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/BasicBlockUtils.h"
18 #include "llvm/ADT/STLExtras.h"
19 #include "llvm/IR/Constants.h"
20 #include "llvm/IR/Function.h"
21 #include "llvm/IR/Instructions.h"
22 #include "llvm/IR/LLVMContext.h"
23 #include "llvm/IR/CFG.h"
24 #include "llvm/Pass.h"
25 #include "llvm/Support/Compiler.h"
26 #include "llvm/Support/Debug.h"
27 #include "llvm/Support/raw_ostream.h"
28 #include "llvm/Transforms/Utils/UnifyFunctionExitNodes.h"
32 #define DEBUG_TYPE "lower-switch"
35 /// LowerSwitch Pass - Replace all SwitchInst instructions with chained branch
37 class LowerSwitch : public FunctionPass {
39 static char ID; // Pass identification, replacement for typeid
40 LowerSwitch() : FunctionPass(ID) {
41 initializeLowerSwitchPass(*PassRegistry::getPassRegistry());
44 bool runOnFunction(Function &F) override;
46 void getAnalysisUsage(AnalysisUsage &AU) const override {
47 // This is a cluster of orthogonal Transforms
48 AU.addPreserved<UnifyFunctionExitNodes>();
49 AU.addPreserved("mem2reg");
50 AU.addPreservedID(LowerInvokePassID);
58 CaseRange(Constant *low = nullptr, Constant *high = nullptr,
59 BasicBlock *bb = nullptr) :
60 Low(low), High(high), BB(bb) { }
63 typedef std::vector<CaseRange> CaseVector;
64 typedef std::vector<CaseRange>::iterator CaseItr;
66 void processSwitchInst(SwitchInst *SI);
68 BasicBlock *switchConvert(CaseItr Begin, CaseItr End,
69 ConstantInt *LowerBound, ConstantInt *UpperBound,
70 Value *Val, BasicBlock *Predecessor,
71 BasicBlock *OrigBlock, BasicBlock *Default);
72 BasicBlock *newLeafBlock(CaseRange &Leaf, Value *Val, BasicBlock *OrigBlock,
74 unsigned Clusterify(CaseVector &Cases, SwitchInst *SI);
77 /// The comparison function for sorting the switch case values in the vector.
78 /// WARNING: Case ranges should be disjoint!
80 bool operator () (const LowerSwitch::CaseRange& C1,
81 const LowerSwitch::CaseRange& C2) {
83 const ConstantInt* CI1 = cast<const ConstantInt>(C1.Low);
84 const ConstantInt* CI2 = cast<const ConstantInt>(C2.High);
85 return CI1->getValue().slt(CI2->getValue());
90 char LowerSwitch::ID = 0;
91 INITIALIZE_PASS(LowerSwitch, "lowerswitch",
92 "Lower SwitchInst's to branches", false, false)
94 // Publicly exposed interface to pass...
95 char &llvm::LowerSwitchID = LowerSwitch::ID;
96 // createLowerSwitchPass - Interface to this file...
97 FunctionPass *llvm::createLowerSwitchPass() {
98 return new LowerSwitch();
101 bool LowerSwitch::runOnFunction(Function &F) {
102 bool Changed = false;
104 for (Function::iterator I = F.begin(), E = F.end(); I != E; ) {
105 BasicBlock *Cur = I++; // Advance over block so we don't traverse new blocks
107 if (SwitchInst *SI = dyn_cast<SwitchInst>(Cur->getTerminator())) {
109 processSwitchInst(SI);
116 // operator<< - Used for debugging purposes.
118 static raw_ostream& operator<<(raw_ostream &O,
119 const LowerSwitch::CaseVector &C)
121 static raw_ostream& operator<<(raw_ostream &O,
122 const LowerSwitch::CaseVector &C) {
125 for (LowerSwitch::CaseVector::const_iterator B = C.begin(),
126 E = C.end(); B != E; ) {
127 O << *B->Low << " -" << *B->High;
128 if (++B != E) O << ", ";
134 /// \brief Update the first occurrence of the "switch statement" BB in the PHI
135 /// node with the "new" BB. The other occurrences will be updated by subsequent
136 /// calls to this function.
138 /// Switch statements may have more than one incoming edge into the same BB if
139 /// they all have the same value. When the switch statement is converted these
140 /// incoming edges are now coming from multiple BBs.
141 static void fixPhis(BasicBlock *SuccBB, BasicBlock *OrigBB, BasicBlock *NewBB) {
142 for (BasicBlock::iterator I = SuccBB->begin(), E = SuccBB->getFirstNonPHI();
144 PHINode *PN = cast<PHINode>(I);
146 // Only update the first occurence.
147 for (unsigned Idx = 0, E = PN->getNumIncomingValues(); Idx != E; ++Idx) {
148 if (PN->getIncomingBlock(Idx) == OrigBB) {
149 PN->setIncomingBlock(Idx, NewBB);
156 // switchConvert - Convert the switch statement into a binary lookup of
157 // the case values. The function recursively builds this tree.
158 // LowerBound and UpperBound are used to keep track of the bounds for Val
159 // that have already been checked by a block emitted by one of the previous
160 // calls to switchConvert in the call stack.
161 BasicBlock *LowerSwitch::switchConvert(CaseItr Begin, CaseItr End,
162 ConstantInt *LowerBound,
163 ConstantInt *UpperBound, Value *Val,
164 BasicBlock *Predecessor,
165 BasicBlock *OrigBlock,
166 BasicBlock *Default) {
167 unsigned Size = End - Begin;
170 // Check if the Case Range is perfectly squeezed in between
171 // already checked Upper and Lower bounds. If it is then we can avoid
172 // emitting the code that checks if the value actually falls in the range
173 // because the bounds already tell us so.
174 if (Begin->Low == LowerBound && Begin->High == UpperBound) {
175 fixPhis(Begin->BB, OrigBlock, Predecessor);
178 return newLeafBlock(*Begin, Val, OrigBlock, Default);
181 unsigned Mid = Size / 2;
182 std::vector<CaseRange> LHS(Begin, Begin + Mid);
183 DEBUG(dbgs() << "LHS: " << LHS << "\n");
184 std::vector<CaseRange> RHS(Begin + Mid, End);
185 DEBUG(dbgs() << "RHS: " << RHS << "\n");
187 CaseRange &Pivot = *(Begin + Mid);
188 DEBUG(dbgs() << "Pivot ==> "
189 << cast<ConstantInt>(Pivot.Low)->getValue()
190 << " -" << cast<ConstantInt>(Pivot.High)->getValue() << "\n");
192 // NewLowerBound here should never be the integer minimal value.
193 // This is because it is computed from a case range that is never
194 // the smallest, so there is always a case range that has at least
196 ConstantInt *NewLowerBound = cast<ConstantInt>(Pivot.Low);
197 ConstantInt *NewUpperBound;
199 // If we don't have a Default block then it means that we can never
200 // have a value outside of a case range, so set the UpperBound to the highest
201 // value in the LHS part of the case ranges.
202 if (Default != nullptr) {
203 // Because NewLowerBound is never the smallest representable integer
204 // it is safe here to subtract one.
205 NewUpperBound = ConstantInt::get(NewLowerBound->getContext(),
206 NewLowerBound->getValue() - 1);
208 CaseItr LastLHS = LHS.begin() + LHS.size() - 1;
209 NewUpperBound = cast<ConstantInt>(LastLHS->High);
212 DEBUG(dbgs() << "LHS Bounds ==> ";
214 dbgs() << cast<ConstantInt>(LowerBound)->getSExtValue();
218 dbgs() << " - " << NewUpperBound->getSExtValue() << "\n";
219 dbgs() << "RHS Bounds ==> ";
220 dbgs() << NewLowerBound->getSExtValue() << " - ";
222 dbgs() << cast<ConstantInt>(UpperBound)->getSExtValue() << "\n";
227 // Create a new node that checks if the value is < pivot. Go to the
228 // left branch if it is and right branch if not.
229 Function* F = OrigBlock->getParent();
230 BasicBlock* NewNode = BasicBlock::Create(Val->getContext(), "NodeBlock");
232 ICmpInst* Comp = new ICmpInst(ICmpInst::ICMP_SLT,
233 Val, Pivot.Low, "Pivot");
235 BasicBlock *LBranch = switchConvert(LHS.begin(), LHS.end(), LowerBound,
236 NewUpperBound, Val, NewNode, OrigBlock,
238 BasicBlock *RBranch = switchConvert(RHS.begin(), RHS.end(), NewLowerBound,
239 UpperBound, Val, NewNode, OrigBlock,
242 Function::iterator FI = OrigBlock;
243 F->getBasicBlockList().insert(++FI, NewNode);
244 NewNode->getInstList().push_back(Comp);
246 BranchInst::Create(LBranch, RBranch, Comp, NewNode);
250 // newLeafBlock - Create a new leaf block for the binary lookup tree. It
251 // checks if the switch's value == the case's value. If not, then it
252 // jumps to the default branch. At this point in the tree, the value
253 // can't be another valid case value, so the jump to the "default" branch
256 BasicBlock* LowerSwitch::newLeafBlock(CaseRange& Leaf, Value* Val,
257 BasicBlock* OrigBlock,
260 Function* F = OrigBlock->getParent();
261 BasicBlock* NewLeaf = BasicBlock::Create(Val->getContext(), "LeafBlock");
262 Function::iterator FI = OrigBlock;
263 F->getBasicBlockList().insert(++FI, NewLeaf);
266 ICmpInst* Comp = nullptr;
267 if (Leaf.Low == Leaf.High) {
268 // Make the seteq instruction...
269 Comp = new ICmpInst(*NewLeaf, ICmpInst::ICMP_EQ, Val,
270 Leaf.Low, "SwitchLeaf");
272 // Make range comparison
273 if (cast<ConstantInt>(Leaf.Low)->isMinValue(true /*isSigned*/)) {
274 // Val >= Min && Val <= Hi --> Val <= Hi
275 Comp = new ICmpInst(*NewLeaf, ICmpInst::ICMP_SLE, Val, Leaf.High,
277 } else if (cast<ConstantInt>(Leaf.Low)->isZero()) {
278 // Val >= 0 && Val <= Hi --> Val <=u Hi
279 Comp = new ICmpInst(*NewLeaf, ICmpInst::ICMP_ULE, Val, Leaf.High,
282 // Emit V-Lo <=u Hi-Lo
283 Constant* NegLo = ConstantExpr::getNeg(Leaf.Low);
284 Instruction* Add = BinaryOperator::CreateAdd(Val, NegLo,
285 Val->getName()+".off",
287 Constant *UpperBound = ConstantExpr::getAdd(NegLo, Leaf.High);
288 Comp = new ICmpInst(*NewLeaf, ICmpInst::ICMP_ULE, Add, UpperBound,
293 // Make the conditional branch...
294 BasicBlock* Succ = Leaf.BB;
295 BranchInst::Create(Succ, Default, Comp, NewLeaf);
297 // If there were any PHI nodes in this successor, rewrite one entry
298 // from OrigBlock to come from NewLeaf.
299 for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) {
300 PHINode* PN = cast<PHINode>(I);
301 // Remove all but one incoming entries from the cluster
302 uint64_t Range = cast<ConstantInt>(Leaf.High)->getSExtValue() -
303 cast<ConstantInt>(Leaf.Low)->getSExtValue();
304 for (uint64_t j = 0; j < Range; ++j) {
305 PN->removeIncomingValue(OrigBlock);
308 int BlockIdx = PN->getBasicBlockIndex(OrigBlock);
309 assert(BlockIdx != -1 && "Switch didn't go to this successor??");
310 PN->setIncomingBlock((unsigned)BlockIdx, NewLeaf);
316 // Clusterify - Transform simple list of Cases into list of CaseRange's
317 unsigned LowerSwitch::Clusterify(CaseVector& Cases, SwitchInst *SI) {
318 unsigned numCmps = 0;
320 // Start with "simple" cases
321 for (SwitchInst::CaseIt i = SI->case_begin(), e = SI->case_end(); i != e; ++i)
322 Cases.push_back(CaseRange(i.getCaseValue(), i.getCaseValue(),
323 i.getCaseSuccessor()));
325 std::sort(Cases.begin(), Cases.end(), CaseCmp());
327 // Merge case into clusters
329 for (CaseItr I = Cases.begin(), J = std::next(Cases.begin());
331 int64_t nextValue = cast<ConstantInt>(J->Low)->getSExtValue();
332 int64_t currentValue = cast<ConstantInt>(I->High)->getSExtValue();
333 BasicBlock* nextBB = J->BB;
334 BasicBlock* currentBB = I->BB;
336 // If the two neighboring cases go to the same destination, merge them
337 // into a single case.
338 if ((nextValue-currentValue==1) && (currentBB == nextBB)) {
346 for (CaseItr I=Cases.begin(), E=Cases.end(); I!=E; ++I, ++numCmps) {
347 if (I->Low != I->High)
348 // A range counts double, since it requires two compares.
355 // processSwitchInst - Replace the specified switch instruction with a sequence
356 // of chained if-then insts in a balanced binary search.
358 void LowerSwitch::processSwitchInst(SwitchInst *SI) {
359 BasicBlock *CurBlock = SI->getParent();
360 BasicBlock *OrigBlock = CurBlock;
361 Function *F = CurBlock->getParent();
362 Value *Val = SI->getCondition(); // The value we are switching on...
363 BasicBlock* Default = SI->getDefaultDest();
365 // If there is only the default destination, don't bother with the code below.
366 if (!SI->getNumCases()) {
367 BranchInst::Create(SI->getDefaultDest(), CurBlock);
368 CurBlock->getInstList().erase(SI);
372 const bool DefaultIsUnreachable =
373 Default->size() == 1 && isa<UnreachableInst>(Default->getTerminator());
374 // Create a new, empty default block so that the new hierarchy of
375 // if-then statements go to this and the PHI nodes are happy.
376 // if the default block is set as an unreachable we avoid creating one
377 // because will never be a valid target.
378 BasicBlock *NewDefault = nullptr;
379 if (!DefaultIsUnreachable) {
380 NewDefault = BasicBlock::Create(SI->getContext(), "NewDefault");
381 F->getBasicBlockList().insert(Default, NewDefault);
383 BranchInst::Create(Default, NewDefault);
385 // If there is an entry in any PHI nodes for the default edge, make sure
386 // to update them as well.
387 for (BasicBlock::iterator I = Default->begin(); isa<PHINode>(I); ++I) {
388 PHINode *PN = cast<PHINode>(I);
389 int BlockIdx = PN->getBasicBlockIndex(OrigBlock);
390 assert(BlockIdx != -1 && "Switch didn't go to this successor??");
391 PN->setIncomingBlock((unsigned)BlockIdx, NewDefault);
394 // Prepare cases vector.
396 unsigned numCmps = Clusterify(Cases, SI);
398 DEBUG(dbgs() << "Clusterify finished. Total clusters: " << Cases.size()
399 << ". Total compares: " << numCmps << "\n");
400 DEBUG(dbgs() << "Cases: " << Cases << "\n");
403 ConstantInt *UpperBound = nullptr;
404 ConstantInt *LowerBound = nullptr;
406 // Optimize the condition where Default is an unreachable block. In this case
407 // we can make the bounds tightly fitted around the case value ranges,
408 // because we know that the value passed to the switch should always be
409 // exactly one of the case values.
410 if (DefaultIsUnreachable) {
411 CaseItr LastCase = Cases.begin() + Cases.size() - 1;
412 UpperBound = cast<ConstantInt>(LastCase->High);
413 LowerBound = cast<ConstantInt>(Cases.begin()->Low);
415 BasicBlock *SwitchBlock =
416 switchConvert(Cases.begin(), Cases.end(), LowerBound, UpperBound, Val,
417 OrigBlock, OrigBlock, NewDefault);
419 // Branch to our shiny new if-then stuff...
420 BranchInst::Create(SwitchBlock, OrigBlock);
422 // We are now done with the switch instruction, delete it.
423 CurBlock->getInstList().erase(SI);
425 pred_iterator PI = pred_begin(Default), E = pred_end(Default);
426 // If the Default block has no more predecessors just remove it
428 DeleteDeadBlock(Default);