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/Constants.h"
18 #include "llvm/Function.h"
19 #include "llvm/iTerminators.h"
20 #include "llvm/iOperators.h"
21 #include "llvm/iPHINode.h"
22 #include "llvm/Pass.h"
23 #include "Support/Debug.h"
24 #include "Support/Statistic.h"
27 Statistic<> NumLowered("lowerswitch", "Number of SwitchInst's replaced");
29 /// LowerSwitch Pass - Replace all SwitchInst instructions with chained branch
30 /// instructions. Note that this cannot be a BasicBlock pass because it
32 class LowerSwitch : public FunctionPass {
34 bool runOnFunction(Function &F);
35 typedef std::pair<Constant*, BasicBlock*> Case;
36 typedef std::vector<Case>::iterator CaseItr;
38 void processSwitchInst(SwitchInst *SI);
40 BasicBlock* switchConvert(CaseItr Begin, CaseItr End, Value* Val,
41 BasicBlock* OrigBlock, BasicBlock* Default);
42 BasicBlock* newLeafBlock(Case& Leaf, Value* Val,
43 BasicBlock* OrigBlock, BasicBlock* Default);
46 /// The comparison function for sorting the switch case values in the vector.
48 bool operator () (const LowerSwitch::Case& C1,
49 const LowerSwitch::Case& C2) {
50 if (const ConstantUInt* U1 = dyn_cast<const ConstantUInt>(C1.first))
51 return U1->getValue() < cast<const ConstantUInt>(C2.first)->getValue();
53 const ConstantSInt* S1 = dyn_cast<const ConstantSInt>(C1.first);
54 return S1->getValue() < cast<const ConstantSInt>(C2.first)->getValue();
58 RegisterOpt<LowerSwitch>
59 X("lowerswitch", "Lower SwitchInst's to branches");
62 // createLowerSwitchPass - Interface to this file...
63 FunctionPass *createLowerSwitchPass() {
64 return new LowerSwitch();
67 bool LowerSwitch::runOnFunction(Function &F) {
70 for (Function::iterator I = F.begin(), E = F.end(); I != E; ) {
71 BasicBlock *Cur = I++; // Advance over block so we don't traverse new blocks
73 if (SwitchInst *SI = dyn_cast<SwitchInst>(Cur->getTerminator())) {
75 processSwitchInst(SI);
82 // operator<< - Used for debugging purposes.
84 std::ostream& operator << (std::ostream& O, std::vector<LowerSwitch::Case>& C)
88 for (std::vector<LowerSwitch::Case>::iterator B = C.begin(), E = C.end();
91 if (++B != E) O << ", ";
97 // switchConvert - Convert the switch statement into a binary lookup of
98 // the case values. The function recursively builds this tree.
100 BasicBlock* LowerSwitch::switchConvert(CaseItr Begin, CaseItr End,
101 Value* Val, BasicBlock* OrigBlock,
104 unsigned Size = End - Begin;
107 return newLeafBlock(*Begin, Val, OrigBlock, Default);
109 unsigned Mid = Size / 2;
110 std::vector<Case> LHS(Begin, Begin + Mid);
111 DEBUG(std::cerr << "LHS: " << LHS << "\n");
112 std::vector<Case> RHS(Begin + Mid, End);
113 DEBUG(std::cerr << "RHS: " << RHS << "\n");
115 Case& Pivot = *(Begin + Mid);
116 DEBUG(std::cerr << "Pivot ==> "
117 << cast<ConstantUInt>(Pivot.first)->getValue() << "\n");
119 BasicBlock* LBranch = switchConvert(LHS.begin(), LHS.end(), Val,
121 BasicBlock* RBranch = switchConvert(RHS.begin(), RHS.end(), Val,
124 // Create a new node that checks if the value is < pivot. Go to the
125 // left branch if it is and right branch if not.
126 Function* F = OrigBlock->getParent();
127 BasicBlock* NewNode = new BasicBlock("NodeBlock");
128 F->getBasicBlockList().insert(OrigBlock->getNext(), NewNode);
130 SetCondInst* Comp = new SetCondInst(Instruction::SetLT, Val, Pivot.first,
132 NewNode->getInstList().push_back(Comp);
133 BranchInst* Br = new BranchInst(LBranch, RBranch, Comp);
134 NewNode->getInstList().push_back(Br);
138 // newLeafBlock - Create a new leaf block for the binary lookup tree. It
139 // checks if the switch's value == the case's value. If not, then it
140 // jumps to the default branch. At this point in the tree, the value
141 // can't be another valid case value, so the jump to the "default" branch
144 BasicBlock* LowerSwitch::newLeafBlock(Case& Leaf, Value* Val,
145 BasicBlock* OrigBlock,
148 Function* F = OrigBlock->getParent();
149 BasicBlock* NewLeaf = new BasicBlock("LeafBlock");
150 F->getBasicBlockList().insert(OrigBlock->getNext(), NewLeaf);
152 // Make the seteq instruction...
153 SetCondInst* Comp = new SetCondInst(Instruction::SetEQ, Val,
154 Leaf.first, "SwitchLeaf");
155 NewLeaf->getInstList().push_back(Comp);
157 // Make the conditional branch...
158 BasicBlock* Succ = Leaf.second;
159 Instruction* Br = new BranchInst(Succ, Default, Comp);
160 NewLeaf->getInstList().push_back(Br);
162 // If there were any PHI nodes in this successor, rewrite one entry
163 // from OrigBlock to come from NewLeaf.
164 for (BasicBlock::iterator I = Succ->begin();
165 PHINode* PN = dyn_cast<PHINode>(I); ++I) {
166 int BlockIdx = PN->getBasicBlockIndex(OrigBlock);
167 assert(BlockIdx != -1 && "Switch didn't go to this successor??");
168 PN->setIncomingBlock((unsigned)BlockIdx, NewLeaf);
174 // processSwitchInst - Replace the specified switch instruction with a sequence
175 // of chained if-then insts in a balanced binary search.
177 void LowerSwitch::processSwitchInst(SwitchInst *SI) {
178 BasicBlock *CurBlock = SI->getParent();
179 BasicBlock *OrigBlock = CurBlock;
180 Function *F = CurBlock->getParent();
181 Value *Val = SI->getOperand(0); // The value we are switching on...
182 BasicBlock* Default = SI->getDefaultDest();
184 // Unlink the switch instruction from it's block.
185 CurBlock->getInstList().remove(SI);
187 // If there is only the default destination, don't bother with the code below.
188 if (SI->getNumOperands() == 2) {
189 CurBlock->getInstList().push_back(new BranchInst(SI->getDefaultDest()));
194 // Create a new, empty default block so that the new hierarchy of
195 // if-then statements go to this and the PHI nodes are happy.
196 BasicBlock* NewDefault = new BasicBlock("NewDefault");
197 F->getBasicBlockList().insert(Default, NewDefault);
199 NewDefault->getInstList().push_back(new BranchInst(Default));
201 // If there is an entry in any PHI nodes for the default edge, make sure
202 // to update them as well.
203 for (BasicBlock::iterator I = Default->begin();
204 PHINode *PN = dyn_cast<PHINode>(I); ++I) {
205 int BlockIdx = PN->getBasicBlockIndex(OrigBlock);
206 assert(BlockIdx != -1 && "Switch didn't go to this successor??");
207 PN->setIncomingBlock((unsigned)BlockIdx, NewDefault);
210 std::vector<Case> Cases;
212 // Expand comparisons for all of the non-default cases...
213 for (unsigned i = 1; i < SI->getNumSuccessors(); ++i)
214 Cases.push_back(Case(SI->getSuccessorValue(i), SI->getSuccessor(i)));
216 std::sort(Cases.begin(), Cases.end(), CaseCmp());
217 DEBUG(std::cerr << "Cases: " << Cases << "\n");
218 BasicBlock* SwitchBlock = switchConvert(Cases.begin(), Cases.end(), Val,
219 OrigBlock, NewDefault);
221 // Branch to our shiny new if-then stuff...
222 OrigBlock->getInstList().push_back(new BranchInst(SwitchBlock));
224 // We are now done with the switch instruction, delete it.