1 //===- LowerSwitch.cpp - Eliminate Switch instructions --------------------===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
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.
14 //===----------------------------------------------------------------------===//
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/Compiler.h"
25 #include "llvm/Support/Debug.h"
26 #include "llvm/Support/raw_ostream.h"
31 /// LowerSwitch Pass - Replace all SwitchInst instructions with chained branch
33 class LowerSwitch : public FunctionPass {
35 static char ID; // Pass identification, replacement for typeid
36 LowerSwitch() : FunctionPass(ID) {}
38 virtual bool runOnFunction(Function &F);
40 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
41 // This is a cluster of orthogonal Transforms
42 AU.addPreserved<UnifyFunctionExitNodes>();
43 AU.addPreserved("mem2reg");
44 AU.addPreservedID(LowerInvokePassID);
52 CaseRange(Constant *low = 0, Constant *high = 0, BasicBlock *bb = 0) :
53 Low(low), High(high), BB(bb) { }
56 typedef std::vector<CaseRange> CaseVector;
57 typedef std::vector<CaseRange>::iterator CaseItr;
59 void processSwitchInst(SwitchInst *SI);
61 BasicBlock* switchConvert(CaseItr Begin, CaseItr End, Value* Val,
62 BasicBlock* OrigBlock, BasicBlock* Default);
63 BasicBlock* newLeafBlock(CaseRange& Leaf, Value* Val,
64 BasicBlock* OrigBlock, BasicBlock* Default);
65 unsigned Clusterify(CaseVector& Cases, SwitchInst *SI);
68 /// The comparison function for sorting the switch case values in the vector.
69 /// WARNING: Case ranges should be disjoint!
71 bool operator () (const LowerSwitch::CaseRange& C1,
72 const LowerSwitch::CaseRange& C2) {
74 const ConstantInt* CI1 = cast<const ConstantInt>(C1.Low);
75 const ConstantInt* CI2 = cast<const ConstantInt>(C2.High);
76 return CI1->getValue().slt(CI2->getValue());
81 char LowerSwitch::ID = 0;
82 INITIALIZE_PASS(LowerSwitch, "lowerswitch",
83 "Lower SwitchInst's to branches", false, false);
85 // Publically exposed interface to pass...
86 char &llvm::LowerSwitchID = LowerSwitch::ID;
87 // createLowerSwitchPass - Interface to this file...
88 FunctionPass *llvm::createLowerSwitchPass() {
89 return new LowerSwitch();
92 bool LowerSwitch::runOnFunction(Function &F) {
95 for (Function::iterator I = F.begin(), E = F.end(); I != E; ) {
96 BasicBlock *Cur = I++; // Advance over block so we don't traverse new blocks
98 if (SwitchInst *SI = dyn_cast<SwitchInst>(Cur->getTerminator())) {
100 processSwitchInst(SI);
107 // operator<< - Used for debugging purposes.
109 static raw_ostream& operator<<(raw_ostream &O,
110 const LowerSwitch::CaseVector &C) ATTRIBUTE_USED;
111 static raw_ostream& operator<<(raw_ostream &O,
112 const LowerSwitch::CaseVector &C) {
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 << ", ";
124 // switchConvert - Convert the switch statement into a binary lookup of
125 // the case values. The function recursively builds this tree.
127 BasicBlock* LowerSwitch::switchConvert(CaseItr Begin, CaseItr End,
128 Value* Val, BasicBlock* OrigBlock,
131 unsigned Size = End - Begin;
134 return newLeafBlock(*Begin, Val, OrigBlock, Default);
136 unsigned Mid = Size / 2;
137 std::vector<CaseRange> LHS(Begin, Begin + Mid);
138 DEBUG(dbgs() << "LHS: " << LHS << "\n");
139 std::vector<CaseRange> RHS(Begin + Mid, End);
140 DEBUG(dbgs() << "RHS: " << RHS << "\n");
142 CaseRange& Pivot = *(Begin + Mid);
143 DEBUG(dbgs() << "Pivot ==> "
144 << cast<ConstantInt>(Pivot.Low)->getValue() << " -"
145 << cast<ConstantInt>(Pivot.High)->getValue() << "\n");
147 BasicBlock* LBranch = switchConvert(LHS.begin(), LHS.end(), Val,
149 BasicBlock* RBranch = switchConvert(RHS.begin(), RHS.end(), Val,
152 // Create a new node that checks if the value is < pivot. Go to the
153 // left branch if it is and right branch if not.
154 Function* F = OrigBlock->getParent();
155 BasicBlock* NewNode = BasicBlock::Create(Val->getContext(), "NodeBlock");
156 Function::iterator FI = OrigBlock;
157 F->getBasicBlockList().insert(++FI, NewNode);
159 ICmpInst* Comp = new ICmpInst(ICmpInst::ICMP_SLT,
160 Val, Pivot.Low, "Pivot");
161 NewNode->getInstList().push_back(Comp);
162 BranchInst::Create(LBranch, RBranch, Comp, NewNode);
166 // newLeafBlock - Create a new leaf block for the binary lookup tree. It
167 // checks if the switch's value == the case's value. If not, then it
168 // jumps to the default branch. At this point in the tree, the value
169 // can't be another valid case value, so the jump to the "default" branch
172 BasicBlock* LowerSwitch::newLeafBlock(CaseRange& Leaf, Value* Val,
173 BasicBlock* OrigBlock,
176 Function* F = OrigBlock->getParent();
177 BasicBlock* NewLeaf = BasicBlock::Create(Val->getContext(), "LeafBlock");
178 Function::iterator FI = OrigBlock;
179 F->getBasicBlockList().insert(++FI, NewLeaf);
182 ICmpInst* Comp = NULL;
183 if (Leaf.Low == Leaf.High) {
184 // Make the seteq instruction...
185 Comp = new ICmpInst(*NewLeaf, ICmpInst::ICMP_EQ, Val,
186 Leaf.Low, "SwitchLeaf");
188 // Make range comparison
189 if (cast<ConstantInt>(Leaf.Low)->isMinValue(true /*isSigned*/)) {
190 // Val >= Min && Val <= Hi --> Val <= Hi
191 Comp = new ICmpInst(*NewLeaf, ICmpInst::ICMP_SLE, Val, Leaf.High,
193 } else if (cast<ConstantInt>(Leaf.Low)->isZero()) {
194 // Val >= 0 && Val <= Hi --> Val <=u Hi
195 Comp = new ICmpInst(*NewLeaf, ICmpInst::ICMP_ULE, Val, Leaf.High,
198 // Emit V-Lo <=u Hi-Lo
199 Constant* NegLo = ConstantExpr::getNeg(Leaf.Low);
200 Instruction* Add = BinaryOperator::CreateAdd(Val, NegLo,
201 Val->getName()+".off",
203 Constant *UpperBound = ConstantExpr::getAdd(NegLo, Leaf.High);
204 Comp = new ICmpInst(*NewLeaf, ICmpInst::ICMP_ULE, Add, UpperBound,
209 // Make the conditional branch...
210 BasicBlock* Succ = Leaf.BB;
211 BranchInst::Create(Succ, Default, Comp, NewLeaf);
213 // If there were any PHI nodes in this successor, rewrite one entry
214 // from OrigBlock to come from NewLeaf.
215 for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) {
216 PHINode* PN = cast<PHINode>(I);
217 // Remove all but one incoming entries from the cluster
218 uint64_t Range = cast<ConstantInt>(Leaf.High)->getSExtValue() -
219 cast<ConstantInt>(Leaf.Low)->getSExtValue();
220 for (uint64_t j = 0; j < Range; ++j) {
221 PN->removeIncomingValue(OrigBlock);
224 int BlockIdx = PN->getBasicBlockIndex(OrigBlock);
225 assert(BlockIdx != -1 && "Switch didn't go to this successor??");
226 PN->setIncomingBlock((unsigned)BlockIdx, NewLeaf);
232 // Clusterify - Transform simple list of Cases into list of CaseRange's
233 unsigned LowerSwitch::Clusterify(CaseVector& Cases, SwitchInst *SI) {
234 unsigned numCmps = 0;
236 // Start with "simple" cases
237 for (unsigned i = 1; i < SI->getNumSuccessors(); ++i)
238 Cases.push_back(CaseRange(SI->getSuccessorValue(i),
239 SI->getSuccessorValue(i),
240 SI->getSuccessor(i)));
241 std::sort(Cases.begin(), Cases.end(), CaseCmp());
243 // Merge case into clusters
245 for (CaseItr I=Cases.begin(), J=llvm::next(Cases.begin()); J!=Cases.end(); ) {
246 int64_t nextValue = cast<ConstantInt>(J->Low)->getSExtValue();
247 int64_t currentValue = cast<ConstantInt>(I->High)->getSExtValue();
248 BasicBlock* nextBB = J->BB;
249 BasicBlock* currentBB = I->BB;
251 // If the two neighboring cases go to the same destination, merge them
252 // into a single case.
253 if ((nextValue-currentValue==1) && (currentBB == nextBB)) {
261 for (CaseItr I=Cases.begin(), E=Cases.end(); I!=E; ++I, ++numCmps) {
262 if (I->Low != I->High)
263 // A range counts double, since it requires two compares.
270 // processSwitchInst - Replace the specified switch instruction with a sequence
271 // of chained if-then insts in a balanced binary search.
273 void LowerSwitch::processSwitchInst(SwitchInst *SI) {
274 BasicBlock *CurBlock = SI->getParent();
275 BasicBlock *OrigBlock = CurBlock;
276 Function *F = CurBlock->getParent();
277 Value *Val = SI->getOperand(0); // The value we are switching on...
278 BasicBlock* Default = SI->getDefaultDest();
280 // If there is only the default destination, don't bother with the code below.
281 if (SI->getNumOperands() == 2) {
282 BranchInst::Create(SI->getDefaultDest(), CurBlock);
283 CurBlock->getInstList().erase(SI);
287 // Create a new, empty default block so that the new hierarchy of
288 // if-then statements go to this and the PHI nodes are happy.
289 BasicBlock* NewDefault = BasicBlock::Create(SI->getContext(), "NewDefault");
290 F->getBasicBlockList().insert(Default, NewDefault);
292 BranchInst::Create(Default, NewDefault);
294 // If there is an entry in any PHI nodes for the default edge, make sure
295 // to update them as well.
296 for (BasicBlock::iterator I = Default->begin(); isa<PHINode>(I); ++I) {
297 PHINode *PN = cast<PHINode>(I);
298 int BlockIdx = PN->getBasicBlockIndex(OrigBlock);
299 assert(BlockIdx != -1 && "Switch didn't go to this successor??");
300 PN->setIncomingBlock((unsigned)BlockIdx, NewDefault);
303 // Prepare cases vector.
305 unsigned numCmps = Clusterify(Cases, SI);
307 DEBUG(dbgs() << "Clusterify finished. Total clusters: " << Cases.size()
308 << ". Total compares: " << numCmps << "\n");
309 DEBUG(dbgs() << "Cases: " << Cases << "\n");
312 BasicBlock* SwitchBlock = switchConvert(Cases.begin(), Cases.end(), Val,
313 OrigBlock, NewDefault);
315 // Branch to our shiny new if-then stuff...
316 BranchInst::Create(SwitchBlock, OrigBlock);
318 // We are now done with the switch instruction, delete it.
319 CurBlock->getInstList().erase(SI);