LowerSwitch.cpp revision 108e4ab159b59a616b0868e396dc7ddc1fb48616 
1//===- LowerSwitch.cpp - Eliminate Switch instructions --------------------===//
2//
3//                     The LLVM Compiler Infrastructure
4//
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.
7//
8//===----------------------------------------------------------------------===//
9//
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.
13//
14//===----------------------------------------------------------------------===//
15
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"
25
26namespace llvm {
27
28namespace {
29  Statistic<> NumLowered("lowerswitch", "Number of SwitchInst's replaced");
30
31  /// LowerSwitch Pass - Replace all SwitchInst instructions with chained branch
32  /// instructions.  Note that this cannot be a BasicBlock pass because it
33  /// modifies the CFG!
34  class LowerSwitch : public FunctionPass {
35  public:
36    bool runOnFunction(Function &F);
37    typedef std::pair<Constant*, BasicBlock*> Case;
38    typedef std::vector<Case>::iterator       CaseItr;
39  private:
40    void processSwitchInst(SwitchInst *SI);
41
42    BasicBlock* switchConvert(CaseItr Begin, CaseItr End, Value* Val,
43                              BasicBlock* OrigBlock, BasicBlock* Default);
44    BasicBlock* newLeafBlock(Case& Leaf, Value* Val,
45                             BasicBlock* OrigBlock, BasicBlock* Default);
46  };
47
48  /// The comparison function for sorting the switch case values in the vector.
49  struct CaseCmp {
50    bool operator () (const LowerSwitch::Case& C1,
51                      const LowerSwitch::Case& C2) {
52      if (const ConstantUInt* U1 = dyn_cast<const ConstantUInt>(C1.first))
53        return U1->getValue() < cast<const ConstantUInt>(C2.first)->getValue();
54
55      const ConstantSInt* S1 = dyn_cast<const ConstantSInt>(C1.first);
56      return S1->getValue() < cast<const ConstantSInt>(C2.first)->getValue();
57    }
58  };
59
60  RegisterOpt<LowerSwitch>
61  X("lowerswitch", "Lower SwitchInst's to branches");
62}
63
64// createLowerSwitchPass - Interface to this file...
65FunctionPass *createLowerSwitchPass() {
66  return new LowerSwitch();
67}
68
69bool LowerSwitch::runOnFunction(Function &F) {
70  bool Changed = false;
71
72  for (Function::iterator I = F.begin(), E = F.end(); I != E; ) {
73    BasicBlock *Cur = I++; // Advance over block so we don't traverse new blocks
74
75    if (SwitchInst *SI = dyn_cast<SwitchInst>(Cur->getTerminator())) {
76      Changed = true;
77      processSwitchInst(SI);
78    }
79  }
80
81  return Changed;
82}
83
84// operator<< - Used for debugging purposes.
85//
86std::ostream& operator << (std::ostream& O, std::vector<LowerSwitch::Case>& C)
87{
88  O << "[";
89
90  for (std::vector<LowerSwitch::Case>::iterator B = C.begin(), E = C.end();
91      B != E; ) {
92    O << *B->first;
93    if (++B != E) O << ", ";
94  }
95
96  return O << "]";
97}
98
99// switchConvert - Convert the switch statement into a binary lookup of
100// the case values. The function recursively builds this tree.
101//
102BasicBlock* LowerSwitch::switchConvert(CaseItr Begin, CaseItr End,
103                                       Value* Val, BasicBlock* OrigBlock,
104                                       BasicBlock* Default)
105{
106  unsigned Size = End - Begin;
107
108  if (Size == 1)
109    return newLeafBlock(*Begin, Val, OrigBlock, Default);
110
111  unsigned Mid = Size / 2;
112  std::vector<Case> LHS(Begin, Begin + Mid);
113  DEBUG(std::cerr << "LHS: " << LHS << "\n");
114  std::vector<Case> RHS(Begin + Mid, End);
115  DEBUG(std::cerr << "RHS: " << RHS << "\n");
116
117  Case& Pivot = *(Begin + Mid);
118  DEBUG(std::cerr << "Pivot ==> "
119                  << cast<ConstantUInt>(Pivot.first)->getValue() << "\n");
120
121  BasicBlock* LBranch = switchConvert(LHS.begin(), LHS.end(), Val,
122                                      OrigBlock, Default);
123  BasicBlock* RBranch = switchConvert(RHS.begin(), RHS.end(), Val,
124                                      OrigBlock, Default);
125
126  // Create a new node that checks if the value is < pivot. Go to the
127  // left branch if it is and right branch if not.
128  Function* F = OrigBlock->getParent();
129  BasicBlock* NewNode = new BasicBlock("NodeBlock");
130  F->getBasicBlockList().insert(OrigBlock->getNext(), NewNode);
131
132  SetCondInst* Comp = new SetCondInst(Instruction::SetLT, Val, Pivot.first,
133                                      "Pivot");
134  NewNode->getInstList().push_back(Comp);
135  new BranchInst(LBranch, RBranch, Comp, NewNode);
136  return NewNode;
137}
138
139// newLeafBlock - Create a new leaf block for the binary lookup tree. It
140// checks if the switch's value == the case's value. If not, then it
141// jumps to the default branch. At this point in the tree, the value
142// can't be another valid case value, so the jump to the "default" branch
143// is warranted.
144//
145BasicBlock* LowerSwitch::newLeafBlock(Case& Leaf, Value* Val,
146                                      BasicBlock* OrigBlock,
147                                      BasicBlock* Default)
148{
149  Function* F = OrigBlock->getParent();
150  BasicBlock* NewLeaf = new BasicBlock("LeafBlock");
151  F->getBasicBlockList().insert(OrigBlock->getNext(), NewLeaf);
152
153  // Make the seteq instruction...
154  SetCondInst* Comp = new SetCondInst(Instruction::SetEQ, Val,
155                                      Leaf.first, "SwitchLeaf");
156  NewLeaf->getInstList().push_back(Comp);
157
158  // Make the conditional branch...
159  BasicBlock* Succ = Leaf.second;
160  new BranchInst(Succ, Default, Comp, NewLeaf);
161
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);
169  }
170
171  return NewLeaf;
172}
173
174// processSwitchInst - Replace the specified switch instruction with a sequence
175// of chained if-then insts in a balanced binary search.
176//
177void 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();
183
184  // Unlink the switch instruction from it's block.
185  CurBlock->getInstList().remove(SI);
186
187  // If there is only the default destination, don't bother with the code below.
188  if (SI->getNumOperands() == 2) {
189    new BranchInst(SI->getDefaultDest(), CurBlock);
190    delete SI;
191    return;
192  }
193
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);
198
199  new BranchInst(Default, NewDefault);
200
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);
208  }
209
210  std::vector<Case> Cases;
211
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)));
215
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);
220
221  // Branch to our shiny new if-then stuff...
222  new BranchInst(SwitchBlock, OrigBlock);
223
224  // We are now done with the switch instruction, delete it.
225  delete SI;
226}
227
228} // End llvm namespace
229