return new CFGSimplifyPass(Threshold);
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233 }
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234