LowerSwitch.cpp revision d216e8ba60494caacf919cbf5fef110d48f0d162 
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/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 <algorithm>
25using namespace llvm;
26
27namespace {
28  /// LowerSwitch Pass - Replace all SwitchInst instructions with chained branch
29  /// instructions.  Note that this cannot be a BasicBlock pass because it
30  /// modifies the CFG!
31  class VISIBILITY_HIDDEN LowerSwitch : public FunctionPass {
32  public:
33    virtual bool runOnFunction(Function &F);
34
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);
42    }
43
44    typedef std::pair<Constant*, BasicBlock*> Case;
45    typedef std::vector<Case>::iterator       CaseItr;
46  private:
47    void processSwitchInst(SwitchInst *SI);
48
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);
53  };
54
55  /// The comparison function for sorting the switch case values in the vector.
56  struct CaseCmp {
57    bool operator () (const LowerSwitch::Case& C1,
58                      const LowerSwitch::Case& C2) {
59
60      const ConstantInt* CI1 = cast<const ConstantInt>(C1.first);
61      const ConstantInt* CI2 = cast<const ConstantInt>(C2.first);
62      if (CI1->getType()->isUnsigned())
63        return CI1->getZExtValue() < CI2->getZExtValue();
64      return CI1->getSExtValue() < CI2->getSExtValue();
65    }
66  };
67
68  RegisterPass<LowerSwitch>
69  X("lowerswitch", "Lower SwitchInst's to branches");
70}
71
72// Publically exposed interface to pass...
73const PassInfo *llvm::LowerSwitchID = X.getPassInfo();
74// createLowerSwitchPass - Interface to this file...
75FunctionPass *llvm::createLowerSwitchPass() {
76  return new LowerSwitch();
77}
78
79bool LowerSwitch::runOnFunction(Function &F) {
80  bool Changed = false;
81
82  for (Function::iterator I = F.begin(), E = F.end(); I != E; ) {
83    BasicBlock *Cur = I++; // Advance over block so we don't traverse new blocks
84
85    if (SwitchInst *SI = dyn_cast<SwitchInst>(Cur->getTerminator())) {
86      Changed = true;
87      processSwitchInst(SI);
88    }
89  }
90
91  return Changed;
92}
93
94// operator<< - Used for debugging purposes.
95//
96std::ostream& operator<<(std::ostream &O,
97                         const std::vector<LowerSwitch::Case> &C) {
98  O << "[";
99
100  for (std::vector<LowerSwitch::Case>::const_iterator B = C.begin(),
101         E = C.end(); B != E; ) {
102    O << *B->first;
103    if (++B != E) O << ", ";
104  }
105
106  return O << "]";
107}
108OStream& operator<<(OStream &O, const std::vector<LowerSwitch::Case> &C) {
109  if (O.stream()) *O.stream() << C;
110  return O;
111}
112
113// switchConvert - Convert the switch statement into a binary lookup of
114// the case values. The function recursively builds this tree.
115//
116BasicBlock* LowerSwitch::switchConvert(CaseItr Begin, CaseItr End,
117                                       Value* Val, BasicBlock* OrigBlock,
118                                       BasicBlock* Default)
119{
120  unsigned Size = End - Begin;
121
122  if (Size == 1)
123    return newLeafBlock(*Begin, Val, OrigBlock, Default);
124
125  unsigned Mid = Size / 2;
126  std::vector<Case> LHS(Begin, Begin + Mid);
127  DOUT << "LHS: " << LHS << "\n";
128  std::vector<Case> RHS(Begin + Mid, End);
129  DOUT << "RHS: " << RHS << "\n";
130
131  Case& Pivot = *(Begin + Mid);
132  DOUT << "Pivot ==> "
133       << cast<ConstantInt>(Pivot.first)->getSExtValue() << "\n";
134
135  BasicBlock* LBranch = switchConvert(LHS.begin(), LHS.end(), Val,
136                                      OrigBlock, Default);
137  BasicBlock* RBranch = switchConvert(RHS.begin(), RHS.end(), Val,
138                                      OrigBlock, Default);
139
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);
145
146  SetCondInst* Comp = new SetCondInst(Instruction::SetLT, Val, Pivot.first,
147                                      "Pivot");
148  NewNode->getInstList().push_back(Comp);
149  new BranchInst(LBranch, RBranch, Comp, NewNode);
150  return NewNode;
151}
152
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
157// is warranted.
158//
159BasicBlock* LowerSwitch::newLeafBlock(Case& Leaf, Value* Val,
160                                      BasicBlock* OrigBlock,
161                                      BasicBlock* Default)
162{
163  Function* F = OrigBlock->getParent();
164  BasicBlock* NewLeaf = new BasicBlock("LeafBlock");
165  F->getBasicBlockList().insert(OrigBlock->getNext(), NewLeaf);
166
167  // Make the seteq instruction...
168  SetCondInst* Comp = new SetCondInst(Instruction::SetEQ, Val,
169                                      Leaf.first, "SwitchLeaf");
170  NewLeaf->getInstList().push_back(Comp);
171
172  // Make the conditional branch...
173  BasicBlock* Succ = Leaf.second;
174  new BranchInst(Succ, Default, Comp, NewLeaf);
175
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);
183  }
184
185  return NewLeaf;
186}
187
188// processSwitchInst - Replace the specified switch instruction with a sequence
189// of chained if-then insts in a balanced binary search.
190//
191void 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();
197
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);
202    return;
203  }
204
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);
209
210  new BranchInst(Default, NewDefault);
211
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);
219  }
220
221  std::vector<Case> Cases;
222
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)));
226
227  std::sort(Cases.begin(), Cases.end(), CaseCmp());
228  DOUT << "Cases: " << Cases << "\n";
229  BasicBlock* SwitchBlock = switchConvert(Cases.begin(), Cases.end(), Val,
230                                          OrigBlock, NewDefault);
231
232  // Branch to our shiny new if-then stuff...
233  new BranchInst(SwitchBlock, OrigBlock);
234
235  // We are now done with the switch instruction, delete it.
236  CurBlock->getInstList().erase(SI);
237}
238