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