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"
28 /// LowerSwitch Pass - Replace all SwitchInst instructions with chained branch
29 /// instructions. Note that this cannot be a BasicBlock pass because it
31 class VISIBILITY_HIDDEN LowerSwitch : public FunctionPass {
33 virtual bool runOnFunction(Function &F);
35 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
36 // This is a cluster of orthogonal Transforms
37 AU.addPreserved<UnifyFunctionExitNodes>();
38 AU.addPreservedID(PromoteMemoryToRegisterID);
39 AU.addPreservedID(LowerSelectID);
40 AU.addPreservedID(LowerInvokePassID);
41 AU.addPreservedID(LowerAllocationsID);
44 typedef std::pair<Constant*, BasicBlock*> Case;
45 typedef std::vector<Case>::iterator CaseItr;
47 void processSwitchInst(SwitchInst *SI);
49 BasicBlock* switchConvert(CaseItr Begin, CaseItr End, Value* Val,
50 BasicBlock* OrigBlock, BasicBlock* Default);
51 BasicBlock* newLeafBlock(Case& Leaf, Value* Val,
52 BasicBlock* OrigBlock, BasicBlock* Default);
55 /// The comparison function for sorting the switch case values in the vector.
57 bool operator () (const LowerSwitch::Case& C1,
58 const LowerSwitch::Case& C2) {
60 const ConstantInt* CI1 = cast<const ConstantInt>(C1.first);
61 const ConstantInt* CI2 = cast<const ConstantInt>(C2.first);
62 return CI1->getValue().ult(CI2->getValue());
66 RegisterPass<LowerSwitch>
67 X("lowerswitch", "Lower SwitchInst's to branches");
70 // Publically exposed interface to pass...
71 const PassInfo *llvm::LowerSwitchID = X.getPassInfo();
72 // createLowerSwitchPass - Interface to this file...
73 FunctionPass *llvm::createLowerSwitchPass() {
74 return new LowerSwitch();
77 bool LowerSwitch::runOnFunction(Function &F) {
80 for (Function::iterator I = F.begin(), E = F.end(); I != E; ) {
81 BasicBlock *Cur = I++; // Advance over block so we don't traverse new blocks
83 if (SwitchInst *SI = dyn_cast<SwitchInst>(Cur->getTerminator())) {
85 processSwitchInst(SI);
92 // operator<< - Used for debugging purposes.
94 std::ostream& operator<<(std::ostream &O,
95 const std::vector<LowerSwitch::Case> &C) {
98 for (std::vector<LowerSwitch::Case>::const_iterator B = C.begin(),
99 E = C.end(); B != E; ) {
101 if (++B != E) O << ", ";
106 OStream& operator<<(OStream &O, const std::vector<LowerSwitch::Case> &C) {
107 if (O.stream()) *O.stream() << C;
111 // switchConvert - Convert the switch statement into a binary lookup of
112 // the case values. The function recursively builds this tree.
114 BasicBlock* LowerSwitch::switchConvert(CaseItr Begin, CaseItr End,
115 Value* Val, BasicBlock* OrigBlock,
118 unsigned Size = End - Begin;
121 return newLeafBlock(*Begin, Val, OrigBlock, Default);
123 unsigned Mid = Size / 2;
124 std::vector<Case> LHS(Begin, Begin + Mid);
125 DOUT << "LHS: " << LHS << "\n";
126 std::vector<Case> RHS(Begin + Mid, End);
127 DOUT << "RHS: " << RHS << "\n";
129 Case& Pivot = *(Begin + Mid);
131 << cast<ConstantInt>(Pivot.first)->getValue().toStringSigned(10) << "\n";
133 BasicBlock* LBranch = switchConvert(LHS.begin(), LHS.end(), Val,
135 BasicBlock* RBranch = switchConvert(RHS.begin(), RHS.end(), Val,
138 // Create a new node that checks if the value is < pivot. Go to the
139 // left branch if it is and right branch if not.
140 Function* F = OrigBlock->getParent();
141 BasicBlock* NewNode = new BasicBlock("NodeBlock");
142 F->getBasicBlockList().insert(OrigBlock->getNext(), NewNode);
144 ICmpInst* Comp = new ICmpInst(ICmpInst::ICMP_ULT, Val, Pivot.first, "Pivot");
145 NewNode->getInstList().push_back(Comp);
146 new BranchInst(LBranch, RBranch, Comp, NewNode);
150 // newLeafBlock - Create a new leaf block for the binary lookup tree. It
151 // checks if the switch's value == the case's value. If not, then it
152 // jumps to the default branch. At this point in the tree, the value
153 // can't be another valid case value, so the jump to the "default" branch
156 BasicBlock* LowerSwitch::newLeafBlock(Case& Leaf, Value* Val,
157 BasicBlock* OrigBlock,
160 Function* F = OrigBlock->getParent();
161 BasicBlock* NewLeaf = new BasicBlock("LeafBlock");
162 F->getBasicBlockList().insert(OrigBlock->getNext(), NewLeaf);
164 // Make the seteq instruction...
165 ICmpInst* Comp = new ICmpInst(ICmpInst::ICMP_EQ, Val,
166 Leaf.first, "SwitchLeaf");
167 NewLeaf->getInstList().push_back(Comp);
169 // Make the conditional branch...
170 BasicBlock* Succ = Leaf.second;
171 new BranchInst(Succ, Default, Comp, NewLeaf);
173 // If there were any PHI nodes in this successor, rewrite one entry
174 // from OrigBlock to come from NewLeaf.
175 for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) {
176 PHINode* PN = cast<PHINode>(I);
177 int BlockIdx = PN->getBasicBlockIndex(OrigBlock);
178 assert(BlockIdx != -1 && "Switch didn't go to this successor??");
179 PN->setIncomingBlock((unsigned)BlockIdx, NewLeaf);
185 // processSwitchInst - Replace the specified switch instruction with a sequence
186 // of chained if-then insts in a balanced binary search.
188 void LowerSwitch::processSwitchInst(SwitchInst *SI) {
189 BasicBlock *CurBlock = SI->getParent();
190 BasicBlock *OrigBlock = CurBlock;
191 Function *F = CurBlock->getParent();
192 Value *Val = SI->getOperand(0); // The value we are switching on...
193 BasicBlock* Default = SI->getDefaultDest();
195 // If there is only the default destination, don't bother with the code below.
196 if (SI->getNumOperands() == 2) {
197 new BranchInst(SI->getDefaultDest(), CurBlock);
198 CurBlock->getInstList().erase(SI);
202 // Create a new, empty default block so that the new hierarchy of
203 // if-then statements go to this and the PHI nodes are happy.
204 BasicBlock* NewDefault = new BasicBlock("NewDefault");
205 F->getBasicBlockList().insert(Default, NewDefault);
207 new BranchInst(Default, NewDefault);
209 // If there is an entry in any PHI nodes for the default edge, make sure
210 // to update them as well.
211 for (BasicBlock::iterator I = Default->begin(); isa<PHINode>(I); ++I) {
212 PHINode *PN = cast<PHINode>(I);
213 int BlockIdx = PN->getBasicBlockIndex(OrigBlock);
214 assert(BlockIdx != -1 && "Switch didn't go to this successor??");
215 PN->setIncomingBlock((unsigned)BlockIdx, NewDefault);
218 std::vector<Case> Cases;
220 // Expand comparisons for all of the non-default cases...
221 for (unsigned i = 1; i < SI->getNumSuccessors(); ++i)
222 Cases.push_back(Case(SI->getSuccessorValue(i), SI->getSuccessor(i)));
224 std::sort(Cases.begin(), Cases.end(), CaseCmp());
225 DOUT << "Cases: " << Cases << "\n";
226 BasicBlock* SwitchBlock = switchConvert(Cases.begin(), Cases.end(), Val,
227 OrigBlock, NewDefault);
229 // Branch to our shiny new if-then stuff...
230 new BranchInst(SwitchBlock, OrigBlock);
232 // We are now done with the switch instruction, delete it.
233 CurBlock->getInstList().erase(SI);