LowerSwitch.cpp revision 193323
1//===- LowerSwitch.cpp - Eliminate Switch instructions --------------------===//
2//
3//                     The LLVM Compiler Infrastructure
4//
5// This file is distributed under the University of Illinois Open Source
6// License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9//
10// The LowerSwitch transformation rewrites switch instructions with a sequence
11// of branches, which allows targets to get away with not implementing the
12// switch instruction 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/ADT/STLExtras.h"
23#include "llvm/Support/Debug.h"
24#include "llvm/Support/Compiler.h"
25#include "llvm/Support/raw_ostream.h"
26#include <algorithm>
27using namespace llvm;
28
29namespace {
30  /// LowerSwitch Pass - Replace all SwitchInst instructions with chained branch
31  /// instructions.  Note that this cannot be a BasicBlock pass because it
32  /// modifies the CFG!
33  class VISIBILITY_HIDDEN LowerSwitch : public FunctionPass {
34  public:
35    static char ID; // Pass identification, replacement for typeid
36    LowerSwitch() : FunctionPass(&ID) {}
37
38    virtual bool runOnFunction(Function &F);
39
40    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
41      // This is a cluster of orthogonal Transforms
42      AU.addPreserved<UnifyFunctionExitNodes>();
43      AU.addPreservedID(PromoteMemoryToRegisterID);
44      AU.addPreservedID(LowerInvokePassID);
45      AU.addPreservedID(LowerAllocationsID);
46    }
47
48    struct CaseRange {
49      Constant* Low;
50      Constant* High;
51      BasicBlock* BB;
52
53      CaseRange() : Low(0), High(0), BB(0) { }
54      CaseRange(Constant* low, Constant* high, BasicBlock* bb) :
55        Low(low), High(high), BB(bb) { }
56    };
57
58    typedef std::vector<CaseRange>           CaseVector;
59    typedef std::vector<CaseRange>::iterator CaseItr;
60  private:
61    void processSwitchInst(SwitchInst *SI);
62
63    BasicBlock* switchConvert(CaseItr Begin, CaseItr End, Value* Val,
64                              BasicBlock* OrigBlock, BasicBlock* Default);
65    BasicBlock* newLeafBlock(CaseRange& Leaf, Value* Val,
66                             BasicBlock* OrigBlock, BasicBlock* Default);
67    unsigned Clusterify(CaseVector& Cases, SwitchInst *SI);
68  };
69
70  /// The comparison function for sorting the switch case values in the vector.
71  /// WARNING: Case ranges should be disjoint!
72  struct CaseCmp {
73    bool operator () (const LowerSwitch::CaseRange& C1,
74                      const LowerSwitch::CaseRange& C2) {
75
76      const ConstantInt* CI1 = cast<const ConstantInt>(C1.Low);
77      const ConstantInt* CI2 = cast<const ConstantInt>(C2.High);
78      return CI1->getValue().slt(CI2->getValue());
79    }
80  };
81}
82
83char LowerSwitch::ID = 0;
84static RegisterPass<LowerSwitch>
85X("lowerswitch", "Lower SwitchInst's to branches");
86
87// Publically exposed interface to pass...
88const PassInfo *const llvm::LowerSwitchID = &X;
89// createLowerSwitchPass - Interface to this file...
90FunctionPass *llvm::createLowerSwitchPass() {
91  return new LowerSwitch();
92}
93
94bool LowerSwitch::runOnFunction(Function &F) {
95  bool Changed = false;
96
97  for (Function::iterator I = F.begin(), E = F.end(); I != E; ) {
98    BasicBlock *Cur = I++; // Advance over block so we don't traverse new blocks
99
100    if (SwitchInst *SI = dyn_cast<SwitchInst>(Cur->getTerminator())) {
101      Changed = true;
102      processSwitchInst(SI);
103    }
104  }
105
106  return Changed;
107}
108
109// operator<< - Used for debugging purposes.
110//
111static std::ostream& operator<<(std::ostream &O,
112                                const LowerSwitch::CaseVector &C) {
113  O << "[";
114
115  for (LowerSwitch::CaseVector::const_iterator B = C.begin(),
116         E = C.end(); B != E; ) {
117    O << *B->Low << " -" << *B->High;
118    if (++B != E) O << ", ";
119  }
120
121  return O << "]";
122}
123
124static OStream& operator<<(OStream &O, const LowerSwitch::CaseVector &C) {
125  if (O.stream()) *O.stream() << C;
126  return O;
127}
128
129// switchConvert - Convert the switch statement into a binary lookup of
130// the case values. The function recursively builds this tree.
131//
132BasicBlock* LowerSwitch::switchConvert(CaseItr Begin, CaseItr End,
133                                       Value* Val, BasicBlock* OrigBlock,
134                                       BasicBlock* Default)
135{
136  unsigned Size = End - Begin;
137
138  if (Size == 1)
139    return newLeafBlock(*Begin, Val, OrigBlock, Default);
140
141  unsigned Mid = Size / 2;
142  std::vector<CaseRange> LHS(Begin, Begin + Mid);
143  DOUT << "LHS: " << LHS << "\n";
144  std::vector<CaseRange> RHS(Begin + Mid, End);
145  DOUT << "RHS: " << RHS << "\n";
146
147  CaseRange& Pivot = *(Begin + Mid);
148  DEBUG(errs() << "Pivot ==> "
149               << cast<ConstantInt>(Pivot.Low)->getValue() << " -"
150               << cast<ConstantInt>(Pivot.High)->getValue() << "\n");
151
152  BasicBlock* LBranch = switchConvert(LHS.begin(), LHS.end(), Val,
153                                      OrigBlock, Default);
154  BasicBlock* RBranch = switchConvert(RHS.begin(), RHS.end(), Val,
155                                      OrigBlock, Default);
156
157  // Create a new node that checks if the value is < pivot. Go to the
158  // left branch if it is and right branch if not.
159  Function* F = OrigBlock->getParent();
160  BasicBlock* NewNode = BasicBlock::Create("NodeBlock");
161  Function::iterator FI = OrigBlock;
162  F->getBasicBlockList().insert(++FI, NewNode);
163
164  ICmpInst* Comp = new ICmpInst(ICmpInst::ICMP_SLT, Val, Pivot.Low, "Pivot");
165  NewNode->getInstList().push_back(Comp);
166  BranchInst::Create(LBranch, RBranch, Comp, NewNode);
167  return NewNode;
168}
169
170// newLeafBlock - Create a new leaf block for the binary lookup tree. It
171// checks if the switch's value == the case's value. If not, then it
172// jumps to the default branch. At this point in the tree, the value
173// can't be another valid case value, so the jump to the "default" branch
174// is warranted.
175//
176BasicBlock* LowerSwitch::newLeafBlock(CaseRange& Leaf, Value* Val,
177                                      BasicBlock* OrigBlock,
178                                      BasicBlock* Default)
179{
180  Function* F = OrigBlock->getParent();
181  BasicBlock* NewLeaf = BasicBlock::Create("LeafBlock");
182  Function::iterator FI = OrigBlock;
183  F->getBasicBlockList().insert(++FI, NewLeaf);
184
185  // Emit comparison
186  ICmpInst* Comp = NULL;
187  if (Leaf.Low == Leaf.High) {
188    // Make the seteq instruction...
189    Comp = new ICmpInst(ICmpInst::ICMP_EQ, Val, Leaf.Low,
190                        "SwitchLeaf", NewLeaf);
191  } else {
192    // Make range comparison
193    if (cast<ConstantInt>(Leaf.Low)->isMinValue(true /*isSigned*/)) {
194      // Val >= Min && Val <= Hi --> Val <= Hi
195      Comp = new ICmpInst(ICmpInst::ICMP_SLE, Val, Leaf.High,
196                          "SwitchLeaf", NewLeaf);
197    } else if (cast<ConstantInt>(Leaf.Low)->isZero()) {
198      // Val >= 0 && Val <= Hi --> Val <=u Hi
199      Comp = new ICmpInst(ICmpInst::ICMP_ULE, Val, Leaf.High,
200                          "SwitchLeaf", NewLeaf);
201    } else {
202      // Emit V-Lo <=u Hi-Lo
203      Constant* NegLo = ConstantExpr::getNeg(Leaf.Low);
204      Instruction* Add = BinaryOperator::CreateAdd(Val, NegLo,
205                                                   Val->getName()+".off",
206                                                   NewLeaf);
207      Constant *UpperBound = ConstantExpr::getAdd(NegLo, Leaf.High);
208      Comp = new ICmpInst(ICmpInst::ICMP_ULE, Add, UpperBound,
209                          "SwitchLeaf", NewLeaf);
210    }
211  }
212
213  // Make the conditional branch...
214  BasicBlock* Succ = Leaf.BB;
215  BranchInst::Create(Succ, Default, Comp, NewLeaf);
216
217  // If there were any PHI nodes in this successor, rewrite one entry
218  // from OrigBlock to come from NewLeaf.
219  for (BasicBlock::iterator I = Succ->begin(); isa<PHINode>(I); ++I) {
220    PHINode* PN = cast<PHINode>(I);
221    // Remove all but one incoming entries from the cluster
222    uint64_t Range = cast<ConstantInt>(Leaf.High)->getSExtValue() -
223                     cast<ConstantInt>(Leaf.Low)->getSExtValue();
224    for (uint64_t j = 0; j < Range; ++j) {
225      PN->removeIncomingValue(OrigBlock);
226    }
227
228    int BlockIdx = PN->getBasicBlockIndex(OrigBlock);
229    assert(BlockIdx != -1 && "Switch didn't go to this successor??");
230    PN->setIncomingBlock((unsigned)BlockIdx, NewLeaf);
231  }
232
233  return NewLeaf;
234}
235
236// Clusterify - Transform simple list of Cases into list of CaseRange's
237unsigned LowerSwitch::Clusterify(CaseVector& Cases, SwitchInst *SI) {
238  unsigned numCmps = 0;
239
240  // Start with "simple" cases
241  for (unsigned i = 1; i < SI->getNumSuccessors(); ++i)
242    Cases.push_back(CaseRange(SI->getSuccessorValue(i),
243                              SI->getSuccessorValue(i),
244                              SI->getSuccessor(i)));
245  std::sort(Cases.begin(), Cases.end(), CaseCmp());
246
247  // Merge case into clusters
248  if (Cases.size()>=2)
249    for (CaseItr I=Cases.begin(), J=next(Cases.begin()); J!=Cases.end(); ) {
250      int64_t nextValue = cast<ConstantInt>(J->Low)->getSExtValue();
251      int64_t currentValue = cast<ConstantInt>(I->High)->getSExtValue();
252      BasicBlock* nextBB = J->BB;
253      BasicBlock* currentBB = I->BB;
254
255      // If the two neighboring cases go to the same destination, merge them
256      // into a single case.
257      if ((nextValue-currentValue==1) && (currentBB == nextBB)) {
258        I->High = J->High;
259        J = Cases.erase(J);
260      } else {
261        I = J++;
262      }
263    }
264
265  for (CaseItr I=Cases.begin(), E=Cases.end(); I!=E; ++I, ++numCmps) {
266    if (I->Low != I->High)
267      // A range counts double, since it requires two compares.
268      ++numCmps;
269  }
270
271  return numCmps;
272}
273
274// processSwitchInst - Replace the specified switch instruction with a sequence
275// of chained if-then insts in a balanced binary search.
276//
277void LowerSwitch::processSwitchInst(SwitchInst *SI) {
278  BasicBlock *CurBlock = SI->getParent();
279  BasicBlock *OrigBlock = CurBlock;
280  Function *F = CurBlock->getParent();
281  Value *Val = SI->getOperand(0);  // The value we are switching on...
282  BasicBlock* Default = SI->getDefaultDest();
283
284  // If there is only the default destination, don't bother with the code below.
285  if (SI->getNumOperands() == 2) {
286    BranchInst::Create(SI->getDefaultDest(), CurBlock);
287    CurBlock->getInstList().erase(SI);
288    return;
289  }
290
291  // Create a new, empty default block so that the new hierarchy of
292  // if-then statements go to this and the PHI nodes are happy.
293  BasicBlock* NewDefault = BasicBlock::Create("NewDefault");
294  F->getBasicBlockList().insert(Default, NewDefault);
295
296  BranchInst::Create(Default, NewDefault);
297
298  // If there is an entry in any PHI nodes for the default edge, make sure
299  // to update them as well.
300  for (BasicBlock::iterator I = Default->begin(); isa<PHINode>(I); ++I) {
301    PHINode *PN = cast<PHINode>(I);
302    int BlockIdx = PN->getBasicBlockIndex(OrigBlock);
303    assert(BlockIdx != -1 && "Switch didn't go to this successor??");
304    PN->setIncomingBlock((unsigned)BlockIdx, NewDefault);
305  }
306
307  // Prepare cases vector.
308  CaseVector Cases;
309  unsigned numCmps = Clusterify(Cases, SI);
310
311  DOUT << "Clusterify finished. Total clusters: " << Cases.size()
312       << ". Total compares: " << numCmps << "\n";
313  DOUT << "Cases: " << Cases << "\n";
314
315  BasicBlock* SwitchBlock = switchConvert(Cases.begin(), Cases.end(), Val,
316                                          OrigBlock, NewDefault);
317
318  // Branch to our shiny new if-then stuff...
319  BranchInst::Create(SwitchBlock, OrigBlock);
320
321  // We are now done with the switch instruction, delete it.
322  CurBlock->getInstList().erase(SI);
323}
324