LowerSwitch.cpp revision 261cdfbe5e6e11d56ca1c49a75f26fece3b139c8
1//===- LowerSwitch.cpp - Eliminate Switch instructions --------------------===// 2// 3// The LLVM Compiler Infrastructure 4// 5// This file was developed by the LLVM research group and is distributed under 6// the University of Illinois Open Source License. See LICENSE.TXT for details. 7// 8//===----------------------------------------------------------------------===// 9// 10// The LowerSwitch transformation rewrites switch statements with a sequence of 11// branches, which allows targets to get away with not implementing the switch 12// statement until it is convenient. 13// 14//===----------------------------------------------------------------------===// 15 16#include "llvm/Transforms/Scalar.h" 17#include "llvm/Transforms/Utils/UnifyFunctionExitNodes.h" 18#include "llvm/Constants.h" 19#include "llvm/Function.h" 20#include "llvm/Instructions.h" 21#include "llvm/Pass.h" 22#include "llvm/Support/Debug.h" 23#include "llvm/Support/Compiler.h" 24#include <algorithm> 25using namespace llvm; 26 27namespace { 28 /// LowerSwitch Pass - Replace all SwitchInst instructions with chained branch 29 /// instructions. Note that this cannot be a BasicBlock pass because it 30 /// modifies the CFG! 31 class VISIBILITY_HIDDEN LowerSwitch : public FunctionPass { 32 public: 33 virtual bool runOnFunction(Function &F); 34 35 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 36 // This is a cluster of orthogonal Transforms 37 AU.addPreserved<UnifyFunctionExitNodes>(); 38 AU.addPreservedID(PromoteMemoryToRegisterID); 39 AU.addPreservedID(LowerSelectID); 40 AU.addPreservedID(LowerInvokePassID); 41 AU.addPreservedID(LowerAllocationsID); 42 } 43 44 struct CaseRange { 45 Constant* Low; 46 Constant* High; 47 BasicBlock* BB; 48 49 CaseRange() : Low(0), High(0), BB(0) { } 50 CaseRange(Constant* low, Constant* high, BasicBlock* bb) : 51 Low(low), High(high), BB(bb) { } 52 }; 53 54 typedef std::vector<CaseRange> CaseVector; 55 typedef std::vector<CaseRange>::iterator CaseItr; 56 private: 57 void processSwitchInst(SwitchInst *SI); 58 59 BasicBlock* switchConvert(CaseItr Begin, CaseItr End, Value* Val, 60 BasicBlock* OrigBlock, BasicBlock* Default); 61 BasicBlock* newLeafBlock(CaseRange& Leaf, Value* Val, 62 BasicBlock* OrigBlock, BasicBlock* Default); 63 unsigned Clusterify(CaseVector& Cases, SwitchInst *SI); 64 }; 65 66 /// The comparison function for sorting the switch case values in the vector. 67 /// WARNING: Case ranges should be disjoint! 68 struct CaseCmp { 69 bool operator () (const LowerSwitch::CaseRange& C1, 70 const LowerSwitch::CaseRange& C2) { 71 72 const ConstantInt* CI1 = cast<const ConstantInt>(C1.Low); 73 const ConstantInt* CI2 = cast<const ConstantInt>(C2.High); 74 return CI1->getValue().slt(CI2->getValue()); 75 } 76 }; 77 78 RegisterPass<LowerSwitch> 79 X("lowerswitch", "Lower SwitchInst's to branches"); 80} 81 82// Publically exposed interface to pass... 83const PassInfo *llvm::LowerSwitchID = X.getPassInfo(); 84// createLowerSwitchPass - Interface to this file... 85FunctionPass *llvm::createLowerSwitchPass() { 86 return new LowerSwitch(); 87} 88 89bool LowerSwitch::runOnFunction(Function &F) { 90 bool Changed = false; 91 92 for (Function::iterator I = F.begin(), E = F.end(); I != E; ) { 93 BasicBlock *Cur = I++; // Advance over block so we don't traverse new blocks 94 95 if (SwitchInst *SI = dyn_cast<SwitchInst>(Cur->getTerminator())) { 96 Changed = true; 97 processSwitchInst(SI); 98 } 99 } 100 101 return Changed; 102} 103 104// operator<< - Used for debugging purposes. 105// 106static std::ostream& operator<<(std::ostream &O, 107 const LowerSwitch::CaseVector &C) { 108 O << "["; 109 110 for (LowerSwitch::CaseVector::const_iterator B = C.begin(), 111 E = C.end(); B != E; ) { 112 O << *B->Low << " -" << *B->High; 113 if (++B != E) O << ", "; 114 } 115 116 return O << "]"; 117} 118 119static OStream& operator<<(OStream &O, const LowerSwitch::CaseVector &C) { 120 if (O.stream()) *O.stream() << C; 121 return O; 122} 123 124// switchConvert - Convert the switch statement into a binary lookup of 125// the case values. The function recursively builds this tree. 126// 127BasicBlock* LowerSwitch::switchConvert(CaseItr Begin, CaseItr End, 128 Value* Val, BasicBlock* OrigBlock, 129 BasicBlock* Default) 130{ 131 unsigned Size = End - Begin; 132 133 if (Size == 1) 134 return newLeafBlock(*Begin, Val, OrigBlock, Default); 135 136 unsigned Mid = Size / 2; 137 std::vector<CaseRange> LHS(Begin, Begin + Mid); 138 DOUT << "LHS: " << LHS << "\n"; 139 std::vector<CaseRange> RHS(Begin + Mid, End); 140 DOUT << "RHS: " << RHS << "\n"; 141 142 CaseRange& Pivot = *(Begin + Mid); 143 DEBUG( DOUT << "Pivot ==> " 144 << cast<ConstantInt>(Pivot.Low)->getValue().toStringSigned(10) 145 << " -" 146 << cast<ConstantInt>(Pivot.High)->getValue().toStringSigned(10) 147 << "\n"); 148 149 BasicBlock* LBranch = switchConvert(LHS.begin(), LHS.end(), Val, 150 OrigBlock, Default); 151 BasicBlock* RBranch = switchConvert(RHS.begin(), RHS.end(), Val, 152 OrigBlock, Default); 153 154 // Create a new node that checks if the value is < pivot. Go to the 155 // left branch if it is and right branch if not. 156 Function* F = OrigBlock->getParent(); 157 BasicBlock* NewNode = new BasicBlock("NodeBlock"); 158 Function::iterator FI = OrigBlock; 159 F->getBasicBlockList().insert(++FI, NewNode); 160 161 ICmpInst* Comp = new ICmpInst(ICmpInst::ICMP_SLT, Val, Pivot.Low, "Pivot"); 162 NewNode->getInstList().push_back(Comp); 163 new BranchInst(LBranch, RBranch, Comp, NewNode); 164 return NewNode; 165} 166 167// newLeafBlock - Create a new leaf block for the binary lookup tree. It 168// checks if the switch's value == the case's value. If not, then it 169// jumps to the default branch. At this point in the tree, the value 170// can't be another valid case value, so the jump to the "default" branch 171// is warranted. 172// 173BasicBlock* LowerSwitch::newLeafBlock(CaseRange& Leaf, Value* Val, 174 BasicBlock* OrigBlock, 175 BasicBlock* Default) 176{ 177 Function* F = OrigBlock->getParent(); 178 BasicBlock* NewLeaf = new BasicBlock("LeafBlock"); 179 Function::iterator FI = OrigBlock; 180 F->getBasicBlockList().insert(++FI, NewLeaf); 181 182 // Emit comparison 183 ICmpInst* Comp = NULL; 184 if (Leaf.Low == Leaf.High) { 185 // Make the seteq instruction... 186 Comp = new ICmpInst(ICmpInst::ICMP_EQ, Val, Leaf.Low, 187 "SwitchLeaf", NewLeaf); 188 } else { 189 // Make range comparison 190 if (cast<ConstantInt>(Leaf.Low)->isMinValue(true /*isSigned*/)) { 191 // Val >= Min && Val <= Hi --> Val <= Hi 192 Comp = new ICmpInst(ICmpInst::ICMP_SLE, Val, Leaf.High, 193 "SwitchLeaf", NewLeaf); 194 } else if (cast<ConstantInt>(Leaf.Low)->isZero()) { 195 // Val >= 0 && Val <= Hi --> Val <=u Hi 196 Comp = new ICmpInst(ICmpInst::ICMP_ULE, Val, Leaf.High, 197 "SwitchLeaf", NewLeaf); 198 } else { 199 // Emit V-Lo <=u Hi-Lo 200 Constant* NegLo = ConstantExpr::getNeg(Leaf.Low); 201 Instruction* Add = BinaryOperator::createAdd(Val, NegLo, 202 Val->getName()+".off", 203 NewLeaf); 204 Constant *UpperBound = ConstantExpr::getAdd(NegLo, Leaf.High); 205 Comp = new ICmpInst(ICmpInst::ICMP_ULE, Add, UpperBound, 206 "SwitchLeaf", NewLeaf); 207 } 208 } 209 210 // Make the conditional branch... 211 BasicBlock* Succ = Leaf.BB; 212 new BranchInst(Succ, Default, Comp, NewLeaf); 213 214 // If there were any PHI nodes in this successor, rewrite one entry 215 // from OrigBlock to come from NewLeaf. 216 for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) { 217 PHINode* PN = cast<PHINode>(I); 218 // Remove all but one incoming entries from the cluster 219 uint64_t Range = cast<ConstantInt>(Leaf.High)->getSExtValue() - 220 cast<ConstantInt>(Leaf.Low)->getSExtValue(); 221 for (uint64_t j = 0; j < Range; ++j) { 222 PN->removeIncomingValue(OrigBlock); 223 } 224 225 int BlockIdx = PN->getBasicBlockIndex(OrigBlock); 226 assert(BlockIdx != -1 && "Switch didn't go to this successor??"); 227 PN->setIncomingBlock((unsigned)BlockIdx, NewLeaf); 228 } 229 230 return NewLeaf; 231} 232 233// Clusterify - Transform simple list of Cases into list of CaseRange's 234unsigned LowerSwitch::Clusterify(CaseVector& Cases, SwitchInst *SI) { 235 unsigned numCmps = 0; 236 237 // Start with "simple" cases 238 for (unsigned i = 1; i < SI->getNumSuccessors(); ++i) 239 Cases.push_back(CaseRange(SI->getSuccessorValue(i), 240 SI->getSuccessorValue(i), 241 SI->getSuccessor(i))); 242 sort(Cases.begin(), Cases.end(), CaseCmp()); 243 244 // Merge case into clusters 245 if (Cases.size()>=2) 246 for (CaseItr I=Cases.begin(), J=++(Cases.begin()), E=Cases.end(); J!=E; ) { 247 int64_t nextValue = cast<ConstantInt>(J->Low)->getSExtValue(); 248 int64_t currentValue = cast<ConstantInt>(I->High)->getSExtValue(); 249 BasicBlock* nextBB = J->BB; 250 BasicBlock* currentBB = I->BB; 251 252 // If the two neighboring cases go to the same destination, merge them 253 // into a single case. 254 if ((nextValue-currentValue==1) && (currentBB == nextBB)) { 255 I->High = J->High; 256 J = Cases.erase(J); 257 } else { 258 I = J++; 259 } 260 } 261 262 for (CaseItr I=Cases.begin(), E=Cases.end(); I!=E; ++I, ++numCmps) { 263 if (I->Low != I->High) 264 // A range counts double, since it requires two compares. 265 ++numCmps; 266 } 267 268 return numCmps; 269} 270 271// processSwitchInst - Replace the specified switch instruction with a sequence 272// of chained if-then insts in a balanced binary search. 273// 274void LowerSwitch::processSwitchInst(SwitchInst *SI) { 275 BasicBlock *CurBlock = SI->getParent(); 276 BasicBlock *OrigBlock = CurBlock; 277 Function *F = CurBlock->getParent(); 278 Value *Val = SI->getOperand(0); // The value we are switching on... 279 BasicBlock* Default = SI->getDefaultDest(); 280 281 // If there is only the default destination, don't bother with the code below. 282 if (SI->getNumOperands() == 2) { 283 new BranchInst(SI->getDefaultDest(), CurBlock); 284 CurBlock->getInstList().erase(SI); 285 return; 286 } 287 288 // Create a new, empty default block so that the new hierarchy of 289 // if-then statements go to this and the PHI nodes are happy. 290 BasicBlock* NewDefault = new BasicBlock("NewDefault"); 291 F->getBasicBlockList().insert(Default, NewDefault); 292 293 new BranchInst(Default, NewDefault); 294 295 // If there is an entry in any PHI nodes for the default edge, make sure 296 // to update them as well. 297 for (BasicBlock::iterator I = Default->begin(); isa<PHINode>(I); ++I) { 298 PHINode *PN = cast<PHINode>(I); 299 int BlockIdx = PN->getBasicBlockIndex(OrigBlock); 300 assert(BlockIdx != -1 && "Switch didn't go to this successor??"); 301 PN->setIncomingBlock((unsigned)BlockIdx, NewDefault); 302 } 303 304 // Prepare cases vector. 305 CaseVector Cases; 306 unsigned numCmps = Clusterify(Cases, SI); 307 308 DOUT << "Clusterify finished. Total clusters: " << Cases.size() 309 << ". Total compares: " << numCmps << "\n"; 310 DOUT << "Cases: " << Cases << "\n"; 311 312 BasicBlock* SwitchBlock = switchConvert(Cases.begin(), Cases.end(), Val, 313 OrigBlock, NewDefault); 314 315 // Branch to our shiny new if-then stuff... 316 new BranchInst(SwitchBlock, OrigBlock); 317 318 // We are now done with the switch instruction, delete it. 319 CurBlock->getInstList().erase(SI); 320} 321