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"
29 Statistic<> NumLowered("lowerswitch", "Number of SwitchInst's replaced");
31 /// LowerSwitch Pass - Replace all SwitchInst instructions with chained branch
32 /// instructions. Note that this cannot be a BasicBlock pass because it
34 class VISIBILITY_HIDDEN LowerSwitch : public FunctionPass {
36 virtual bool runOnFunction(Function &F);
38 virtual void getAnalysisUsage(AnalysisUsage &AU) const {
39 // This is a cluster of orthogonal Transforms
40 AU.addPreserved<UnifyFunctionExitNodes>();
41 AU.addPreservedID(PromoteMemoryToRegisterID);
42 AU.addPreservedID(LowerSelectID);
43 AU.addPreservedID(LowerInvokePassID);
44 AU.addPreservedID(LowerAllocationsID);
47 typedef std::pair<Constant*, BasicBlock*> Case;
48 typedef std::vector<Case>::iterator CaseItr;
50 void processSwitchInst(SwitchInst *SI);
52 BasicBlock* switchConvert(CaseItr Begin, CaseItr End, Value* Val,
53 BasicBlock* OrigBlock, BasicBlock* Default);
54 BasicBlock* newLeafBlock(Case& Leaf, Value* Val,
55 BasicBlock* OrigBlock, BasicBlock* Default);
58 /// The comparison function for sorting the switch case values in the vector.
60 bool operator () (const LowerSwitch::Case& C1,
61 const LowerSwitch::Case& C2) {
63 const ConstantInt* CI1 = cast<const ConstantInt>(C1.first);
64 const ConstantInt* CI2 = cast<const ConstantInt>(C2.first);
65 if (CI1->getType()->isUnsigned())
66 return CI1->getZExtValue() < CI2->getZExtValue();
67 return CI1->getSExtValue() < CI2->getSExtValue();
71 RegisterPass<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 llvm_ostream& operator<<(llvm_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 DOUT << "LHS: " << LHS << "\n";
127 std::vector<Case> RHS(Begin + Mid, End);
128 DOUT << "RHS: " << RHS << "\n";
130 Case& Pivot = *(Begin + Mid);
132 << cast<ConstantInt>(Pivot.first)->getSExtValue() << "\n";
134 BasicBlock* LBranch = switchConvert(LHS.begin(), LHS.end(), Val,
136 BasicBlock* RBranch = switchConvert(RHS.begin(), RHS.end(), Val,
139 // Create a new node that checks if the value is < pivot. Go to the
140 // left branch if it is and right branch if not.
141 Function* F = OrigBlock->getParent();
142 BasicBlock* NewNode = new BasicBlock("NodeBlock");
143 F->getBasicBlockList().insert(OrigBlock->getNext(), NewNode);
145 SetCondInst* Comp = new SetCondInst(Instruction::SetLT, Val, Pivot.first,
147 NewNode->getInstList().push_back(Comp);
148 new BranchInst(LBranch, RBranch, Comp, NewNode);
152 // newLeafBlock - Create a new leaf block for the binary lookup tree. It
153 // checks if the switch's value == the case's value. If not, then it
154 // jumps to the default branch. At this point in the tree, the value
155 // can't be another valid case value, so the jump to the "default" branch
158 BasicBlock* LowerSwitch::newLeafBlock(Case& Leaf, Value* Val,
159 BasicBlock* OrigBlock,
162 Function* F = OrigBlock->getParent();
163 BasicBlock* NewLeaf = new BasicBlock("LeafBlock");
164 F->getBasicBlockList().insert(OrigBlock->getNext(), NewLeaf);
166 // Make the seteq instruction...
167 SetCondInst* Comp = new SetCondInst(Instruction::SetEQ, Val,
168 Leaf.first, "SwitchLeaf");
169 NewLeaf->getInstList().push_back(Comp);
171 // Make the conditional branch...
172 BasicBlock* Succ = Leaf.second;
173 new BranchInst(Succ, Default, Comp, NewLeaf);
175 // If there were any PHI nodes in this successor, rewrite one entry
176 // from OrigBlock to come from NewLeaf.
177 for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) {
178 PHINode* PN = cast<PHINode>(I);
179 int BlockIdx = PN->getBasicBlockIndex(OrigBlock);
180 assert(BlockIdx != -1 && "Switch didn't go to this successor??");
181 PN->setIncomingBlock((unsigned)BlockIdx, NewLeaf);
187 // processSwitchInst - Replace the specified switch instruction with a sequence
188 // of chained if-then insts in a balanced binary search.
190 void LowerSwitch::processSwitchInst(SwitchInst *SI) {
191 BasicBlock *CurBlock = SI->getParent();
192 BasicBlock *OrigBlock = CurBlock;
193 Function *F = CurBlock->getParent();
194 Value *Val = SI->getOperand(0); // The value we are switching on...
195 BasicBlock* Default = SI->getDefaultDest();
197 // If there is only the default destination, don't bother with the code below.
198 if (SI->getNumOperands() == 2) {
199 new BranchInst(SI->getDefaultDest(), CurBlock);
200 CurBlock->getInstList().erase(SI);
204 // Create a new, empty default block so that the new hierarchy of
205 // if-then statements go to this and the PHI nodes are happy.
206 BasicBlock* NewDefault = new BasicBlock("NewDefault");
207 F->getBasicBlockList().insert(Default, NewDefault);
209 new BranchInst(Default, NewDefault);
211 // If there is an entry in any PHI nodes for the default edge, make sure
212 // to update them as well.
213 for (BasicBlock::iterator I = Default->begin(); isa<PHINode>(I); ++I) {
214 PHINode *PN = cast<PHINode>(I);
215 int BlockIdx = PN->getBasicBlockIndex(OrigBlock);
216 assert(BlockIdx != -1 && "Switch didn't go to this successor??");
217 PN->setIncomingBlock((unsigned)BlockIdx, NewDefault);
220 std::vector<Case> Cases;
222 // Expand comparisons for all of the non-default cases...
223 for (unsigned i = 1; i < SI->getNumSuccessors(); ++i)
224 Cases.push_back(Case(SI->getSuccessorValue(i), SI->getSuccessor(i)));
226 std::sort(Cases.begin(), Cases.end(), CaseCmp());
227 DOUT << "Cases: " << Cases << "\n";
228 BasicBlock* SwitchBlock = switchConvert(Cases.begin(), Cases.end(), Val,
229 OrigBlock, NewDefault);
231 // Branch to our shiny new if-then stuff...
232 new BranchInst(SwitchBlock, OrigBlock);
234 // We are now done with the switch instruction, delete it.
235 CurBlock->getInstList().erase(SI);