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a06dee18e5
There was a bug in LoopBoundSplit. The pass should ignore phi node which is not scevable. Differential Revision: https://reviews.llvm.org/D103913
440 lines
15 KiB
C++
440 lines
15 KiB
C++
//===------- LoopBoundSplit.cpp - Split Loop Bound --------------*- C++ -*-===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Transforms/Scalar/LoopBoundSplit.h"
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#include "llvm/Analysis/LoopAccessAnalysis.h"
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#include "llvm/Analysis/LoopAnalysisManager.h"
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#include "llvm/Analysis/LoopInfo.h"
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#include "llvm/Analysis/LoopIterator.h"
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#include "llvm/Analysis/LoopPass.h"
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#include "llvm/Analysis/MemorySSA.h"
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#include "llvm/Analysis/MemorySSAUpdater.h"
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#include "llvm/Analysis/ScalarEvolution.h"
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#include "llvm/Analysis/ScalarEvolutionExpressions.h"
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#include "llvm/IR/PatternMatch.h"
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#include "llvm/Transforms/Utils/BasicBlockUtils.h"
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#include "llvm/Transforms/Utils/Cloning.h"
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#include "llvm/Transforms/Utils/LoopSimplify.h"
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#include "llvm/Transforms/Utils/LoopUtils.h"
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#include "llvm/Transforms/Utils/ScalarEvolutionExpander.h"
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#define DEBUG_TYPE "loop-bound-split"
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namespace llvm {
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using namespace PatternMatch;
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namespace {
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struct ConditionInfo {
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/// Branch instruction with this condition
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BranchInst *BI;
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/// ICmp instruction with this condition
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ICmpInst *ICmp;
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/// Preciate info
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ICmpInst::Predicate Pred;
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/// AddRec llvm value
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Value *AddRecValue;
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/// Bound llvm value
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Value *BoundValue;
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/// AddRec SCEV
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const SCEV *AddRecSCEV;
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/// Bound SCEV
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const SCEV *BoundSCEV;
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ConditionInfo()
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: BI(nullptr), ICmp(nullptr), Pred(ICmpInst::BAD_ICMP_PREDICATE),
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AddRecValue(nullptr), BoundValue(nullptr), AddRecSCEV(nullptr),
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BoundSCEV(nullptr) {}
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};
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} // namespace
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static void analyzeICmp(ScalarEvolution &SE, ICmpInst *ICmp,
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ConditionInfo &Cond) {
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Cond.ICmp = ICmp;
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if (match(ICmp, m_ICmp(Cond.Pred, m_Value(Cond.AddRecValue),
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m_Value(Cond.BoundValue)))) {
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Cond.AddRecSCEV = SE.getSCEV(Cond.AddRecValue);
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Cond.BoundSCEV = SE.getSCEV(Cond.BoundValue);
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// Locate AddRec in LHSSCEV and Bound in RHSSCEV.
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if (isa<SCEVAddRecExpr>(Cond.BoundSCEV) &&
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!isa<SCEVAddRecExpr>(Cond.AddRecSCEV)) {
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std::swap(Cond.AddRecValue, Cond.BoundValue);
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std::swap(Cond.AddRecSCEV, Cond.BoundSCEV);
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Cond.Pred = ICmpInst::getSwappedPredicate(Cond.Pred);
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}
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}
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}
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static bool calculateUpperBound(const Loop &L, ScalarEvolution &SE,
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ConditionInfo &Cond, bool IsExitCond) {
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if (IsExitCond) {
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const SCEV *ExitCount = SE.getExitCount(&L, Cond.ICmp->getParent());
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if (isa<SCEVCouldNotCompute>(ExitCount))
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return false;
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Cond.BoundSCEV = ExitCount;
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return true;
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}
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// For non-exit condtion, if pred is LT, keep existing bound.
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if (Cond.Pred == ICmpInst::ICMP_SLT || Cond.Pred == ICmpInst::ICMP_ULT)
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return true;
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// For non-exit condition, if pre is LE, try to convert it to LT.
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// Range Range
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// AddRec <= Bound --> AddRec < Bound + 1
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if (Cond.Pred != ICmpInst::ICMP_ULE && Cond.Pred != ICmpInst::ICMP_SLE)
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return false;
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if (IntegerType *BoundSCEVIntType =
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dyn_cast<IntegerType>(Cond.BoundSCEV->getType())) {
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unsigned BitWidth = BoundSCEVIntType->getBitWidth();
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APInt Max = ICmpInst::isSigned(Cond.Pred)
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? APInt::getSignedMaxValue(BitWidth)
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: APInt::getMaxValue(BitWidth);
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const SCEV *MaxSCEV = SE.getConstant(Max);
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// Check Bound < INT_MAX
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ICmpInst::Predicate Pred =
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ICmpInst::isSigned(Cond.Pred) ? ICmpInst::ICMP_SLT : ICmpInst::ICMP_ULT;
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if (SE.isKnownPredicate(Pred, Cond.BoundSCEV, MaxSCEV)) {
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const SCEV *BoundPlusOneSCEV =
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SE.getAddExpr(Cond.BoundSCEV, SE.getOne(BoundSCEVIntType));
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Cond.BoundSCEV = BoundPlusOneSCEV;
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Cond.Pred = Pred;
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return true;
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}
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}
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// ToDo: Support ICMP_NE/EQ.
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return false;
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}
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static bool hasProcessableCondition(const Loop &L, ScalarEvolution &SE,
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ICmpInst *ICmp, ConditionInfo &Cond,
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bool IsExitCond) {
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analyzeICmp(SE, ICmp, Cond);
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// The BoundSCEV should be evaluated at loop entry.
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if (!SE.isAvailableAtLoopEntry(Cond.BoundSCEV, &L))
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return false;
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const SCEVAddRecExpr *AddRecSCEV = dyn_cast<SCEVAddRecExpr>(Cond.AddRecSCEV);
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// Allowed AddRec as induction variable.
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if (!AddRecSCEV)
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return false;
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if (!AddRecSCEV->isAffine())
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return false;
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const SCEV *StepRecSCEV = AddRecSCEV->getStepRecurrence(SE);
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// Allowed constant step.
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if (!isa<SCEVConstant>(StepRecSCEV))
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return false;
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ConstantInt *StepCI = cast<SCEVConstant>(StepRecSCEV)->getValue();
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// Allowed positive step for now.
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// TODO: Support negative step.
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if (StepCI->isNegative() || StepCI->isZero())
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return false;
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// Calculate upper bound.
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if (!calculateUpperBound(L, SE, Cond, IsExitCond))
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return false;
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return true;
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}
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static bool isProcessableCondBI(const ScalarEvolution &SE,
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const BranchInst *BI) {
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BasicBlock *TrueSucc = nullptr;
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BasicBlock *FalseSucc = nullptr;
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ICmpInst::Predicate Pred;
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Value *LHS, *RHS;
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if (!match(BI, m_Br(m_ICmp(Pred, m_Value(LHS), m_Value(RHS)),
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m_BasicBlock(TrueSucc), m_BasicBlock(FalseSucc))))
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return false;
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if (!SE.isSCEVable(LHS->getType()))
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return false;
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assert(SE.isSCEVable(RHS->getType()) && "Expected RHS's type is SCEVable");
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if (TrueSucc == FalseSucc)
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return false;
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return true;
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}
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static bool canSplitLoopBound(const Loop &L, const DominatorTree &DT,
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ScalarEvolution &SE, ConditionInfo &Cond) {
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// Skip function with optsize.
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if (L.getHeader()->getParent()->hasOptSize())
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return false;
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// Split only innermost loop.
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if (!L.isInnermost())
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return false;
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// Check loop is in simplified form.
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if (!L.isLoopSimplifyForm())
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return false;
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// Check loop is in LCSSA form.
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if (!L.isLCSSAForm(DT))
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return false;
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// Skip loop that cannot be cloned.
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if (!L.isSafeToClone())
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return false;
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BasicBlock *ExitingBB = L.getExitingBlock();
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// Assumed only one exiting block.
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if (!ExitingBB)
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return false;
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BranchInst *ExitingBI = dyn_cast<BranchInst>(ExitingBB->getTerminator());
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if (!ExitingBI)
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return false;
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// Allowed only conditional branch with ICmp.
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if (!isProcessableCondBI(SE, ExitingBI))
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return false;
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// Check the condition is processable.
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ICmpInst *ICmp = cast<ICmpInst>(ExitingBI->getCondition());
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if (!hasProcessableCondition(L, SE, ICmp, Cond, /*IsExitCond*/ true))
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return false;
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Cond.BI = ExitingBI;
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return true;
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}
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static bool isProfitableToTransform(const Loop &L, const BranchInst *BI) {
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// If the conditional branch splits a loop into two halves, we could
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// generally say it is profitable.
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//
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// ToDo: Add more profitable cases here.
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// Check this branch causes diamond CFG.
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BasicBlock *Succ0 = BI->getSuccessor(0);
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BasicBlock *Succ1 = BI->getSuccessor(1);
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BasicBlock *Succ0Succ = Succ0->getSingleSuccessor();
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BasicBlock *Succ1Succ = Succ1->getSingleSuccessor();
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if (!Succ0Succ || !Succ1Succ || Succ0Succ != Succ1Succ)
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return false;
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// ToDo: Calculate each successor's instruction cost.
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return true;
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}
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static BranchInst *findSplitCandidate(const Loop &L, ScalarEvolution &SE,
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ConditionInfo &ExitingCond,
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ConditionInfo &SplitCandidateCond) {
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for (auto *BB : L.blocks()) {
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// Skip condition of backedge.
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if (L.getLoopLatch() == BB)
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continue;
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auto *BI = dyn_cast<BranchInst>(BB->getTerminator());
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if (!BI)
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continue;
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// Check conditional branch with ICmp.
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if (!isProcessableCondBI(SE, BI))
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continue;
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// Skip loop invariant condition.
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if (L.isLoopInvariant(BI->getCondition()))
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continue;
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// Check the condition is processable.
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ICmpInst *ICmp = cast<ICmpInst>(BI->getCondition());
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if (!hasProcessableCondition(L, SE, ICmp, SplitCandidateCond,
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/*IsExitCond*/ false))
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continue;
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if (ExitingCond.BoundSCEV->getType() !=
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SplitCandidateCond.BoundSCEV->getType())
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continue;
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SplitCandidateCond.BI = BI;
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return BI;
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}
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return nullptr;
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}
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static bool splitLoopBound(Loop &L, DominatorTree &DT, LoopInfo &LI,
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ScalarEvolution &SE, LPMUpdater &U) {
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ConditionInfo SplitCandidateCond;
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ConditionInfo ExitingCond;
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// Check we can split this loop's bound.
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if (!canSplitLoopBound(L, DT, SE, ExitingCond))
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return false;
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if (!findSplitCandidate(L, SE, ExitingCond, SplitCandidateCond))
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return false;
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if (!isProfitableToTransform(L, SplitCandidateCond.BI))
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return false;
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// Now, we have a split candidate. Let's build a form as below.
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// +--------------------+
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// | preheader |
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// | set up newbound |
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// +--------------------+
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// | /----------------\
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// +--------v----v------+ |
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// | header |---\ |
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// | with true condition| | |
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// +--------------------+ | |
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// | | |
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// +--------v-----------+ | |
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// | if.then.BB | | |
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// +--------------------+ | |
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// | | |
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// +--------v-----------<---/ |
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// | latch >----------/
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// | with newbound |
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// +--------------------+
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// |
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// +--------v-----------+
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// | preheader2 |--------------\
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// | if (AddRec i != | |
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// | org bound) | |
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// +--------------------+ |
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// | /----------------\ |
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// +--------v----v------+ | |
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// | header2 |---\ | |
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// | conditional branch | | | |
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// |with false condition| | | |
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// +--------------------+ | | |
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// | | | |
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// +--------v-----------+ | | |
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// | if.then.BB2 | | | |
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// +--------------------+ | | |
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// | | | |
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// +--------v-----------<---/ | |
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// | latch2 >----------/ |
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// | with org bound | |
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// +--------v-----------+ |
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// | |
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// | +---------------+ |
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// +--> exit <-------/
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// +---------------+
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// Let's create post loop.
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SmallVector<BasicBlock *, 8> PostLoopBlocks;
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Loop *PostLoop;
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ValueToValueMapTy VMap;
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BasicBlock *PreHeader = L.getLoopPreheader();
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BasicBlock *SplitLoopPH = SplitEdge(PreHeader, L.getHeader(), &DT, &LI);
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PostLoop = cloneLoopWithPreheader(L.getExitBlock(), SplitLoopPH, &L, VMap,
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".split", &LI, &DT, PostLoopBlocks);
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remapInstructionsInBlocks(PostLoopBlocks, VMap);
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// Add conditional branch to check we can skip post-loop in its preheader.
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BasicBlock *PostLoopPreHeader = PostLoop->getLoopPreheader();
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IRBuilder<> Builder(PostLoopPreHeader);
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Instruction *OrigBI = PostLoopPreHeader->getTerminator();
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ICmpInst::Predicate Pred = ICmpInst::ICMP_NE;
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Value *Cond =
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Builder.CreateICmp(Pred, ExitingCond.AddRecValue, ExitingCond.BoundValue);
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Builder.CreateCondBr(Cond, PostLoop->getHeader(), PostLoop->getExitBlock());
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OrigBI->eraseFromParent();
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// Create new loop bound and add it into preheader of pre-loop.
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const SCEV *NewBoundSCEV = ExitingCond.BoundSCEV;
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const SCEV *SplitBoundSCEV = SplitCandidateCond.BoundSCEV;
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NewBoundSCEV = ICmpInst::isSigned(ExitingCond.Pred)
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? SE.getSMinExpr(NewBoundSCEV, SplitBoundSCEV)
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: SE.getUMinExpr(NewBoundSCEV, SplitBoundSCEV);
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SCEVExpander Expander(
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SE, L.getHeader()->getParent()->getParent()->getDataLayout(), "split");
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Instruction *InsertPt = SplitLoopPH->getTerminator();
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Value *NewBoundValue =
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Expander.expandCodeFor(NewBoundSCEV, NewBoundSCEV->getType(), InsertPt);
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NewBoundValue->setName("new.bound");
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// Replace exiting bound value of pre-loop NewBound.
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ExitingCond.ICmp->setOperand(1, NewBoundValue);
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// Replace IV's start value of post-loop by NewBound.
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for (PHINode &PN : L.getHeader()->phis()) {
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// Find PHI with exiting condition from pre-loop.
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if (SE.isSCEVable(PN.getType()) && isa<SCEVAddRecExpr>(SE.getSCEV(&PN))) {
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for (Value *Op : PN.incoming_values()) {
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if (Op == ExitingCond.AddRecValue) {
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// Find cloned PHI for post-loop.
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PHINode *PostLoopPN = cast<PHINode>(VMap[&PN]);
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PostLoopPN->setIncomingValueForBlock(PostLoopPreHeader,
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NewBoundValue);
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}
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}
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}
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}
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// Replace SplitCandidateCond.BI's condition of pre-loop by True.
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LLVMContext &Context = PreHeader->getContext();
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SplitCandidateCond.BI->setCondition(ConstantInt::getTrue(Context));
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// Replace cloned SplitCandidateCond.BI's condition in post-loop by False.
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BranchInst *ClonedSplitCandidateBI =
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cast<BranchInst>(VMap[SplitCandidateCond.BI]);
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ClonedSplitCandidateBI->setCondition(ConstantInt::getFalse(Context));
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// Replace exit branch target of pre-loop by post-loop's preheader.
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if (L.getExitBlock() == ExitingCond.BI->getSuccessor(0))
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ExitingCond.BI->setSuccessor(0, PostLoopPreHeader);
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else
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ExitingCond.BI->setSuccessor(1, PostLoopPreHeader);
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// Update dominator tree.
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DT.changeImmediateDominator(PostLoopPreHeader, L.getExitingBlock());
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DT.changeImmediateDominator(PostLoop->getExitBlock(), PostLoopPreHeader);
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// Invalidate cached SE information.
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SE.forgetLoop(&L);
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// Canonicalize loops.
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// TODO: Try to update LCSSA information according to above change.
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formLCSSA(L, DT, &LI, &SE);
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simplifyLoop(&L, &DT, &LI, &SE, nullptr, nullptr, true);
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formLCSSA(*PostLoop, DT, &LI, &SE);
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simplifyLoop(PostLoop, &DT, &LI, &SE, nullptr, nullptr, true);
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// Add new post-loop to loop pass manager.
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U.addSiblingLoops(PostLoop);
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return true;
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}
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PreservedAnalyses LoopBoundSplitPass::run(Loop &L, LoopAnalysisManager &AM,
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LoopStandardAnalysisResults &AR,
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LPMUpdater &U) {
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Function &F = *L.getHeader()->getParent();
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(void)F;
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LLVM_DEBUG(dbgs() << "Spliting bound of loop in " << F.getName() << ": " << L
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<< "\n");
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if (!splitLoopBound(L, AR.DT, AR.LI, AR.SE, U))
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return PreservedAnalyses::all();
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assert(AR.DT.verify(DominatorTree::VerificationLevel::Fast));
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AR.LI.verify(AR.DT);
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return getLoopPassPreservedAnalyses();
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}
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} // end namespace llvm
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