296 lines
10 KiB
C++
296 lines
10 KiB
C++
//===- InductionVariable.cpp - Induction variable classification ----------===//
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//
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// This file implements identification and classification of induction
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// variables. Induction variables must contain a PHI node that exists in a
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// loop header. Because of this, they are identified an managed by this PHI
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// node.
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//
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// Induction variables are classified into a type. Knowing that an induction
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// variable is of a specific type can constrain the values of the start and
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// step. For example, a SimpleLinear induction variable must have a start and
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// step values that are constants.
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//
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// Induction variables can be created with or without loop information. If no
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// loop information is available, induction variables cannot be recognized to be
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// more than SimpleLinear variables.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Analysis/InductionVariable.h"
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#include "llvm/Analysis/LoopInfo.h"
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#include "llvm/Analysis/Expressions.h"
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#include "llvm/BasicBlock.h"
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#include "llvm/iPHINode.h"
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#include "llvm/iOperators.h"
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#include "llvm/iTerminators.h"
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#include "llvm/Type.h"
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#include "llvm/Constants.h"
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#include "llvm/Support/CFG.h"
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#include "llvm/Assembly/Writer.h"
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#include "Support/Debug.h"
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static bool isLoopInvariant(const Value *V, const Loop *L) {
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if (const Instruction *I = dyn_cast<Instruction>(V))
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return !L->contains(I->getParent());
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// non-instructions all dominate instructions/blocks
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return true;
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}
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enum InductionVariable::iType
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InductionVariable::Classify(const Value *Start, const Value *Step,
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const Loop *L) {
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// Check for canonical and simple linear expressions now...
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if (const ConstantInt *CStart = dyn_cast<ConstantInt>(Start))
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if (const ConstantInt *CStep = dyn_cast<ConstantInt>(Step)) {
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if (CStart->isNullValue() && CStep->equalsInt(1))
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return Canonical;
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else
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return SimpleLinear;
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}
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// Without loop information, we cannot do any better, so bail now...
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if (L == 0) return Unknown;
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if (isLoopInvariant(Start, L) && isLoopInvariant(Step, L))
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return Linear;
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return Unknown;
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}
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// Create an induction variable for the specified value. If it is a PHI, and
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// if it's recognizable, classify it and fill in instance variables.
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//
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InductionVariable::InductionVariable(PHINode *P, LoopInfo *LoopInfo): End(0) {
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InductionType = Unknown; // Assume the worst
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Phi = P;
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// If the PHI node has more than two predecessors, we don't know how to
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// handle it.
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//
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if (Phi->getNumIncomingValues() != 2) return;
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// FIXME: Handle FP induction variables.
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if (Phi->getType() == Type::FloatTy || Phi->getType() == Type::DoubleTy)
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return;
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// If we have loop information, make sure that this PHI node is in the header
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// of a loop...
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//
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const Loop *L = LoopInfo ? LoopInfo->getLoopFor(Phi->getParent()) : 0;
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if (L && L->getHeader() != Phi->getParent())
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return;
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Value *V1 = Phi->getIncomingValue(0);
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Value *V2 = Phi->getIncomingValue(1);
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if (L == 0) { // No loop information? Base everything on expression analysis
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ExprType E1 = ClassifyExpression(V1);
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ExprType E2 = ClassifyExpression(V2);
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if (E1.ExprTy > E2.ExprTy) // Make E1 be the simpler expression
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std::swap(E1, E2);
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// E1 must be a constant incoming value, and E2 must be a linear expression
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// with respect to the PHI node.
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//
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if (E1.ExprTy > ExprType::Constant || E2.ExprTy != ExprType::Linear ||
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E2.Var != Phi)
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return;
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// Okay, we have found an induction variable. Save the start and step values
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const Type *ETy = Phi->getType();
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if (isa<PointerType>(ETy)) ETy = Type::ULongTy;
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Start = (Value*)(E1.Offset ? E1.Offset : ConstantInt::get(ETy, 0));
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Step = (Value*)(E2.Offset ? E2.Offset : ConstantInt::get(ETy, 0));
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} else {
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// Okay, at this point, we know that we have loop information...
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// Make sure that V1 is the incoming value, and V2 is from the backedge of
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// the loop.
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if (L->contains(Phi->getIncomingBlock(0))) // Wrong order. Swap now.
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std::swap(V1, V2);
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Start = V1; // We know that Start has to be loop invariant...
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Step = 0;
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if (V2 == Phi) { // referencing the PHI directly? Must have zero step
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Step = Constant::getNullValue(Phi->getType());
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} else if (BinaryOperator *I = dyn_cast<BinaryOperator>(V2)) {
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// TODO: This could be much better...
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if (I->getOpcode() == Instruction::Add) {
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if (I->getOperand(0) == Phi)
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Step = I->getOperand(1);
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else if (I->getOperand(1) == Phi)
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Step = I->getOperand(0);
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}
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}
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if (Step == 0) { // Unrecognized step value...
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ExprType StepE = ClassifyExpression(V2);
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if (StepE.ExprTy != ExprType::Linear ||
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StepE.Var != Phi) return;
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const Type *ETy = Phi->getType();
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if (isa<PointerType>(ETy)) ETy = Type::ULongTy;
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Step = (Value*)(StepE.Offset ? StepE.Offset : ConstantInt::get(ETy, 0));
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} else { // We were able to get a step value, simplify with expr analysis
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ExprType StepE = ClassifyExpression(Step);
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if (StepE.ExprTy == ExprType::Linear && StepE.Offset == 0) {
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// No offset from variable? Grab the variable
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Step = StepE.Var;
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} else if (StepE.ExprTy == ExprType::Constant) {
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if (StepE.Offset)
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Step = (Value*)StepE.Offset;
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else
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Step = Constant::getNullValue(Step->getType());
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const Type *ETy = Phi->getType();
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if (isa<PointerType>(ETy)) ETy = Type::ULongTy;
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Step = (Value*)(StepE.Offset ? StepE.Offset : ConstantInt::get(ETy,0));
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}
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}
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}
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// Classify the induction variable type now...
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InductionType = InductionVariable::Classify(Start, Step, L);
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}
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Value *InductionVariable::getExecutionCount(LoopInfo *LoopInfo) {
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if (InductionType != Canonical) return 0;
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DEBUG(std::cerr << "entering getExecutionCount\n");
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// Don't recompute if already available
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if (End) {
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DEBUG(std::cerr << "returning cached End value.\n");
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return End;
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}
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const Loop *L = LoopInfo ? LoopInfo->getLoopFor(Phi->getParent()) : 0;
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if (!L) {
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DEBUG(std::cerr << "null loop. oops\n");
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return 0;
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}
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// >1 backedge => cannot predict number of iterations
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if (Phi->getNumIncomingValues() != 2) {
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DEBUG(std::cerr << ">2 incoming values. oops\n");
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return 0;
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}
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// Find final node: predecesor of the loop header that's also an exit
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BasicBlock *terminator = 0;
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for (pred_iterator PI = pred_begin(L->getHeader()),
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PE = pred_end(L->getHeader()); PI != PE; ++PI)
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if (L->isLoopExit(*PI)) {
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terminator = *PI;
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break;
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}
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// Break in the loop => cannot predict number of iterations
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// break: any block which is an exit node whose successor is not in loop,
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// and this block is not marked as the terminator
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//
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const std::vector<BasicBlock*> &blocks = L->getBlocks();
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for (std::vector<BasicBlock*>::const_iterator I = blocks.begin(),
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e = blocks.end(); I != e; ++I)
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if (L->isLoopExit(*I) && *I != terminator)
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for (succ_iterator SI = succ_begin(*I), SE = succ_end(*I); SI != SE; ++SI)
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if (!L->contains(*SI)) {
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DEBUG(std::cerr << "break found in loop");
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return 0;
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}
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BranchInst *B = dyn_cast<BranchInst>(terminator->getTerminator());
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if (!B) {
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DEBUG(std::cerr << "Terminator is not a cond branch!");
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return 0;
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}
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SetCondInst *SCI = dyn_cast<SetCondInst>(B->getCondition());
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if (!SCI) {
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DEBUG(std::cerr << "Not a cond branch on setcc!\n");
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return 0;
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}
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DEBUG(std::cerr << "sci:" << *SCI);
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Value *condVal0 = SCI->getOperand(0);
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Value *condVal1 = SCI->getOperand(1);
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Value *indVar = 0;
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// the induction variable is the one coming from the backedge
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indVar = Phi->getIncomingValue(L->contains(Phi->getIncomingBlock(1)));
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// Check to see if indVar is one of the parameters in SCI and if the other is
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// loop-invariant, it is the UB
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if (indVar == condVal0) {
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if (isLoopInvariant(condVal1, L))
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End = condVal1;
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else {
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DEBUG(std::cerr << "not loop invariant 1\n");
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return 0;
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}
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} else if (indVar == condVal1) {
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if (isLoopInvariant(condVal0, L))
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End = condVal0;
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else {
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DEBUG(std::cerr << "not loop invariant 0\n");
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return 0;
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}
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} else {
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DEBUG(std::cerr << "Loop condition doesn't directly uses indvar\n");
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return 0;
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}
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switch (SCI->getOpcode()) {
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case Instruction::SetLT:
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case Instruction::SetNE: return End; // already done
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case Instruction::SetLE:
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// if compared to a constant int N, then predict N+1 iterations
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if (ConstantSInt *ubSigned = dyn_cast<ConstantSInt>(End)) {
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DEBUG(std::cerr << "signed int constant\n");
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return ConstantSInt::get(ubSigned->getType(), ubSigned->getValue()+1);
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} else if (ConstantUInt *ubUnsigned = dyn_cast<ConstantUInt>(End)) {
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DEBUG(std::cerr << "unsigned int constant\n");
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return ConstantUInt::get(ubUnsigned->getType(),
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ubUnsigned->getValue()+1);
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} else {
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DEBUG(std::cerr << "symbolic bound\n");
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// new expression N+1, insert right before the SCI. FIXME: If End is loop
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// invariant, then so is this expression. We should insert it in the loop
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// preheader if it exists.
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return BinaryOperator::create(Instruction::Add, End,
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ConstantInt::get(End->getType(), 1),
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"tripcount", SCI);
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}
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default:
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return 0; // cannot predict
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}
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}
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void InductionVariable::print(std::ostream &o) const {
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switch (InductionType) {
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case InductionVariable::Canonical: o << "Canonical "; break;
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case InductionVariable::SimpleLinear: o << "SimpleLinear "; break;
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case InductionVariable::Linear: o << "Linear "; break;
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case InductionVariable::Unknown: o << "Unrecognized "; break;
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}
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o << "Induction Variable: ";
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if (Phi) {
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WriteAsOperand(o, Phi);
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o << ":\n" << Phi;
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} else {
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o << "\n";
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}
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if (InductionType == InductionVariable::Unknown) return;
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o << " Start = "; WriteAsOperand(o, Start);
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o << " Step = " ; WriteAsOperand(o, Step);
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if (End) {
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o << " End = " ; WriteAsOperand(o, End);
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}
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o << "\n";
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}
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