lib/Transforms/Utils/LowerSwitch.cpp

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