1 //===- InductionVariable.cpp - Induction variable classification ----------===//
3 // The LLVM Compiler Infrastructure
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.
8 //===----------------------------------------------------------------------===//
10 // This file implements identification and classification of induction
11 // variables. Induction variables must contain a PHI node that exists in a
12 // loop header. Because of this, they are identified an managed by this PHI
15 // Induction variables are classified into a type. Knowing that an induction
16 // variable is of a specific type can constrain the values of the start and
17 // step. For example, a SimpleLinear induction variable must have a start and
18 // step values that are constants.
20 // Induction variables can be created with or without loop information. If no
21 // loop information is available, induction variables cannot be recognized to be
22 // more than SimpleLinear variables.
24 //===----------------------------------------------------------------------===//
26 #include "llvm/Analysis/InductionVariable.h"
27 #include "llvm/Analysis/LoopInfo.h"
28 #include "llvm/Analysis/Expressions.h"
29 #include "llvm/BasicBlock.h"
30 #include "llvm/iPHINode.h"
31 #include "llvm/iOperators.h"
32 #include "llvm/iTerminators.h"
33 #include "llvm/Type.h"
34 #include "llvm/Constants.h"
35 #include "llvm/Support/CFG.h"
36 #include "llvm/Assembly/Writer.h"
37 #include "Support/Debug.h"
39 static bool isLoopInvariant(const Value *V, const Loop *L) {
40 if (const Instruction *I = dyn_cast<Instruction>(V))
41 return !L->contains(I->getParent());
42 // non-instructions all dominate instructions/blocks
46 enum InductionVariable::iType
47 InductionVariable::Classify(const Value *Start, const Value *Step,
49 // Check for canonical and simple linear expressions now...
50 if (const ConstantInt *CStart = dyn_cast<ConstantInt>(Start))
51 if (const ConstantInt *CStep = dyn_cast<ConstantInt>(Step)) {
52 if (CStart->isNullValue() && CStep->equalsInt(1))
58 // Without loop information, we cannot do any better, so bail now...
59 if (L == 0) return Unknown;
61 if (isLoopInvariant(Start, L) && isLoopInvariant(Step, L))
66 // Create an induction variable for the specified value. If it is a PHI, and
67 // if it's recognizable, classify it and fill in instance variables.
69 InductionVariable::InductionVariable(PHINode *P, LoopInfo *LoopInfo): End(0) {
70 InductionType = Unknown; // Assume the worst
73 // If the PHI node has more than two predecessors, we don't know how to
76 if (Phi->getNumIncomingValues() != 2) return;
78 // FIXME: Handle FP induction variables.
79 if (Phi->getType() == Type::FloatTy || Phi->getType() == Type::DoubleTy)
82 // If we have loop information, make sure that this PHI node is in the header
85 const Loop *L = LoopInfo ? LoopInfo->getLoopFor(Phi->getParent()) : 0;
86 if (L && L->getHeader() != Phi->getParent())
89 Value *V1 = Phi->getIncomingValue(0);
90 Value *V2 = Phi->getIncomingValue(1);
92 if (L == 0) { // No loop information? Base everything on expression analysis
93 ExprType E1 = ClassifyExpression(V1);
94 ExprType E2 = ClassifyExpression(V2);
96 if (E1.ExprTy > E2.ExprTy) // Make E1 be the simpler expression
99 // E1 must be a constant incoming value, and E2 must be a linear expression
100 // with respect to the PHI node.
102 if (E1.ExprTy > ExprType::Constant || E2.ExprTy != ExprType::Linear ||
106 // Okay, we have found an induction variable. Save the start and step values
107 const Type *ETy = Phi->getType();
108 if (isa<PointerType>(ETy)) ETy = Type::ULongTy;
110 Start = (Value*)(E1.Offset ? E1.Offset : ConstantInt::get(ETy, 0));
111 Step = (Value*)(E2.Offset ? E2.Offset : ConstantInt::get(ETy, 0));
113 // Okay, at this point, we know that we have loop information...
115 // Make sure that V1 is the incoming value, and V2 is from the backedge of
117 if (L->contains(Phi->getIncomingBlock(0))) // Wrong order. Swap now.
120 Start = V1; // We know that Start has to be loop invariant...
123 if (V2 == Phi) { // referencing the PHI directly? Must have zero step
124 Step = Constant::getNullValue(Phi->getType());
125 } else if (BinaryOperator *I = dyn_cast<BinaryOperator>(V2)) {
126 // TODO: This could be much better...
127 if (I->getOpcode() == Instruction::Add) {
128 if (I->getOperand(0) == Phi)
129 Step = I->getOperand(1);
130 else if (I->getOperand(1) == Phi)
131 Step = I->getOperand(0);
135 if (Step == 0) { // Unrecognized step value...
136 ExprType StepE = ClassifyExpression(V2);
137 if (StepE.ExprTy != ExprType::Linear ||
138 StepE.Var != Phi) return;
140 const Type *ETy = Phi->getType();
141 if (isa<PointerType>(ETy)) ETy = Type::ULongTy;
142 Step = (Value*)(StepE.Offset ? StepE.Offset : ConstantInt::get(ETy, 0));
143 } else { // We were able to get a step value, simplify with expr analysis
144 ExprType StepE = ClassifyExpression(Step);
145 if (StepE.ExprTy == ExprType::Linear && StepE.Offset == 0) {
146 // No offset from variable? Grab the variable
148 } else if (StepE.ExprTy == ExprType::Constant) {
150 Step = (Value*)StepE.Offset;
152 Step = Constant::getNullValue(Step->getType());
153 const Type *ETy = Phi->getType();
154 if (isa<PointerType>(ETy)) ETy = Type::ULongTy;
155 Step = (Value*)(StepE.Offset ? StepE.Offset : ConstantInt::get(ETy,0));
160 // Classify the induction variable type now...
161 InductionType = InductionVariable::Classify(Start, Step, L);
165 Value *InductionVariable::getExecutionCount(LoopInfo *LoopInfo) {
166 if (InductionType != Canonical) return 0;
168 DEBUG(std::cerr << "entering getExecutionCount\n");
170 // Don't recompute if already available
172 DEBUG(std::cerr << "returning cached End value.\n");
176 const Loop *L = LoopInfo ? LoopInfo->getLoopFor(Phi->getParent()) : 0;
178 DEBUG(std::cerr << "null loop. oops\n");
182 // >1 backedge => cannot predict number of iterations
183 if (Phi->getNumIncomingValues() != 2) {
184 DEBUG(std::cerr << ">2 incoming values. oops\n");
188 // Find final node: predecessor of the loop header that's also an exit
189 BasicBlock *terminator = 0;
190 for (pred_iterator PI = pred_begin(L->getHeader()),
191 PE = pred_end(L->getHeader()); PI != PE; ++PI)
192 if (L->isLoopExit(*PI)) {
197 // Break in the loop => cannot predict number of iterations
198 // break: any block which is an exit node whose successor is not in loop,
199 // and this block is not marked as the terminator
201 const std::vector<BasicBlock*> &blocks = L->getBlocks();
202 for (std::vector<BasicBlock*>::const_iterator I = blocks.begin(),
203 e = blocks.end(); I != e; ++I)
204 if (L->isLoopExit(*I) && *I != terminator)
205 for (succ_iterator SI = succ_begin(*I), SE = succ_end(*I); SI != SE; ++SI)
206 if (!L->contains(*SI)) {
207 DEBUG(std::cerr << "break found in loop");
211 BranchInst *B = dyn_cast<BranchInst>(terminator->getTerminator());
213 DEBUG(std::cerr << "Terminator is not a cond branch!");
216 SetCondInst *SCI = dyn_cast<SetCondInst>(B->getCondition());
218 DEBUG(std::cerr << "Not a cond branch on setcc!\n");
222 DEBUG(std::cerr << "sci:" << *SCI);
223 Value *condVal0 = SCI->getOperand(0);
224 Value *condVal1 = SCI->getOperand(1);
226 // The induction variable is the one coming from the backedge
227 Value *indVar = Phi->getIncomingValue(L->contains(Phi->getIncomingBlock(1)));
230 // Check to see if indVar is one of the parameters in SCI and if the other is
231 // loop-invariant, it is the UB
232 if (indVar == condVal0) {
233 if (isLoopInvariant(condVal1, L))
236 DEBUG(std::cerr << "not loop invariant 1\n");
239 } else if (indVar == condVal1) {
240 if (isLoopInvariant(condVal0, L))
243 DEBUG(std::cerr << "not loop invariant 0\n");
247 DEBUG(std::cerr << "Loop condition doesn't directly uses indvar\n");
251 switch (SCI->getOpcode()) {
252 case Instruction::SetLT:
253 case Instruction::SetNE: return End; // already done
254 case Instruction::SetLE:
255 // if compared to a constant int N, then predict N+1 iterations
256 if (ConstantSInt *ubSigned = dyn_cast<ConstantSInt>(End)) {
257 DEBUG(std::cerr << "signed int constant\n");
258 return ConstantSInt::get(ubSigned->getType(), ubSigned->getValue()+1);
259 } else if (ConstantUInt *ubUnsigned = dyn_cast<ConstantUInt>(End)) {
260 DEBUG(std::cerr << "unsigned int constant\n");
261 return ConstantUInt::get(ubUnsigned->getType(),
262 ubUnsigned->getValue()+1);
264 DEBUG(std::cerr << "symbolic bound\n");
265 // new expression N+1, insert right before the SCI. FIXME: If End is loop
266 // invariant, then so is this expression. We should insert it in the loop
267 // preheader if it exists.
268 return BinaryOperator::create(Instruction::Add, End,
269 ConstantInt::get(End->getType(), 1),
274 return 0; // cannot predict
279 void InductionVariable::print(std::ostream &o) const {
280 switch (InductionType) {
281 case InductionVariable::Canonical: o << "Canonical "; break;
282 case InductionVariable::SimpleLinear: o << "SimpleLinear "; break;
283 case InductionVariable::Linear: o << "Linear "; break;
284 case InductionVariable::Unknown: o << "Unrecognized "; break;
286 o << "Induction Variable: ";
288 WriteAsOperand(o, Phi);
293 if (InductionType == InductionVariable::Unknown) return;
295 o << " Start = "; WriteAsOperand(o, Start);
296 o << " Step = " ; WriteAsOperand(o, Step);
298 o << " End = " ; WriteAsOperand(o, End);