IndVarSimplify.cpp revision 5e76140536ba66fadeced1cd892f79616f407e3c 
1//===- IndVarSimplify.cpp - Induction Variable Elimination ----------------===//
2//
3// InductionVariableSimplify - Transform induction variables in a program
4//   to all use a single cannonical induction variable per loop.
5//
6//===----------------------------------------------------------------------===//
7
8#include "llvm/Transforms/Scalar.h"
9#include "llvm/Analysis/InductionVariable.h"
10#include "llvm/Analysis/LoopInfo.h"
11#include "llvm/iPHINode.h"
12#include "llvm/iOther.h"
13#include "llvm/Type.h"
14#include "llvm/Constants.h"
15#include "llvm/Support/CFG.h"
16#include "Support/STLExtras.h"
17#include "Support/StatisticReporter.h"
18
19namespace {
20  Statistic<> NumRemoved ("indvars\t\t- Number of aux indvars removed");
21  Statistic<> NumInserted("indvars\t\t- Number of cannonical indvars added");
22}
23
24// InsertCast - Cast Val to Ty, setting a useful name on the cast if Val has a
25// name...
26//
27static Instruction *InsertCast(Value *Val, const Type *Ty,
28                               BasicBlock::iterator It) {
29  Instruction *Cast = new CastInst(Val, Ty);
30  if (Val->hasName()) Cast->setName(Val->getName()+"-casted");
31  It->getParent()->getInstList().insert(It, Cast);
32  return Cast;
33}
34
35static bool TransformLoop(LoopInfo *Loops, Loop *Loop) {
36  // Transform all subloops before this loop...
37  bool Changed = reduce_apply_bool(Loop->getSubLoops().begin(),
38                                   Loop->getSubLoops().end(),
39                              std::bind1st(std::ptr_fun(TransformLoop), Loops));
40  // Get the header node for this loop.  All of the phi nodes that could be
41  // induction variables must live in this basic block.
42  //
43  BasicBlock *Header = Loop->getBlocks().front();
44
45  // Loop over all of the PHI nodes in the basic block, calculating the
46  // induction variables that they represent... stuffing the induction variable
47  // info into a vector...
48  //
49  std::vector<InductionVariable> IndVars;    // Induction variables for block
50  BasicBlock::iterator AfterPHIIt = Header->begin();
51  for (; PHINode *PN = dyn_cast<PHINode>(&*AfterPHIIt); ++AfterPHIIt)
52    IndVars.push_back(InductionVariable(PN, Loops));
53  // AfterPHIIt now points to first nonphi instruction...
54
55  // If there are no phi nodes in this basic block, there can't be indvars...
56  if (IndVars.empty()) return Changed;
57
58  // Loop over the induction variables, looking for a cannonical induction
59  // variable, and checking to make sure they are not all unknown induction
60  // variables.
61  //
62  bool FoundIndVars = false;
63  InductionVariable *Cannonical = 0;
64  for (unsigned i = 0; i < IndVars.size(); ++i) {
65    if (IndVars[i].InductionType == InductionVariable::Cannonical &&
66        !isa<PointerType>(IndVars[i].Phi->getType()))
67      Cannonical = &IndVars[i];
68    if (IndVars[i].InductionType != InductionVariable::Unknown)
69      FoundIndVars = true;
70  }
71
72  // No induction variables, bail early... don't add a cannonnical indvar
73  if (!FoundIndVars) return Changed;
74
75  // Okay, we want to convert other induction variables to use a cannonical
76  // indvar.  If we don't have one, add one now...
77  if (!Cannonical) {
78    // Create the PHI node for the new induction variable
79    PHINode *PN = new PHINode(Type::UIntTy, "cann-indvar");
80
81    // Insert the phi node at the end of the other phi nodes...
82    AfterPHIIt = ++Header->getInstList().insert(AfterPHIIt, PN);
83
84    // Create the increment instruction to add one to the counter...
85    Instruction *Add = BinaryOperator::create(Instruction::Add, PN,
86                                              ConstantUInt::get(Type::UIntTy,1),
87                                              "add1-indvar");
88
89    // Insert the add instruction after all of the PHI nodes...
90    Header->getInstList().insert(AfterPHIIt, Add);
91
92    // Figure out which block is incoming and which is the backedge for the loop
93    BasicBlock *Incoming, *BackEdgeBlock;
94    pred_iterator PI = pred_begin(Header);
95    assert(PI != pred_end(Header) && "Loop headers should have 2 preds!");
96    if (Loop->contains(*PI)) {  // First pred is back edge...
97      BackEdgeBlock = *PI++;
98      Incoming      = *PI++;
99    } else {
100      Incoming      = *PI++;
101      BackEdgeBlock = *PI++;
102    }
103    assert(PI == pred_end(Header) && "Loop headers should have 2 preds!");
104
105    // Add incoming values for the PHI node...
106    PN->addIncoming(Constant::getNullValue(Type::UIntTy), Incoming);
107    PN->addIncoming(Add, BackEdgeBlock);
108
109    // Analyze the new induction variable...
110    IndVars.push_back(InductionVariable(PN, Loops));
111    assert(IndVars.back().InductionType == InductionVariable::Cannonical &&
112           "Just inserted cannonical indvar that is not cannonical!");
113    Cannonical = &IndVars.back();
114    ++NumInserted;
115    Changed = true;
116  }
117
118  DEBUG(std::cerr << "Induction variables:\n");
119
120  // Get the current loop iteration count, which is always the value of the
121  // cannonical phi node...
122  //
123  PHINode *IterCount = Cannonical->Phi;
124
125  // Loop through and replace all of the auxillary induction variables with
126  // references to the primary induction variable...
127  //
128  for (unsigned i = 0; i < IndVars.size(); ++i) {
129    InductionVariable *IV = &IndVars[i];
130
131    DEBUG(IV->print(std::cerr));
132
133    // Don't do math with pointers...
134    const Type *IVTy = IV->Phi->getType();
135    if (isa<PointerType>(IVTy)) IVTy = Type::ULongTy;
136
137    // Don't modify the cannonical indvar or unrecognized indvars...
138    if (IV != Cannonical && IV->InductionType != InductionVariable::Unknown) {
139      Instruction *Val = IterCount;
140      if (!isa<ConstantInt>(IV->Step) ||   // If the step != 1
141          !cast<ConstantInt>(IV->Step)->equalsInt(1)) {
142
143        // If the types are not compatible, insert a cast now...
144        if (Val->getType() != IVTy)
145          Val = InsertCast(Val, IVTy, AfterPHIIt);
146        if (IV->Step->getType() != IVTy)
147          IV->Step = InsertCast(IV->Step, IVTy, AfterPHIIt);
148
149        Val = BinaryOperator::create(Instruction::Mul, Val, IV->Step,
150                                     IV->Phi->getName()+"-scale");
151        // Insert the phi node at the end of the other phi nodes...
152        Header->getInstList().insert(AfterPHIIt, Val);
153      }
154
155      // If the start != 0
156      if (IV->Start != Constant::getNullValue(IV->Start->getType())) {
157        // If the types are not compatible, insert a cast now...
158        if (Val->getType() != IVTy)
159          Val = InsertCast(Val, IVTy, AfterPHIIt);
160        if (IV->Start->getType() != IVTy)
161          IV->Start = InsertCast(IV->Start, IVTy, AfterPHIIt);
162
163        Val = BinaryOperator::create(Instruction::Add, Val, IV->Start,
164                                     IV->Phi->getName()+"-offset");
165
166        // Insert the phi node at the end of the other phi nodes...
167        Header->getInstList().insert(AfterPHIIt, Val);
168      }
169
170      // If the PHI node has a different type than val is, insert a cast now...
171      if (Val->getType() != IV->Phi->getType())
172        Val = InsertCast(Val, IV->Phi->getType(), AfterPHIIt);
173
174      // Replace all uses of the old PHI node with the new computed value...
175      IV->Phi->replaceAllUsesWith(Val);
176
177      // Move the PHI name to it's new equivalent value...
178      std::string OldName = IV->Phi->getName();
179      IV->Phi->setName("");
180      Val->setName(OldName);
181
182      // Delete the old, now unused, phi node...
183      Header->getInstList().erase(IV->Phi);
184      Changed = true;
185      ++NumRemoved;
186    }
187  }
188
189  return Changed;
190}
191
192namespace {
193  struct InductionVariableSimplify : public FunctionPass {
194    virtual bool runOnFunction(Function &) {
195      LoopInfo &LI = getAnalysis<LoopInfo>();
196
197      // Induction Variables live in the header nodes of loops
198      return reduce_apply_bool(LI.getTopLevelLoops().begin(),
199                               LI.getTopLevelLoops().end(),
200                               std::bind1st(std::ptr_fun(TransformLoop), &LI));
201    }
202
203    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
204      AU.addRequired<LoopInfo>();
205      AU.preservesCFG();
206    }
207  };
208  RegisterOpt<InductionVariableSimplify> X("indvars",
209                                           "Cannonicalize Induction Variables");
210}
211
212Pass *createIndVarSimplifyPass() {
213  return new InductionVariableSimplify();
214}
215