IndVarSimplify.cpp revision a59cbb2043c08f3cfb8fb379f0d336e21e070be8 
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
19static Statistic<> NumRemoved ("indvars\t\t- Number of aux indvars removed");
20static Statistic<> NumInserted("indvars\t\t- Number of cannonical indvars added");
21
22
23// InsertCast - Cast Val to Ty, setting a useful name on the cast if Val has a
24// name...
25//
26static Instruction *InsertCast(Instruction *Val, const Type *Ty,
27                               BasicBlock::iterator It) {
28  Instruction *Cast = new CastInst(Val, Ty);
29  if (Val->hasName()) Cast->setName(Val->getName()+"-casted");
30  Val->getParent()->getInstList().insert(It, Cast);
31  return Cast;
32}
33
34static bool TransformLoop(LoopInfo *Loops, Loop *Loop) {
35  // Transform all subloops before this loop...
36  bool Changed = reduce_apply_bool(Loop->getSubLoops().begin(),
37                                   Loop->getSubLoops().end(),
38                              std::bind1st(std::ptr_fun(TransformLoop), Loops));
39  // Get the header node for this loop.  All of the phi nodes that could be
40  // induction variables must live in this basic block.
41  //
42  BasicBlock *Header = Loop->getBlocks().front();
43
44  // Loop over all of the PHI nodes in the basic block, calculating the
45  // induction variables that they represent... stuffing the induction variable
46  // info into a vector...
47  //
48  std::vector<InductionVariable> IndVars;    // Induction variables for block
49  BasicBlock::iterator AfterPHIIt = Header->begin();
50  for (; PHINode *PN = dyn_cast<PHINode>(&*AfterPHIIt); ++AfterPHIIt)
51    IndVars.push_back(InductionVariable(PN, Loops));
52  // AfterPHIIt now points to first nonphi instruction...
53
54  // If there are no phi nodes in this basic block, there can't be indvars...
55  if (IndVars.empty()) return Changed;
56
57  // Loop over the induction variables, looking for a cannonical induction
58  // variable, and checking to make sure they are not all unknown induction
59  // variables.
60  //
61  bool FoundIndVars = false;
62  InductionVariable *Cannonical = 0;
63  for (unsigned i = 0; i < IndVars.size(); ++i) {
64    if (IndVars[i].InductionType == InductionVariable::Cannonical)
65      Cannonical = &IndVars[i];
66    if (IndVars[i].InductionType != InductionVariable::Unknown)
67      FoundIndVars = true;
68  }
69
70  // No induction variables, bail early... don't add a cannonnical indvar
71  if (!FoundIndVars) return Changed;
72
73  // Okay, we want to convert other induction variables to use a cannonical
74  // indvar.  If we don't have one, add one now...
75  if (!Cannonical) {
76    // Create the PHI node for the new induction variable
77    PHINode *PN = new PHINode(Type::UIntTy, "cann-indvar");
78
79    // Insert the phi node at the end of the other phi nodes...
80    AfterPHIIt = ++Header->getInstList().insert(AfterPHIIt, PN);
81
82    // Create the increment instruction to add one to the counter...
83    Instruction *Add = BinaryOperator::create(Instruction::Add, PN,
84                                              ConstantUInt::get(Type::UIntTy,1),
85                                              "add1-indvar");
86
87    // Insert the add instruction after all of the PHI nodes...
88    Header->getInstList().insert(AfterPHIIt, Add);
89
90    // Figure out which block is incoming and which is the backedge for the loop
91    BasicBlock *Incoming, *BackEdgeBlock;
92    pred_iterator PI = pred_begin(Header);
93    assert(PI != pred_end(Header) && "Loop headers should have 2 preds!");
94    if (Loop->contains(*PI)) {  // First pred is back edge...
95      BackEdgeBlock = *PI++;
96      Incoming      = *PI++;
97    } else {
98      Incoming      = *PI++;
99      BackEdgeBlock = *PI++;
100    }
101    assert(PI == pred_end(Header) && "Loop headers should have 2 preds!");
102
103    // Add incoming values for the PHI node...
104    PN->addIncoming(Constant::getNullValue(Type::UIntTy), Incoming);
105    PN->addIncoming(Add, BackEdgeBlock);
106
107    // Analyze the new induction variable...
108    IndVars.push_back(InductionVariable(PN, Loops));
109    assert(IndVars.back().InductionType == InductionVariable::Cannonical &&
110           "Just inserted cannonical indvar that is not cannonical!");
111    Cannonical = &IndVars.back();
112    ++NumInserted;
113    Changed = true;
114  }
115
116  DEBUG(std::cerr << "Induction variables:\n");
117
118  // Get the current loop iteration count, which is always the value of the
119  // cannonical phi node...
120  //
121  PHINode *IterCount = Cannonical->Phi;
122
123  // Loop through and replace all of the auxillary induction variables with
124  // references to the primary induction variable...
125  //
126  for (unsigned i = 0; i < IndVars.size(); ++i) {
127    InductionVariable *IV = &IndVars[i];
128
129    DEBUG(IV->print(std::cerr));
130
131    // Don't modify the cannonical indvar or unrecognized indvars...
132    if (IV != Cannonical && IV->InductionType != InductionVariable::Unknown) {
133      Instruction *Val = IterCount;
134      if (!isa<ConstantInt>(IV->Step) ||   // If the step != 1
135          !cast<ConstantInt>(IV->Step)->equalsInt(1)) {
136        std::string Name;   // Create a scale by the step value...
137        if (IV->Phi->hasName()) Name = IV->Phi->getName()+"-scale";
138
139        // If the types are not compatible, insert a cast now...
140        if (Val->getType() != IV->Step->getType())
141          Val = InsertCast(Val, IV->Step->getType(), AfterPHIIt);
142
143        Val = BinaryOperator::create(Instruction::Mul, Val, IV->Step, Name);
144        // Insert the phi node at the end of the other phi nodes...
145        Header->getInstList().insert(AfterPHIIt, Val);
146      }
147
148      if (!isa<Constant>(IV->Start) ||   // If the start != 0
149          !cast<Constant>(IV->Start)->isNullValue()) {
150        std::string Name;   // Create a offset by the start value...
151        if (IV->Phi->hasName()) Name = IV->Phi->getName()+"-offset";
152
153        // If the types are not compatible, insert a cast now...
154        if (Val->getType() != IV->Start->getType())
155          Val = InsertCast(Val, IV->Start->getType(), AfterPHIIt);
156
157        Val = BinaryOperator::create(Instruction::Add, Val, IV->Start, Name);
158        // Insert the phi node at the end of the other phi nodes...
159        Header->getInstList().insert(AfterPHIIt, Val);
160      }
161
162      // If the PHI node has a different type than val is, insert a cast now...
163      if (Val->getType() != IV->Phi->getType())
164        Val = InsertCast(Val, IV->Phi->getType(), AfterPHIIt);
165
166      // Replace all uses of the old PHI node with the new computed value...
167      IV->Phi->replaceAllUsesWith(Val);
168
169      // Move the PHI name to it's new equivalent value...
170      std::string OldName = IV->Phi->getName();
171      IV->Phi->setName("");
172      Val->setName(OldName);
173
174      // Delete the old, now unused, phi node...
175      Header->getInstList().erase(IV->Phi);
176      Changed = true;
177      ++NumRemoved;
178    }
179  }
180
181  return Changed;
182}
183
184namespace {
185  struct InductionVariableSimplify : public FunctionPass {
186    virtual bool runOnFunction(Function &) {
187      LoopInfo &LI = getAnalysis<LoopInfo>();
188
189      // Induction Variables live in the header nodes of loops
190      return reduce_apply_bool(LI.getTopLevelLoops().begin(),
191                               LI.getTopLevelLoops().end(),
192                               std::bind1st(std::ptr_fun(TransformLoop), &LI));
193    }
194
195    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
196      AU.addRequired(LoopInfo::ID);
197      AU.preservesCFG();
198    }
199  };
200  RegisterOpt<InductionVariableSimplify> X("indvars",
201                                           "Cannonicalize Induction Variables");
202}
203
204Pass *createIndVarSimplifyPass() {
205  return new InductionVariableSimplify();
206}
207