LowerSwitch.cpp revision 7cbd8a3e92221437048b484d5ef9c0a22d0f8c58
1//===- LowerSwitch.cpp - Eliminate Switch instructions --------------------===// 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// The LowerSwitch transformation rewrites switch instructions with a sequence 11// of branches, which allows targets to get away with not implementing the 12// switch instruction 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/ADT/STLExtras.h" 23#include "llvm/Support/Debug.h" 24#include "llvm/Support/Compiler.h" 25#include <algorithm> 26using namespace llvm; 27 28namespace { 29 /// LowerSwitch Pass - Replace all SwitchInst instructions with chained branch 30 /// instructions. Note that this cannot be a BasicBlock pass because it 31 /// modifies the CFG! 32 class VISIBILITY_HIDDEN LowerSwitch : public FunctionPass { 33 public: 34 static char ID; // Pass identification, replacement for typeid 35 LowerSwitch() : FunctionPass((intptr_t) &ID) {} 36 37 virtual bool runOnFunction(Function &F); 38 39 virtual void getAnalysisUsage(AnalysisUsage &AU) const { 40 // This is a cluster of orthogonal Transforms 41 AU.addPreserved<UnifyFunctionExitNodes>(); 42 AU.addPreservedID(PromoteMemoryToRegisterID); 43 AU.addPreservedID(LowerInvokePassID); 44 AU.addPreservedID(LowerAllocationsID); 45 } 46 47 struct CaseRange { 48 Constant* Low; 49 Constant* High; 50 BasicBlock* BB; 51 52 CaseRange() : Low(0), High(0), BB(0) { } 53 CaseRange(Constant* low, Constant* high, BasicBlock* bb) : 54 Low(low), High(high), BB(bb) { } 55 }; 56 57 typedef std::vector<CaseRange> CaseVector; 58 typedef std::vector<CaseRange>::iterator CaseItr; 59 private: 60 void processSwitchInst(SwitchInst *SI); 61 62 BasicBlock* switchConvert(CaseItr Begin, CaseItr End, Value* Val, 63 BasicBlock* OrigBlock, BasicBlock* Default); 64 BasicBlock* newLeafBlock(CaseRange& Leaf, Value* Val, 65 BasicBlock* OrigBlock, BasicBlock* Default); 66 unsigned Clusterify(CaseVector& Cases, SwitchInst *SI); 67 }; 68 69 /// The comparison function for sorting the switch case values in the vector. 70 /// WARNING: Case ranges should be disjoint! 71 struct CaseCmp { 72 bool operator () (const LowerSwitch::CaseRange& C1, 73 const LowerSwitch::CaseRange& C2) { 74 75 const ConstantInt* CI1 = cast<const ConstantInt>(C1.Low); 76 const ConstantInt* CI2 = cast<const ConstantInt>(C2.High); 77 return CI1->getValue().slt(CI2->getValue()); 78 } 79 }; 80} 81 82char LowerSwitch::ID = 0; 83static RegisterPass<LowerSwitch> 84X("lowerswitch", "Lower SwitchInst's to branches"); 85 86// Publically exposed interface to pass... 87const PassInfo *const llvm::LowerSwitchID = &X; 88// createLowerSwitchPass - Interface to this file... 89FunctionPass *llvm::createLowerSwitchPass() { 90 return new LowerSwitch(); 91} 92 93bool LowerSwitch::runOnFunction(Function &F) { 94 bool Changed = false; 95 96 for (Function::iterator I = F.begin(), E = F.end(); I != E; ) { 97 BasicBlock *Cur = I++; // Advance over block so we don't traverse new blocks 98 99 if (SwitchInst *SI = dyn_cast<SwitchInst>(Cur->getTerminator())) { 100 Changed = true; 101 processSwitchInst(SI); 102 } 103 } 104 105 return Changed; 106} 107 108// operator<< - Used for debugging purposes. 109// 110static std::ostream& operator<<(std::ostream &O, 111 const LowerSwitch::CaseVector &C) { 112 O << "["; 113 114 for (LowerSwitch::CaseVector::const_iterator B = C.begin(), 115 E = C.end(); B != E; ) { 116 O << *B->Low << " -" << *B->High; 117 if (++B != E) O << ", "; 118 } 119 120 return O << "]"; 121} 122 123static OStream& operator<<(OStream &O, const LowerSwitch::CaseVector &C) { 124 if (O.stream()) *O.stream() << C; 125 return O; 126} 127 128// switchConvert - Convert the switch statement into a binary lookup of 129// the case values. The function recursively builds this tree. 130// 131BasicBlock* LowerSwitch::switchConvert(CaseItr Begin, CaseItr End, 132 Value* Val, BasicBlock* OrigBlock, 133 BasicBlock* Default) 134{ 135 unsigned Size = End - Begin; 136 137 if (Size == 1) 138 return newLeafBlock(*Begin, Val, OrigBlock, Default); 139 140 unsigned Mid = Size / 2; 141 std::vector<CaseRange> LHS(Begin, Begin + Mid); 142 DOUT << "LHS: " << LHS << "\n"; 143 std::vector<CaseRange> RHS(Begin + Mid, End); 144 DOUT << "RHS: " << RHS << "\n"; 145 146 CaseRange& Pivot = *(Begin + Mid); 147 DEBUG( DOUT << "Pivot ==> " 148 << cast<ConstantInt>(Pivot.Low)->getValue().toStringSigned(10) 149 << " -" 150 << cast<ConstantInt>(Pivot.High)->getValue().toStringSigned(10) 151 << "\n"); 152 153 BasicBlock* LBranch = switchConvert(LHS.begin(), LHS.end(), Val, 154 OrigBlock, Default); 155 BasicBlock* RBranch = switchConvert(RHS.begin(), RHS.end(), Val, 156 OrigBlock, Default); 157 158 // Create a new node that checks if the value is < pivot. Go to the 159 // left branch if it is and right branch if not. 160 Function* F = OrigBlock->getParent(); 161 BasicBlock* NewNode = BasicBlock::Create("NodeBlock"); 162 Function::iterator FI = OrigBlock; 163 F->getBasicBlockList().insert(++FI, NewNode); 164 165 ICmpInst* Comp = new ICmpInst(ICmpInst::ICMP_SLT, Val, Pivot.Low, "Pivot"); 166 NewNode->getInstList().push_back(Comp); 167 BranchInst::Create(LBranch, RBranch, Comp, NewNode); 168 return NewNode; 169} 170 171// newLeafBlock - Create a new leaf block for the binary lookup tree. It 172// checks if the switch's value == the case's value. If not, then it 173// jumps to the default branch. At this point in the tree, the value 174// can't be another valid case value, so the jump to the "default" branch 175// is warranted. 176// 177BasicBlock* LowerSwitch::newLeafBlock(CaseRange& Leaf, Value* Val, 178 BasicBlock* OrigBlock, 179 BasicBlock* Default) 180{ 181 Function* F = OrigBlock->getParent(); 182 BasicBlock* NewLeaf = BasicBlock::Create("LeafBlock"); 183 Function::iterator FI = OrigBlock; 184 F->getBasicBlockList().insert(++FI, NewLeaf); 185 186 // Emit comparison 187 ICmpInst* Comp = NULL; 188 if (Leaf.Low == Leaf.High) { 189 // Make the seteq instruction... 190 Comp = new ICmpInst(ICmpInst::ICMP_EQ, Val, Leaf.Low, 191 "SwitchLeaf", NewLeaf); 192 } else { 193 // Make range comparison 194 if (cast<ConstantInt>(Leaf.Low)->isMinValue(true /*isSigned*/)) { 195 // Val >= Min && Val <= Hi --> Val <= Hi 196 Comp = new ICmpInst(ICmpInst::ICMP_SLE, Val, Leaf.High, 197 "SwitchLeaf", NewLeaf); 198 } else if (cast<ConstantInt>(Leaf.Low)->isZero()) { 199 // Val >= 0 && Val <= Hi --> Val <=u Hi 200 Comp = new ICmpInst(ICmpInst::ICMP_ULE, Val, Leaf.High, 201 "SwitchLeaf", NewLeaf); 202 } else { 203 // Emit V-Lo <=u Hi-Lo 204 Constant* NegLo = ConstantExpr::getNeg(Leaf.Low); 205 Instruction* Add = BinaryOperator::CreateAdd(Val, NegLo, 206 Val->getName()+".off", 207 NewLeaf); 208 Constant *UpperBound = ConstantExpr::getAdd(NegLo, Leaf.High); 209 Comp = new ICmpInst(ICmpInst::ICMP_ULE, Add, UpperBound, 210 "SwitchLeaf", NewLeaf); 211 } 212 } 213 214 // Make the conditional branch... 215 BasicBlock* Succ = Leaf.BB; 216 BranchInst::Create(Succ, Default, Comp, NewLeaf); 217 218 // If there were any PHI nodes in this successor, rewrite one entry 219 // from OrigBlock to come from NewLeaf. 220 for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) { 221 PHINode* PN = cast<PHINode>(I); 222 // Remove all but one incoming entries from the cluster 223 uint64_t Range = cast<ConstantInt>(Leaf.High)->getSExtValue() - 224 cast<ConstantInt>(Leaf.Low)->getSExtValue(); 225 for (uint64_t j = 0; j < Range; ++j) { 226 PN->removeIncomingValue(OrigBlock); 227 } 228 229 int BlockIdx = PN->getBasicBlockIndex(OrigBlock); 230 assert(BlockIdx != -1 && "Switch didn't go to this successor??"); 231 PN->setIncomingBlock((unsigned)BlockIdx, NewLeaf); 232 } 233 234 return NewLeaf; 235} 236 237// Clusterify - Transform simple list of Cases into list of CaseRange's 238unsigned LowerSwitch::Clusterify(CaseVector& Cases, SwitchInst *SI) { 239 unsigned numCmps = 0; 240 241 // Start with "simple" cases 242 for (unsigned i = 1; i < SI->getNumSuccessors(); ++i) 243 Cases.push_back(CaseRange(SI->getSuccessorValue(i), 244 SI->getSuccessorValue(i), 245 SI->getSuccessor(i))); 246 std::sort(Cases.begin(), Cases.end(), CaseCmp()); 247 248 // Merge case into clusters 249 if (Cases.size()>=2) 250 for (CaseItr I=Cases.begin(), J=next(Cases.begin()); J!=Cases.end(); ) { 251 int64_t nextValue = cast<ConstantInt>(J->Low)->getSExtValue(); 252 int64_t currentValue = cast<ConstantInt>(I->High)->getSExtValue(); 253 BasicBlock* nextBB = J->BB; 254 BasicBlock* currentBB = I->BB; 255 256 // If the two neighboring cases go to the same destination, merge them 257 // into a single case. 258 if ((nextValue-currentValue==1) && (currentBB == nextBB)) { 259 I->High = J->High; 260 J = Cases.erase(J); 261 } else { 262 I = J++; 263 } 264 } 265 266 for (CaseItr I=Cases.begin(), E=Cases.end(); I!=E; ++I, ++numCmps) { 267 if (I->Low != I->High) 268 // A range counts double, since it requires two compares. 269 ++numCmps; 270 } 271 272 return numCmps; 273} 274 275// processSwitchInst - Replace the specified switch instruction with a sequence 276// of chained if-then insts in a balanced binary search. 277// 278void LowerSwitch::processSwitchInst(SwitchInst *SI) { 279 BasicBlock *CurBlock = SI->getParent(); 280 BasicBlock *OrigBlock = CurBlock; 281 Function *F = CurBlock->getParent(); 282 Value *Val = SI->getOperand(0); // The value we are switching on... 283 BasicBlock* Default = SI->getDefaultDest(); 284 285 // If there is only the default destination, don't bother with the code below. 286 if (SI->getNumOperands() == 2) { 287 BranchInst::Create(SI->getDefaultDest(), CurBlock); 288 CurBlock->getInstList().erase(SI); 289 return; 290 } 291 292 // Create a new, empty default block so that the new hierarchy of 293 // if-then statements go to this and the PHI nodes are happy. 294 BasicBlock* NewDefault = BasicBlock::Create("NewDefault"); 295 F->getBasicBlockList().insert(Default, NewDefault); 296 297 BranchInst::Create(Default, NewDefault); 298 299 // If there is an entry in any PHI nodes for the default edge, make sure 300 // to update them as well. 301 for (BasicBlock::iterator I = Default->begin(); isa<PHINode>(I); ++I) { 302 PHINode *PN = cast<PHINode>(I); 303 int BlockIdx = PN->getBasicBlockIndex(OrigBlock); 304 assert(BlockIdx != -1 && "Switch didn't go to this successor??"); 305 PN->setIncomingBlock((unsigned)BlockIdx, NewDefault); 306 } 307 308 // Prepare cases vector. 309 CaseVector Cases; 310 unsigned numCmps = Clusterify(Cases, SI); 311 312 DOUT << "Clusterify finished. Total clusters: " << Cases.size() 313 << ". Total compares: " << numCmps << "\n"; 314 DOUT << "Cases: " << Cases << "\n"; 315 316 BasicBlock* SwitchBlock = switchConvert(Cases.begin(), Cases.end(), Val, 317 OrigBlock, NewDefault); 318 319 // Branch to our shiny new if-then stuff... 320 BranchInst::Create(SwitchBlock, OrigBlock); 321 322 // We are now done with the switch instruction, delete it. 323 CurBlock->getInstList().erase(SI); 324} 325