1 //===- LowerSwitch.cpp - Eliminate Switch instructions --------------------===//
3 // The LLVM Compiler Infrastructure
5 // This file was developed by the LLVM research group and is distributed under
6 // the University of Illinois Open Source License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
10 // The LowerSwitch transformation rewrites switch statements with a sequence of
11 // branches, which allows targets to get away with not implementing the switch
12 // statement 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/Pass.h"
22 #include "llvm/Support/Debug.h"
23 #include "llvm/Support/Compiler.h"
24 #include "llvm/ADT/Statistic.h"
30 Statistic<> NumLowered("lowerswitch", "Number of SwitchInst's replaced");
32 /// LowerSwitch Pass - Replace all SwitchInst instructions with chained branch
33 /// instructions. Note that this cannot be a BasicBlock pass because it
35 class VISIBILITY_HIDDEN LowerSwitch : public FunctionPass {
37 virtual bool runOnFunction(Function &F);
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(LowerSelectID);
44 AU.addPreservedID(LowerInvokePassID);
45 AU.addPreservedID(LowerAllocationsID);
48 typedef std::pair<Constant*, BasicBlock*> Case;
49 typedef std::vector<Case>::iterator CaseItr;
51 void processSwitchInst(SwitchInst *SI);
53 BasicBlock* switchConvert(CaseItr Begin, CaseItr End, Value* Val,
54 BasicBlock* OrigBlock, BasicBlock* Default);
55 BasicBlock* newLeafBlock(Case& Leaf, Value* Val,
56 BasicBlock* OrigBlock, BasicBlock* Default);
59 /// The comparison function for sorting the switch case values in the vector.
61 bool operator () (const LowerSwitch::Case& C1,
62 const LowerSwitch::Case& C2) {
63 if (const ConstantUInt* U1 = dyn_cast<const ConstantUInt>(C1.first))
64 return U1->getValue() < cast<const ConstantUInt>(C2.first)->getValue();
66 const ConstantSInt* S1 = dyn_cast<const ConstantSInt>(C1.first);
67 return S1->getValue() < cast<const ConstantSInt>(C2.first)->getValue();
71 RegisterOpt<LowerSwitch>
72 X("lowerswitch", "Lower SwitchInst's to branches");
75 // Publically exposed interface to pass...
76 const PassInfo *llvm::LowerSwitchID = X.getPassInfo();
77 // createLowerSwitchPass - Interface to this file...
78 FunctionPass *llvm::createLowerSwitchPass() {
79 return new LowerSwitch();
82 bool LowerSwitch::runOnFunction(Function &F) {
85 for (Function::iterator I = F.begin(), E = F.end(); I != E; ) {
86 BasicBlock *Cur = I++; // Advance over block so we don't traverse new blocks
88 if (SwitchInst *SI = dyn_cast<SwitchInst>(Cur->getTerminator())) {
90 processSwitchInst(SI);
97 // operator<< - Used for debugging purposes.
99 std::ostream& operator<<(std::ostream &O,
100 const std::vector<LowerSwitch::Case> &C) {
103 for (std::vector<LowerSwitch::Case>::const_iterator B = C.begin(),
104 E = C.end(); B != E; ) {
106 if (++B != E) O << ", ";
112 // switchConvert - Convert the switch statement into a binary lookup of
113 // the case values. The function recursively builds this tree.
115 BasicBlock* LowerSwitch::switchConvert(CaseItr Begin, CaseItr End,
116 Value* Val, BasicBlock* OrigBlock,
119 unsigned Size = End - Begin;
122 return newLeafBlock(*Begin, Val, OrigBlock, Default);
124 unsigned Mid = Size / 2;
125 std::vector<Case> LHS(Begin, Begin + Mid);
126 DEBUG(std::cerr << "LHS: " << LHS << "\n");
127 std::vector<Case> RHS(Begin + Mid, End);
128 DEBUG(std::cerr << "RHS: " << RHS << "\n");
130 Case& Pivot = *(Begin + Mid);
131 DEBUG(std::cerr << "Pivot ==> "
132 << (int64_t)cast<ConstantInt>(Pivot.first)->getRawValue()
135 BasicBlock* LBranch = switchConvert(LHS.begin(), LHS.end(), Val,
137 BasicBlock* RBranch = switchConvert(RHS.begin(), RHS.end(), Val,
140 // Create a new node that checks if the value is < pivot. Go to the
141 // left branch if it is and right branch if not.
142 Function* F = OrigBlock->getParent();
143 BasicBlock* NewNode = new BasicBlock("NodeBlock");
144 F->getBasicBlockList().insert(OrigBlock->getNext(), NewNode);
146 SetCondInst* Comp = new SetCondInst(Instruction::SetLT, Val, Pivot.first,
148 NewNode->getInstList().push_back(Comp);
149 new BranchInst(LBranch, RBranch, Comp, NewNode);
153 // newLeafBlock - Create a new leaf block for the binary lookup tree. It
154 // checks if the switch's value == the case's value. If not, then it
155 // jumps to the default branch. At this point in the tree, the value
156 // can't be another valid case value, so the jump to the "default" branch
159 BasicBlock* LowerSwitch::newLeafBlock(Case& Leaf, Value* Val,
160 BasicBlock* OrigBlock,
163 Function* F = OrigBlock->getParent();
164 BasicBlock* NewLeaf = new BasicBlock("LeafBlock");
165 F->getBasicBlockList().insert(OrigBlock->getNext(), NewLeaf);
167 // Make the seteq instruction...
168 SetCondInst* Comp = new SetCondInst(Instruction::SetEQ, Val,
169 Leaf.first, "SwitchLeaf");
170 NewLeaf->getInstList().push_back(Comp);
172 // Make the conditional branch...
173 BasicBlock* Succ = Leaf.second;
174 new BranchInst(Succ, Default, Comp, NewLeaf);
176 // If there were any PHI nodes in this successor, rewrite one entry
177 // from OrigBlock to come from NewLeaf.
178 for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) {
179 PHINode* PN = cast<PHINode>(I);
180 int BlockIdx = PN->getBasicBlockIndex(OrigBlock);
181 assert(BlockIdx != -1 && "Switch didn't go to this successor??");
182 PN->setIncomingBlock((unsigned)BlockIdx, NewLeaf);
188 // processSwitchInst - Replace the specified switch instruction with a sequence
189 // of chained if-then insts in a balanced binary search.
191 void LowerSwitch::processSwitchInst(SwitchInst *SI) {
192 BasicBlock *CurBlock = SI->getParent();
193 BasicBlock *OrigBlock = CurBlock;
194 Function *F = CurBlock->getParent();
195 Value *Val = SI->getOperand(0); // The value we are switching on...
196 BasicBlock* Default = SI->getDefaultDest();
198 // If there is only the default destination, don't bother with the code below.
199 if (SI->getNumOperands() == 2) {
200 new BranchInst(SI->getDefaultDest(), CurBlock);
201 CurBlock->getInstList().erase(SI);
205 // Create a new, empty default block so that the new hierarchy of
206 // if-then statements go to this and the PHI nodes are happy.
207 BasicBlock* NewDefault = new BasicBlock("NewDefault");
208 F->getBasicBlockList().insert(Default, NewDefault);
210 new BranchInst(Default, NewDefault);
212 // If there is an entry in any PHI nodes for the default edge, make sure
213 // to update them as well.
214 for (BasicBlock::iterator I = Default->begin(); isa<PHINode>(I); ++I) {
215 PHINode *PN = cast<PHINode>(I);
216 int BlockIdx = PN->getBasicBlockIndex(OrigBlock);
217 assert(BlockIdx != -1 && "Switch didn't go to this successor??");
218 PN->setIncomingBlock((unsigned)BlockIdx, NewDefault);
221 std::vector<Case> Cases;
223 // Expand comparisons for all of the non-default cases...
224 for (unsigned i = 1; i < SI->getNumSuccessors(); ++i)
225 Cases.push_back(Case(SI->getSuccessorValue(i), SI->getSuccessor(i)));
227 std::sort(Cases.begin(), Cases.end(), CaseCmp());
228 DEBUG(std::cerr << "Cases: " << Cases << "\n");
229 BasicBlock* SwitchBlock = switchConvert(Cases.begin(), Cases.end(), Val,
230 OrigBlock, NewDefault);
232 // Branch to our shiny new if-then stuff...
233 new BranchInst(SwitchBlock, OrigBlock);
235 // We are now done with the switch instruction, delete it.
236 CurBlock->getInstList().erase(SI);