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llvm-6502/lib/Transforms/Scalar/LoopIndexSplit.cpp
Devang Patel d15dd8c525 Don't increment invalid iterator. 
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@42178 91177308-0d34-0410-b5e6-96231b3b80d8
2007-09-20 23:01:50 +00:00

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//===- LoopIndexSplit.cpp - Loop Index Splitting Pass ---------------------===//
//
// The LLVM Compiler Infrastructure
//
// This file was developed by Devang Patel and is distributed under
// the University of Illinois Open Source License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements Loop Index Splitting Pass.
//
//===----------------------------------------------------------------------===//
#define DEBUG_TYPE "loop-index-split"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/Analysis/ScalarEvolutionExpander.h"
#include "llvm/Analysis/Dominators.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/Cloning.h"
#include "llvm/Support/Compiler.h"
#include "llvm/ADT/DepthFirstIterator.h"
#include "llvm/ADT/Statistic.h"
using namespace llvm;
STATISTIC(NumIndexSplit, "Number of loops index split");
namespace {
class VISIBILITY_HIDDEN LoopIndexSplit : public LoopPass {
public:
static char ID; // Pass ID, replacement for typeid
LoopIndexSplit() : LoopPass((intptr_t)&ID) {}
// Index split Loop L. Return true if loop is split.
bool runOnLoop(Loop *L, LPPassManager &LPM);
void getAnalysisUsage(AnalysisUsage &AU) const {
AU.addRequired<ScalarEvolution>();
AU.addPreserved<ScalarEvolution>();
AU.addRequiredID(LCSSAID);
AU.addPreservedID(LCSSAID);
AU.addRequired<LoopInfo>();
AU.addPreserved<LoopInfo>();
AU.addRequiredID(LoopSimplifyID);
AU.addPreservedID(LoopSimplifyID);
AU.addRequired<DominatorTree>();
AU.addRequired<DominanceFrontier>();
AU.addPreserved<DominatorTree>();
AU.addPreserved<DominanceFrontier>();
}
private:
class SplitInfo {
public:
SplitInfo() : SplitValue(NULL), SplitCondition(NULL),
UseTrueBranchFirst(true), A_ExitValue(NULL),
B_StartValue(NULL) {}
// Induction variable's range is split at this value.
Value *SplitValue;
// This instruction compares IndVar against SplitValue.
Instruction *SplitCondition;
// True if after loop index split, first loop will execute split condition's
// true branch.
bool UseTrueBranchFirst;
// Exit value for first loop after loop split.
Value *A_ExitValue;
// Start value for second loop after loop split.
Value *B_StartValue;
// Clear split info.
void clear() {
SplitValue = NULL;
SplitCondition = NULL;
UseTrueBranchFirst = true;
A_ExitValue = NULL;
B_StartValue = NULL;
}
};
private:
// safeIcmpInst - CI is considered safe instruction if one of the operand
// is SCEVAddRecExpr based on induction variable and other operand is
// loop invariant. If CI is safe then populate SplitInfo object SD appropriately
// and return true;
bool safeICmpInst(ICmpInst *CI, SplitInfo &SD);
/// Find condition inside a loop that is suitable candidate for index split.
void findSplitCondition();
/// Find loop's exit condition.
void findLoopConditionals();
/// Return induction variable associated with value V.
void findIndVar(Value *V, Loop *L);
/// processOneIterationLoop - Current loop L contains compare instruction
/// that compares induction variable, IndVar, agains loop invariant. If
/// entire (i.e. meaningful) loop body is dominated by this compare
/// instruction then loop body is executed only for one iteration. In
/// such case eliminate loop structure surrounding this loop body. For
bool processOneIterationLoop(SplitInfo &SD);
void updateLoopBounds(ICmpInst *CI);
/// updateLoopIterationSpace - Current loop body is covered by an AND
/// instruction whose operands compares induction variables with loop
/// invariants. If possible, hoist this check outside the loop by
/// updating appropriate start and end values for induction variable.
bool updateLoopIterationSpace(SplitInfo &SD);
/// If loop header includes loop variant instruction operands then
/// this loop may not be eliminated.
bool safeHeader(SplitInfo &SD, BasicBlock *BB);
/// If Exiting block includes loop variant instructions then this
/// loop may not be eliminated.
bool safeExitingBlock(SplitInfo &SD, BasicBlock *BB);
/// removeBlocks - Remove basic block DeadBB and all blocks dominated by DeadBB.
/// This routine is used to remove split condition's dead branch, dominated by
/// DeadBB. LiveBB dominates split conidition's other branch.
void removeBlocks(BasicBlock *DeadBB, Loop *LP, BasicBlock *LiveBB);
/// safeSplitCondition - Return true if it is possible to
/// split loop using given split condition.
bool safeSplitCondition(SplitInfo &SD);
/// calculateLoopBounds - ALoop exit value and BLoop start values are calculated
/// based on split value.
void calculateLoopBounds(SplitInfo &SD);
/// updatePHINodes - CFG has been changed.
/// Before
/// - ExitBB's single predecessor was Latch
/// - Latch's second successor was Header
/// Now
/// - ExitBB's single predecessor was Header
/// - Latch's one and only successor was Header
///
/// Update ExitBB PHINodes' to reflect this change.
void updatePHINodes(BasicBlock *ExitBB, BasicBlock *Latch,
BasicBlock *Header,
PHINode *IV, Instruction *IVIncrement);
/// moveExitCondition - Move exit condition EC into split condition block CondBB.
void moveExitCondition(BasicBlock *CondBB, BasicBlock *ActiveBB,
BasicBlock *ExitBB, ICmpInst *EC, ICmpInst *SC,
PHINode *IV, Instruction *IVAdd, Loop *LP);
/// splitLoop - Split current loop L in two loops using split information
/// SD. Update dominator information. Maintain LCSSA form.
bool splitLoop(SplitInfo &SD);
void initialize() {
IndVar = NULL;
IndVarIncrement = NULL;
ExitCondition = NULL;
StartValue = NULL;
ExitValueNum = 0;
SplitData.clear();
}
private:
// Current Loop.
Loop *L;
LPPassManager *LPM;
LoopInfo *LI;
ScalarEvolution *SE;
DominatorTree *DT;
DominanceFrontier *DF;
SmallVector<SplitInfo, 4> SplitData;
// Induction variable whose range is being split by this transformation.
PHINode *IndVar;
Instruction *IndVarIncrement;
// Loop exit condition.
ICmpInst *ExitCondition;
// Induction variable's initial value.
Value *StartValue;
// Induction variable's final loop exit value operand number in exit condition..
unsigned ExitValueNum;
};
char LoopIndexSplit::ID = 0;
RegisterPass<LoopIndexSplit> X ("loop-index-split", "Index Split Loops");
}
LoopPass *llvm::createLoopIndexSplitPass() {
return new LoopIndexSplit();
}
// Index split Loop L. Return true if loop is split.
bool LoopIndexSplit::runOnLoop(Loop *IncomingLoop, LPPassManager &LPM_Ref) {
bool Changed = false;
L = IncomingLoop;
LPM = &LPM_Ref;
// FIXME - Nested loops make dominator info updates tricky.
if (!L->getSubLoops().empty())
return false;
SE = &getAnalysis<ScalarEvolution>();
DT = &getAnalysis<DominatorTree>();
LI = &getAnalysis<LoopInfo>();
DF = &getAnalysis<DominanceFrontier>();
initialize();
findLoopConditionals();
if (!ExitCondition)
return false;
findSplitCondition();
if (SplitData.empty())
return false;
// First see if it is possible to eliminate loop itself or not.
for (SmallVector<SplitInfo, 4>::iterator SI = SplitData.begin(),
E = SplitData.end(); SI != E;) {
SplitInfo &SD = *SI;
ICmpInst *CI = dyn_cast<ICmpInst>(SD.SplitCondition);
if (SD.SplitCondition->getOpcode() == Instruction::And) {
Changed = updateLoopIterationSpace(SD);
if (Changed) {
++NumIndexSplit;
// If is loop is eliminated then nothing else to do here.
return Changed;
} else {
SmallVector<SplitInfo, 4>::iterator Delete_SI = SI;
++SI;
SplitData.erase(Delete_SI);
}
}
else if (CI && CI->getPredicate() == ICmpInst::ICMP_EQ) {
Changed = processOneIterationLoop(SD);
if (Changed) {
++NumIndexSplit;
// If is loop is eliminated then nothing else to do here.
return Changed;
} else {
SmallVector<SplitInfo, 4>::iterator Delete_SI = SI;
++SI;
SplitData.erase(Delete_SI);
}
} else
++SI;
}
if (SplitData.empty())
return false;
// Split most profitiable condition.
// FIXME : Implement cost analysis.
unsigned MostProfitableSDIndex = 0;
Changed = splitLoop(SplitData[MostProfitableSDIndex]);
if (Changed)
++NumIndexSplit;
return Changed;
}
/// Return true if V is a induction variable or induction variable's
/// increment for loop L.
void LoopIndexSplit::findIndVar(Value *V, Loop *L) {
Instruction *I = dyn_cast<Instruction>(V);
if (!I)
return;
// Check if I is a phi node from loop header or not.
if (PHINode *PN = dyn_cast<PHINode>(V)) {
if (PN->getParent() == L->getHeader()) {
IndVar = PN;
return;
}
}
// Check if I is a add instruction whose one operand is
// phi node from loop header and second operand is constant.
if (I->getOpcode() != Instruction::Add)
return;
Value *Op0 = I->getOperand(0);
Value *Op1 = I->getOperand(1);
if (PHINode *PN = dyn_cast<PHINode>(Op0)) {
if (PN->getParent() == L->getHeader()
&& isa<ConstantInt>(Op1)) {
IndVar = PN;
IndVarIncrement = I;
return;
}
}
if (PHINode *PN = dyn_cast<PHINode>(Op1)) {
if (PN->getParent() == L->getHeader()
&& isa<ConstantInt>(Op0)) {
IndVar = PN;
IndVarIncrement = I;
return;
}
}
return;
}
// Find loop's exit condition and associated induction variable.
void LoopIndexSplit::findLoopConditionals() {
BasicBlock *ExitingBlock = NULL;
for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
I != E; ++I) {
BasicBlock *BB = *I;
if (!L->isLoopExit(BB))
continue;
if (ExitingBlock)
return;
ExitingBlock = BB;
}
if (!ExitingBlock)
return;
// If exiting block is neither loop header nor loop latch then this loop is
// not suitable.
if (ExitingBlock != L->getHeader() && ExitingBlock != L->getLoopLatch())
return;
// If exit block's terminator is conditional branch inst then we have found
// exit condition.
BranchInst *BR = dyn_cast<BranchInst>(ExitingBlock->getTerminator());
if (!BR || BR->isUnconditional())
return;
ICmpInst *CI = dyn_cast<ICmpInst>(BR->getCondition());
if (!CI)
return;
// FIXME
if (CI->getPredicate() == ICmpInst::ICMP_EQ
|| CI->getPredicate() == ICmpInst::ICMP_NE)
return;
ExitCondition = CI;
// Exit condition's one operand is loop invariant exit value and second
// operand is SCEVAddRecExpr based on induction variable.
Value *V0 = CI->getOperand(0);
Value *V1 = CI->getOperand(1);
SCEVHandle SH0 = SE->getSCEV(V0);
SCEVHandle SH1 = SE->getSCEV(V1);
if (SH0->isLoopInvariant(L) && isa<SCEVAddRecExpr>(SH1)) {
ExitValueNum = 0;
findIndVar(V1, L);
}
else if (SH1->isLoopInvariant(L) && isa<SCEVAddRecExpr>(SH0)) {
ExitValueNum = 1;
findIndVar(V0, L);
}
if (!IndVar)
ExitCondition = NULL;
else if (IndVar) {
BasicBlock *Preheader = L->getLoopPreheader();
StartValue = IndVar->getIncomingValueForBlock(Preheader);
}
}
/// Find condition inside a loop that is suitable candidate for index split.
void LoopIndexSplit::findSplitCondition() {
SplitInfo SD;
// Check all basic block's terminators.
for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
I != E; ++I) {
SD.clear();
BasicBlock *BB = *I;
// If this basic block does not terminate in a conditional branch
// then terminator is not a suitable split condition.
BranchInst *BR = dyn_cast<BranchInst>(BB->getTerminator());
if (!BR)
continue;
if (BR->isUnconditional())
continue;
if (Instruction *AndI = dyn_cast<Instruction>(BR->getCondition())) {
if (AndI->getOpcode() == Instruction::And) {
ICmpInst *Op0 = dyn_cast<ICmpInst>(AndI->getOperand(0));
ICmpInst *Op1 = dyn_cast<ICmpInst>(AndI->getOperand(1));
if (!Op0 || !Op1)
continue;
if (!safeICmpInst(Op0, SD))
continue;
SD.clear();
if (!safeICmpInst(Op1, SD))
continue;
SD.clear();
SD.SplitCondition = AndI;
SplitData.push_back(SD);
continue;
}
}
ICmpInst *CI = dyn_cast<ICmpInst>(BR->getCondition());
if (!CI || CI == ExitCondition)
continue;
if (CI->getPredicate() == ICmpInst::ICMP_NE)
continue;
// If split condition predicate is GT or GE then first execute
// false branch of split condition.
if (CI->getPredicate() == ICmpInst::ICMP_UGT
|| CI->getPredicate() == ICmpInst::ICMP_SGT
|| CI->getPredicate() == ICmpInst::ICMP_UGE
|| CI->getPredicate() == ICmpInst::ICMP_SGE)
SD.UseTrueBranchFirst = false;
// If one operand is loop invariant and second operand is SCEVAddRecExpr
// based on induction variable then CI is a candidate split condition.
if (safeICmpInst(CI, SD))
SplitData.push_back(SD);
}
}
// safeIcmpInst - CI is considered safe instruction if one of the operand
// is SCEVAddRecExpr based on induction variable and other operand is
// loop invariant. If CI is safe then populate SplitInfo object SD appropriately
// and return true;
bool LoopIndexSplit::safeICmpInst(ICmpInst *CI, SplitInfo &SD) {
Value *V0 = CI->getOperand(0);
Value *V1 = CI->getOperand(1);
SCEVHandle SH0 = SE->getSCEV(V0);
SCEVHandle SH1 = SE->getSCEV(V1);
if (SH0->isLoopInvariant(L) && isa<SCEVAddRecExpr>(SH1)) {
SD.SplitValue = V0;
SD.SplitCondition = CI;
if (PHINode *PN = dyn_cast<PHINode>(V1)) {
if (PN == IndVar)
return true;
}
else if (Instruction *Insn = dyn_cast<Instruction>(V1)) {
if (IndVarIncrement && IndVarIncrement == Insn)
return true;
}
}
else if (SH1->isLoopInvariant(L) && isa<SCEVAddRecExpr>(SH0)) {
SD.SplitValue = V1;
SD.SplitCondition = CI;
if (PHINode *PN = dyn_cast<PHINode>(V0)) {
if (PN == IndVar)
return true;
}
else if (Instruction *Insn = dyn_cast<Instruction>(V0)) {
if (IndVarIncrement && IndVarIncrement == Insn)
return true;
}
}
return false;
}
/// processOneIterationLoop - Current loop L contains compare instruction
/// that compares induction variable, IndVar, against loop invariant. If
/// entire (i.e. meaningful) loop body is dominated by this compare
/// instruction then loop body is executed only once. In such case eliminate
/// loop structure surrounding this loop body. For example,
/// for (int i = start; i < end; ++i) {
/// if ( i == somevalue) {
/// loop_body
/// }
/// }
/// can be transformed into
/// if (somevalue >= start && somevalue < end) {
/// i = somevalue;
/// loop_body
/// }
bool LoopIndexSplit::processOneIterationLoop(SplitInfo &SD) {
BasicBlock *Header = L->getHeader();
// First of all, check if SplitCondition dominates entire loop body
// or not.
// If SplitCondition is not in loop header then this loop is not suitable
// for this transformation.
if (SD.SplitCondition->getParent() != Header)
return false;
// If loop header includes loop variant instruction operands then
// this loop may not be eliminated.
if (!safeHeader(SD, Header))
return false;
// If Exiting block includes loop variant instructions then this
// loop may not be eliminated.
if (!safeExitingBlock(SD, ExitCondition->getParent()))
return false;
// Filter loops where split condition's false branch is not empty.
if (ExitCondition->getParent() != Header->getTerminator()->getSuccessor(1))
return false;
// If split condition is not safe then do not process this loop.
// For example,
// for(int i = 0; i < N; i++) {
// if ( i == XYZ) {
// A;
// else
// B;
// }
// C;
// D;
// }
if (!safeSplitCondition(SD))
return false;
BasicBlock *Latch = L->getLoopLatch();
BranchInst *BR = dyn_cast<BranchInst>(Latch->getTerminator());
if (!BR)
return false;
// Update CFG.
// Replace index variable with split value in loop body. Loop body is executed
// only when index variable is equal to split value.
IndVar->replaceAllUsesWith(SD.SplitValue);
// Remove Latch to Header edge.
BasicBlock *LatchSucc = NULL;
Header->removePredecessor(Latch);
for (succ_iterator SI = succ_begin(Latch), E = succ_end(Latch);
SI != E; ++SI) {
if (Header != *SI)
LatchSucc = *SI;
}
BR->setUnconditionalDest(LatchSucc);
Instruction *Terminator = Header->getTerminator();
Value *ExitValue = ExitCondition->getOperand(ExitValueNum);
// Replace split condition in header.
// Transform
// SplitCondition : icmp eq i32 IndVar, SplitValue
// into
// c1 = icmp uge i32 SplitValue, StartValue
// c2 = icmp ult i32 SplitValue, ExitValue
// and i32 c1, c2
bool SignedPredicate = ExitCondition->isSignedPredicate();
Instruction *C1 = new ICmpInst(SignedPredicate ?
ICmpInst::ICMP_SGE : ICmpInst::ICMP_UGE,
SD.SplitValue, StartValue, "lisplit",
Terminator);
Instruction *C2 = new ICmpInst(SignedPredicate ?
ICmpInst::ICMP_SLT : ICmpInst::ICMP_ULT,
SD.SplitValue, ExitValue, "lisplit",
Terminator);
Instruction *NSplitCond = BinaryOperator::createAnd(C1, C2, "lisplit",
Terminator);
SD.SplitCondition->replaceAllUsesWith(NSplitCond);
SD.SplitCondition->eraseFromParent();
// Now, clear latch block. Remove instructions that are responsible
// to increment induction variable.
Instruction *LTerminator = Latch->getTerminator();
for (BasicBlock::iterator LB = Latch->begin(), LE = Latch->end();
LB != LE; ) {
Instruction *I = LB;
++LB;
if (isa<PHINode>(I) || I == LTerminator)
continue;
if (I == IndVarIncrement)
I->replaceAllUsesWith(ExitValue);
else
I->replaceAllUsesWith(UndefValue::get(I->getType()));
I->eraseFromParent();
}
LPM->deleteLoopFromQueue(L);
// Update Dominator Info.
// Only CFG change done is to remove Latch to Header edge. This
// does not change dominator tree because Latch did not dominate
// Header.
if (DF) {
DominanceFrontier::iterator HeaderDF = DF->find(Header);
if (HeaderDF != DF->end())
DF->removeFromFrontier(HeaderDF, Header);
DominanceFrontier::iterator LatchDF = DF->find(Latch);
if (LatchDF != DF->end())
DF->removeFromFrontier(LatchDF, Header);
}
return true;
}
// If loop header includes loop variant instruction operands then
// this loop can not be eliminated. This is used by processOneIterationLoop().
bool LoopIndexSplit::safeHeader(SplitInfo &SD, BasicBlock *Header) {
Instruction *Terminator = Header->getTerminator();
for(BasicBlock::iterator BI = Header->begin(), BE = Header->end();
BI != BE; ++BI) {
Instruction *I = BI;
// PHI Nodes are OK.
if (isa<PHINode>(I))
continue;
// SplitCondition itself is OK.
if (I == SD.SplitCondition)
continue;
// Induction variable is OK.
if (I == IndVar)
continue;
// Induction variable increment is OK.
if (I == IndVarIncrement)
continue;
// Terminator is also harmless.
if (I == Terminator)
continue;
// Otherwise we have a instruction that may not be safe.
return false;
}
return true;
}
// If Exiting block includes loop variant instructions then this
// loop may not be eliminated. This is used by processOneIterationLoop().
bool LoopIndexSplit::safeExitingBlock(SplitInfo &SD,
BasicBlock *ExitingBlock) {
for (BasicBlock::iterator BI = ExitingBlock->begin(),
BE = ExitingBlock->end(); BI != BE; ++BI) {
Instruction *I = BI;
// PHI Nodes are OK.
if (isa<PHINode>(I))
continue;
// Induction variable increment is OK.
if (IndVarIncrement && IndVarIncrement == I)
continue;
// Check if I is induction variable increment instruction.
if (!IndVarIncrement && I->getOpcode() == Instruction::Add) {
Value *Op0 = I->getOperand(0);
Value *Op1 = I->getOperand(1);
PHINode *PN = NULL;
ConstantInt *CI = NULL;
if ((PN = dyn_cast<PHINode>(Op0))) {
if ((CI = dyn_cast<ConstantInt>(Op1)))
IndVarIncrement = I;
} else
if ((PN = dyn_cast<PHINode>(Op1))) {
if ((CI = dyn_cast<ConstantInt>(Op0)))
IndVarIncrement = I;
}
if (IndVarIncrement && PN == IndVar && CI->isOne())
continue;
}
// I is an Exit condition if next instruction is block terminator.
// Exit condition is OK if it compares loop invariant exit value,
// which is checked below.
else if (ICmpInst *EC = dyn_cast<ICmpInst>(I)) {
if (EC == ExitCondition)
continue;
}
if (I == ExitingBlock->getTerminator())
continue;
// Otherwise we have instruction that may not be safe.
return false;
}
// We could not find any reason to consider ExitingBlock unsafe.
return true;
}
void LoopIndexSplit::updateLoopBounds(ICmpInst *CI) {
Value *V0 = CI->getOperand(0);
Value *V1 = CI->getOperand(1);
Value *NV = NULL;
SCEVHandle SH0 = SE->getSCEV(V0);
if (SH0->isLoopInvariant(L))
NV = V0;
else
NV = V1;
switch (CI->getPredicate()) {
case ICmpInst::ICMP_ULE:
case ICmpInst::ICMP_SLE:
// for (i = LB; i < UB; ++i)
// if (i <= NV && ...)
// LOOP_BODY
//
// is transformed into
// NUB = min (NV+1, UB)
// for (i = LB; i < NUB ; ++i)
// LOOP_BODY
//
// for (i = LB; i <= UB; ++i)
// if (i <= NV && ...)
// LOOP_BODY
//
// is transformed into
// NUB = min (NV, UB)
// for (i = LB; i <= NUB ; ++i)
// LOOP_BODY
//
break;
case ICmpInst::ICMP_ULT:
case ICmpInst::ICMP_SLT:
// for (i = LB; i < UB; ++i)
// if (i < NV && ...)
// LOOP_BODY
//
// is transformed into
// NUB = min (NV, UB)
// for (i = LB; i < NUB ; ++i)
// LOOP_BODY
//
// for (i = LB; i <= UB; ++i)
// if (i < NV && ...)
// LOOP_BODY
//
// is transformed into
// NUB = min (NV -1 , UB)
// for (i = LB; i <= NUB ; ++i)
// LOOP_BODY
//
break;
case ICmpInst::ICMP_UGE:
case ICmpInst::ICMP_SGE:
// for (i = LB; i (< or <=) UB; ++i)
// if (i >= NV && ...)
// LOOP_BODY
//
// is transformed into
// NLB = max (NV, LB)
// for (i = NLB; i (< or <=) UB ; ++i)
// LOOP_BODY
//
break;
case ICmpInst::ICMP_UGT:
case ICmpInst::ICMP_SGT:
// for (i = LB; i (< or <=) UB; ++i)
// if (i > NV && ...)
// LOOP_BODY
//
// is transformed into
// NLB = max (NV+1, LB)
// for (i = NLB; i (< or <=) UB ; ++i)
// LOOP_BODY
//
break;
default:
assert ( 0 && "Unexpected split condition predicate");
}
}
/// updateLoopIterationSpace - Current loop body is covered by an AND
/// instruction whose operands compares induction variables with loop
/// invariants. If possible, hoist this check outside the loop by
/// updating appropriate start and end values for induction variable.
bool LoopIndexSplit::updateLoopIterationSpace(SplitInfo &SD) {
BasicBlock *Header = L->getHeader();
ICmpInst *Op0 = cast<ICmpInst>(SD.SplitCondition->getOperand(0));
ICmpInst *Op1 = cast<ICmpInst>(SD.SplitCondition->getOperand(1));
if (Op0->getPredicate() == ICmpInst::ICMP_EQ
|| Op0->getPredicate() == ICmpInst::ICMP_NE
|| Op0->getPredicate() == ICmpInst::ICMP_EQ
|| Op0->getPredicate() == ICmpInst::ICMP_NE)
return false;
// Check if SplitCondition dominates entire loop body
// or not.
// If SplitCondition is not in loop header then this loop is not suitable
// for this transformation.
if (SD.SplitCondition->getParent() != Header)
return false;
// If loop header includes loop variant instruction operands then
// this loop may not be eliminated.
Instruction *Terminator = Header->getTerminator();
for(BasicBlock::iterator BI = Header->begin(), BE = Header->end();
BI != BE; ++BI) {
Instruction *I = BI;
// PHI Nodes are OK.
if (isa<PHINode>(I))
continue;
// SplitCondition itself is OK.
if (I == SD.SplitCondition)
continue;
if (I == Op0 || I == Op1)
continue;
// Induction variable is OK.
if (I == IndVar)
continue;
// Induction variable increment is OK.
if (I == IndVarIncrement)
continue;
// Terminator is also harmless.
if (I == Terminator)
continue;
// Otherwise we have a instruction that may not be safe.
return false;
}
// If Exiting block includes loop variant instructions then this
// loop may not be eliminated.
if (!safeExitingBlock(SD, ExitCondition->getParent()))
return false;
updateLoopBounds(Op0);
updateLoopBounds(Op1);
// Update CFG
return false;
}
/// removeBlocks - Remove basic block DeadBB and all blocks dominated by DeadBB.
/// This routine is used to remove split condition's dead branch, dominated by
/// DeadBB. LiveBB dominates split conidition's other branch.
void LoopIndexSplit::removeBlocks(BasicBlock *DeadBB, Loop *LP,
BasicBlock *LiveBB) {
// First update DeadBB's dominance frontier.
SmallVector<BasicBlock *, 8> FrontierBBs;
DominanceFrontier::iterator DeadBBDF = DF->find(DeadBB);
if (DeadBBDF != DF->end()) {
SmallVector<BasicBlock *, 8> PredBlocks;
DominanceFrontier::DomSetType DeadBBSet = DeadBBDF->second;
for (DominanceFrontier::DomSetType::iterator DeadBBSetI = DeadBBSet.begin(),
DeadBBSetE = DeadBBSet.end(); DeadBBSetI != DeadBBSetE; ++DeadBBSetI) {
BasicBlock *FrontierBB = *DeadBBSetI;
FrontierBBs.push_back(FrontierBB);
// Rremove any PHI incoming edge from blocks dominated by DeadBB.
PredBlocks.clear();
for(pred_iterator PI = pred_begin(FrontierBB), PE = pred_end(FrontierBB);
PI != PE; ++PI) {
BasicBlock *P = *PI;
if (P == DeadBB || DT->dominates(DeadBB, P))
PredBlocks.push_back(P);
}
for(BasicBlock::iterator FBI = FrontierBB->begin(), FBE = FrontierBB->end();
FBI != FBE; ++FBI) {
if (PHINode *PN = dyn_cast<PHINode>(FBI)) {
for(SmallVector<BasicBlock *, 8>::iterator PI = PredBlocks.begin(),
PE = PredBlocks.end(); PI != PE; ++PI) {
BasicBlock *P = *PI;
PN->removeIncomingValue(P);
}
}
else
break;
}
}
}
// Now remove DeadBB and all nodes dominated by DeadBB in df order.
SmallVector<BasicBlock *, 32> WorkList;
DomTreeNode *DN = DT->getNode(DeadBB);
for (df_iterator<DomTreeNode*> DI = df_begin(DN),
E = df_end(DN); DI != E; ++DI) {
BasicBlock *BB = DI->getBlock();
WorkList.push_back(BB);
BB->replaceAllUsesWith(UndefValue::get(Type::LabelTy));
}
while (!WorkList.empty()) {
BasicBlock *BB = WorkList.back(); WorkList.pop_back();
for(BasicBlock::iterator BBI = BB->begin(), BBE = BB->end();
BBI != BBE; ) {
Instruction *I = BBI;
++BBI;
I->replaceAllUsesWith(UndefValue::get(I->getType()));
I->eraseFromParent();
}
LPM->deleteSimpleAnalysisValue(BB, LP);
DT->eraseNode(BB);
DF->removeBlock(BB);
LI->removeBlock(BB);
BB->eraseFromParent();
}
// Update Frontier BBs' dominator info.
while (!FrontierBBs.empty()) {
BasicBlock *FBB = FrontierBBs.back(); FrontierBBs.pop_back();
BasicBlock *NewDominator = FBB->getSinglePredecessor();
if (!NewDominator) {
pred_iterator PI = pred_begin(FBB), PE = pred_end(FBB);
NewDominator = *PI;
++PI;
if (NewDominator != LiveBB) {
for(; PI != PE; ++PI) {
BasicBlock *P = *PI;
if (P == LiveBB) {
NewDominator = LiveBB;
break;
}
NewDominator = DT->findNearestCommonDominator(NewDominator, P);
}
}
}
assert (NewDominator && "Unable to fix dominator info.");
DT->changeImmediateDominator(FBB, NewDominator);
DF->changeImmediateDominator(FBB, NewDominator, DT);
}
}
/// safeSplitCondition - Return true if it is possible to
/// split loop using given split condition.
bool LoopIndexSplit::safeSplitCondition(SplitInfo &SD) {
BasicBlock *SplitCondBlock = SD.SplitCondition->getParent();
BasicBlock *Latch = L->getLoopLatch();
BranchInst *SplitTerminator =
cast<BranchInst>(SplitCondBlock->getTerminator());
BasicBlock *Succ0 = SplitTerminator->getSuccessor(0);
BasicBlock *Succ1 = SplitTerminator->getSuccessor(1);
// Finally this split condition is safe only if merge point for
// split condition branch is loop latch. This check along with previous
// check, to ensure that exit condition is in either loop latch or header,
// filters all loops with non-empty loop body between merge point
// and exit condition.
DominanceFrontier::iterator Succ0DF = DF->find(Succ0);
assert (Succ0DF != DF->end() && "Unable to find Succ0 dominance frontier");
if (Succ0DF->second.count(Latch))
return true;
DominanceFrontier::iterator Succ1DF = DF->find(Succ1);
assert (Succ1DF != DF->end() && "Unable to find Succ1 dominance frontier");
if (Succ1DF->second.count(Latch))
return true;
return false;
}
/// calculateLoopBounds - ALoop exit value and BLoop start values are calculated
/// based on split value.
void LoopIndexSplit::calculateLoopBounds(SplitInfo &SD) {
ICmpInst *SC = cast<ICmpInst>(SD.SplitCondition);
ICmpInst::Predicate SP = SC->getPredicate();
const Type *Ty = SD.SplitValue->getType();
bool Sign = ExitCondition->isSignedPredicate();
BasicBlock *Preheader = L->getLoopPreheader();
Instruction *PHTerminator = Preheader->getTerminator();
// Initially use split value as upper loop bound for first loop and lower loop
// bound for second loop.
Value *AEV = SD.SplitValue;
Value *BSV = SD.SplitValue;
if (ExitCondition->getPredicate() == ICmpInst::ICMP_SGT
|| ExitCondition->getPredicate() == ICmpInst::ICMP_UGT
|| ExitCondition->getPredicate() == ICmpInst::ICMP_SGE
|| ExitCondition->getPredicate() == ICmpInst::ICMP_UGE)
ExitCondition->swapOperands();
switch (ExitCondition->getPredicate()) {
case ICmpInst::ICMP_SGT:
case ICmpInst::ICMP_UGT:
case ICmpInst::ICMP_SGE:
case ICmpInst::ICMP_UGE:
default:
assert (0 && "Unexpected exit condition predicate");
case ICmpInst::ICMP_SLT:
case ICmpInst::ICMP_ULT:
{
switch (SP) {
case ICmpInst::ICMP_SLT:
case ICmpInst::ICMP_ULT:
//
// for (i = LB; i < UB; ++i) { if (i < SV) A; else B; }
//
// is transformed into
// AEV = BSV = SV
// for (i = LB; i < min(UB, AEV); ++i)
// A;
// for (i = max(LB, BSV); i < UB; ++i);
// B;
break;
case ICmpInst::ICMP_SLE:
case ICmpInst::ICMP_ULE:
{
//
// for (i = LB; i < UB; ++i) { if (i <= SV) A; else B; }
//
// is transformed into
//
// AEV = SV + 1
// BSV = SV + 1
// for (i = LB; i < min(UB, AEV); ++i)
// A;
// for (i = max(LB, BSV); i < UB; ++i)
// B;
BSV = BinaryOperator::createAdd(SD.SplitValue,
ConstantInt::get(Ty, 1, Sign),
"lsplit.add", PHTerminator);
AEV = BSV;
}
break;
case ICmpInst::ICMP_SGE:
case ICmpInst::ICMP_UGE:
//
// for (i = LB; i < UB; ++i) { if (i >= SV) A; else B; }
//
// is transformed into
// AEV = BSV = SV
// for (i = LB; i < min(UB, AEV); ++i)
// B;
// for (i = max(BSV, LB); i < UB; ++i)
// A;
break;
case ICmpInst::ICMP_SGT:
case ICmpInst::ICMP_UGT:
{
//
// for (i = LB; i < UB; ++i) { if (i > SV) A; else B; }
//
// is transformed into
//
// BSV = AEV = SV + 1
// for (i = LB; i < min(UB, AEV); ++i)
// B;
// for (i = max(LB, BSV); i < UB; ++i)
// A;
BSV = BinaryOperator::createAdd(SD.SplitValue,
ConstantInt::get(Ty, 1, Sign),
"lsplit.add", PHTerminator);
AEV = BSV;
}
break;
default:
assert (0 && "Unexpected split condition predicate");
break;
} // end switch (SP)
}
break;
case ICmpInst::ICMP_SLE:
case ICmpInst::ICMP_ULE:
{
switch (SP) {
case ICmpInst::ICMP_SLT:
case ICmpInst::ICMP_ULT:
//
// for (i = LB; i <= UB; ++i) { if (i < SV) A; else B; }
//
// is transformed into
// AEV = SV - 1;
// BSV = SV;
// for (i = LB; i <= min(UB, AEV); ++i)
// A;
// for (i = max(LB, BSV); i <= UB; ++i)
// B;
AEV = BinaryOperator::createSub(SD.SplitValue,
ConstantInt::get(Ty, 1, Sign),
"lsplit.sub", PHTerminator);
break;
case ICmpInst::ICMP_SLE:
case ICmpInst::ICMP_ULE:
//
// for (i = LB; i <= UB; ++i) { if (i <= SV) A; else B; }
//
// is transformed into
// AEV = SV;
// BSV = SV + 1;
// for (i = LB; i <= min(UB, AEV); ++i)
// A;
// for (i = max(LB, BSV); i <= UB; ++i)
// B;
BSV = BinaryOperator::createAdd(SD.SplitValue,
ConstantInt::get(Ty, 1, Sign),
"lsplit.add", PHTerminator);
break;
case ICmpInst::ICMP_SGT:
case ICmpInst::ICMP_UGT:
//
// for (i = LB; i <= UB; ++i) { if (i > SV) A; else B; }
//
// is transformed into
// AEV = SV;
// BSV = SV + 1;
// for (i = LB; i <= min(AEV, UB); ++i)
// B;
// for (i = max(LB, BSV); i <= UB; ++i)
// A;
BSV = BinaryOperator::createAdd(SD.SplitValue,
ConstantInt::get(Ty, 1, Sign),
"lsplit.add", PHTerminator);
break;
case ICmpInst::ICMP_SGE:
case ICmpInst::ICMP_UGE:
// ** TODO **
//
// for (i = LB; i <= UB; ++i) { if (i >= SV) A; else B; }
//
// is transformed into
// AEV = SV - 1;
// BSV = SV;
// for (i = LB; i <= min(AEV, UB); ++i)
// B;
// for (i = max(LB, BSV); i <= UB; ++i)
// A;
AEV = BinaryOperator::createSub(SD.SplitValue,
ConstantInt::get(Ty, 1, Sign),
"lsplit.sub", PHTerminator);
break;
default:
assert (0 && "Unexpected split condition predicate");
break;
} // end switch (SP)
}
break;
}
// Calculate ALoop induction variable's new exiting value and
// BLoop induction variable's new starting value. Calculuate these
// values in original loop's preheader.
// A_ExitValue = min(SplitValue, OrignalLoopExitValue)
// B_StartValue = max(SplitValue, OriginalLoopStartValue)
Value *C1 = new ICmpInst(Sign ?
ICmpInst::ICMP_SLT : ICmpInst::ICMP_ULT,
AEV,
ExitCondition->getOperand(ExitValueNum),
"lsplit.ev", PHTerminator);
SD.A_ExitValue = new SelectInst(C1, AEV,
ExitCondition->getOperand(ExitValueNum),
"lsplit.ev", PHTerminator);
Value *C2 = new ICmpInst(Sign ?
ICmpInst::ICMP_SLT : ICmpInst::ICMP_ULT,
BSV, StartValue, "lsplit.sv",
PHTerminator);
SD.B_StartValue = new SelectInst(C2, StartValue, BSV,
"lsplit.sv", PHTerminator);
}
/// splitLoop - Split current loop L in two loops using split information
/// SD. Update dominator information. Maintain LCSSA form.
bool LoopIndexSplit::splitLoop(SplitInfo &SD) {
if (!safeSplitCondition(SD))
return false;
BasicBlock *SplitCondBlock = SD.SplitCondition->getParent();
// Unable to handle triange loops at the moment.
// In triangle loop, split condition is in header and one of the
// the split destination is loop latch. If split condition is EQ
// then such loops are already handle in processOneIterationLoop().
BasicBlock *Latch = L->getLoopLatch();
BranchInst *SplitTerminator =
cast<BranchInst>(SplitCondBlock->getTerminator());
BasicBlock *Succ0 = SplitTerminator->getSuccessor(0);
BasicBlock *Succ1 = SplitTerminator->getSuccessor(1);
if (L->getHeader() == SplitCondBlock
&& (Latch == Succ0 || Latch == Succ1))
return false;
// If split condition branches heads do not have single predecessor,
// SplitCondBlock, then is not possible to remove inactive branch.
if (!Succ0->getSinglePredecessor() || !Succ1->getSinglePredecessor())
return false;
// After loop is cloned there are two loops.
//
// First loop, referred as ALoop, executes first part of loop's iteration
// space split. Second loop, referred as BLoop, executes remaining
// part of loop's iteration space.
//
// ALoop's exit edge enters BLoop's header through a forwarding block which
// acts as a BLoop's preheader.
BasicBlock *Preheader = L->getLoopPreheader();
// Calculate ALoop induction variable's new exiting value and
// BLoop induction variable's new starting value.
calculateLoopBounds(SD);
//[*] Clone loop.
DenseMap<const Value *, Value *> ValueMap;
Loop *BLoop = CloneLoop(L, LPM, LI, ValueMap, this);
Loop *ALoop = L;
BasicBlock *B_Header = BLoop->getHeader();
//[*] ALoop's exiting edge BLoop's header.
// ALoop's original exit block becomes BLoop's exit block.
PHINode *B_IndVar = cast<PHINode>(ValueMap[IndVar]);
BasicBlock *A_ExitingBlock = ExitCondition->getParent();
BranchInst *A_ExitInsn =
dyn_cast<BranchInst>(A_ExitingBlock->getTerminator());
assert (A_ExitInsn && "Unable to find suitable loop exit branch");
BasicBlock *B_ExitBlock = A_ExitInsn->getSuccessor(1);
if (L->contains(B_ExitBlock)) {
B_ExitBlock = A_ExitInsn->getSuccessor(0);
A_ExitInsn->setSuccessor(0, B_Header);
} else
A_ExitInsn->setSuccessor(1, B_Header);
//[*] Update ALoop's exit value using new exit value.
ExitCondition->setOperand(ExitValueNum, SD.A_ExitValue);
// [*] Update BLoop's header phi nodes. Remove incoming PHINode's from
// original loop's preheader. Add incoming PHINode values from
// ALoop's exiting block. Update BLoop header's domiantor info.
// Collect inverse map of Header PHINodes.
DenseMap<Value *, Value *> InverseMap;
for (BasicBlock::iterator BI = L->getHeader()->begin(),
BE = L->getHeader()->end(); BI != BE; ++BI) {
if (PHINode *PN = dyn_cast<PHINode>(BI)) {
PHINode *PNClone = cast<PHINode>(ValueMap[PN]);
InverseMap[PNClone] = PN;
} else
break;
}
for (BasicBlock::iterator BI = B_Header->begin(), BE = B_Header->end();
BI != BE; ++BI) {
if (PHINode *PN = dyn_cast<PHINode>(BI)) {
// Remove incoming value from original preheader.
PN->removeIncomingValue(Preheader);
// Add incoming value from A_ExitingBlock.
if (PN == B_IndVar)
PN->addIncoming(SD.B_StartValue, A_ExitingBlock);
else {
PHINode *OrigPN = cast<PHINode>(InverseMap[PN]);
Value *V2 = OrigPN->getIncomingValueForBlock(A_ExitingBlock);
PN->addIncoming(V2, A_ExitingBlock);
}
} else
break;
}
DT->changeImmediateDominator(B_Header, A_ExitingBlock);
DF->changeImmediateDominator(B_Header, A_ExitingBlock, DT);
// [*] Update BLoop's exit block. Its new predecessor is BLoop's exit
// block. Remove incoming PHINode values from ALoop's exiting block.
// Add new incoming values from BLoop's incoming exiting value.
// Update BLoop exit block's dominator info..
BasicBlock *B_ExitingBlock = cast<BasicBlock>(ValueMap[A_ExitingBlock]);
for (BasicBlock::iterator BI = B_ExitBlock->begin(), BE = B_ExitBlock->end();
BI != BE; ++BI) {
if (PHINode *PN = dyn_cast<PHINode>(BI)) {
PN->addIncoming(ValueMap[PN->getIncomingValueForBlock(A_ExitingBlock)],
B_ExitingBlock);
PN->removeIncomingValue(A_ExitingBlock);
} else
break;
}
DT->changeImmediateDominator(B_ExitBlock, B_ExitingBlock);
DF->changeImmediateDominator(B_ExitBlock, B_ExitingBlock, DT);
//[*] Split ALoop's exit edge. This creates a new block which
// serves two purposes. First one is to hold PHINode defnitions
// to ensure that ALoop's LCSSA form. Second use it to act
// as a preheader for BLoop.
BasicBlock *A_ExitBlock = SplitEdge(A_ExitingBlock, B_Header, this);
//[*] Preserve ALoop's LCSSA form. Create new forwarding PHINodes
// in A_ExitBlock to redefine outgoing PHI definitions from ALoop.
for(BasicBlock::iterator BI = B_Header->begin(), BE = B_Header->end();
BI != BE; ++BI) {
if (PHINode *PN = dyn_cast<PHINode>(BI)) {
Value *V1 = PN->getIncomingValueForBlock(A_ExitBlock);
PHINode *newPHI = new PHINode(PN->getType(), PN->getName());
newPHI->addIncoming(V1, A_ExitingBlock);
A_ExitBlock->getInstList().push_front(newPHI);
PN->removeIncomingValue(A_ExitBlock);
PN->addIncoming(newPHI, A_ExitBlock);
} else
break;
}
//[*] Eliminate split condition's inactive branch from ALoop.
BasicBlock *A_SplitCondBlock = SD.SplitCondition->getParent();
BranchInst *A_BR = cast<BranchInst>(A_SplitCondBlock->getTerminator());
BasicBlock *A_InactiveBranch = NULL;
BasicBlock *A_ActiveBranch = NULL;
if (SD.UseTrueBranchFirst) {
A_ActiveBranch = A_BR->getSuccessor(0);
A_InactiveBranch = A_BR->getSuccessor(1);
} else {
A_ActiveBranch = A_BR->getSuccessor(1);
A_InactiveBranch = A_BR->getSuccessor(0);
}
A_BR->setUnconditionalDest(A_ActiveBranch);
removeBlocks(A_InactiveBranch, L, A_ActiveBranch);
//[*] Eliminate split condition's inactive branch in from BLoop.
BasicBlock *B_SplitCondBlock = cast<BasicBlock>(ValueMap[A_SplitCondBlock]);
BranchInst *B_BR = cast<BranchInst>(B_SplitCondBlock->getTerminator());
BasicBlock *B_InactiveBranch = NULL;
BasicBlock *B_ActiveBranch = NULL;
if (SD.UseTrueBranchFirst) {
B_ActiveBranch = B_BR->getSuccessor(1);
B_InactiveBranch = B_BR->getSuccessor(0);
} else {
B_ActiveBranch = B_BR->getSuccessor(0);
B_InactiveBranch = B_BR->getSuccessor(1);
}
B_BR->setUnconditionalDest(B_ActiveBranch);
removeBlocks(B_InactiveBranch, BLoop, B_ActiveBranch);
BasicBlock *A_Header = L->getHeader();
if (A_ExitingBlock == A_Header)
return true;
//[*] Move exit condition into split condition block to avoid
// executing dead loop iteration.
ICmpInst *B_ExitCondition = cast<ICmpInst>(ValueMap[ExitCondition]);
Instruction *B_IndVarIncrement = cast<Instruction>(ValueMap[IndVarIncrement]);
ICmpInst *B_SplitCondition = cast<ICmpInst>(ValueMap[SD.SplitCondition]);
moveExitCondition(A_SplitCondBlock, A_ActiveBranch, A_ExitBlock, ExitCondition,
cast<ICmpInst>(SD.SplitCondition), IndVar, IndVarIncrement,
ALoop);
moveExitCondition(B_SplitCondBlock, B_ActiveBranch, B_ExitBlock, B_ExitCondition,
B_SplitCondition, B_IndVar, B_IndVarIncrement, BLoop);
return true;
}
// moveExitCondition - Move exit condition EC into split condition block CondBB.
void LoopIndexSplit::moveExitCondition(BasicBlock *CondBB, BasicBlock *ActiveBB,
BasicBlock *ExitBB, ICmpInst *EC, ICmpInst *SC,
PHINode *IV, Instruction *IVAdd, Loop *LP) {
BasicBlock *ExitingBB = EC->getParent();
Instruction *CurrentBR = CondBB->getTerminator();
// Move exit condition into split condition block.
EC->moveBefore(CurrentBR);
EC->setOperand(ExitValueNum == 0 ? 1 : 0, IV);
// Move exiting block's branch into split condition block. Update its branch
// destination.
BranchInst *ExitingBR = cast<BranchInst>(ExitingBB->getTerminator());
ExitingBR->moveBefore(CurrentBR);
if (ExitingBR->getSuccessor(0) == ExitBB)
ExitingBR->setSuccessor(1, ActiveBB);
else
ExitingBR->setSuccessor(0, ActiveBB);
// Remove split condition and current split condition branch.
SC->eraseFromParent();
CurrentBR->eraseFromParent();
// Connect exiting block to split condition block.
new BranchInst(CondBB, ExitingBB);
// Update PHINodes
updatePHINodes(ExitBB, ExitingBB, CondBB, IV, IVAdd);
// Fix dominator info.
// ExitBB is now dominated by CondBB
DT->changeImmediateDominator(ExitBB, CondBB);
DF->changeImmediateDominator(ExitBB, CondBB, DT);
// Basicblocks dominated by ActiveBB may have ExitingBB or
// a basic block outside the loop in their DF list. If so,
// replace it with CondBB.
DomTreeNode *Node = DT->getNode(ActiveBB);
for (df_iterator<DomTreeNode *> DI = df_begin(Node), DE = df_end(Node);
DI != DE; ++DI) {
BasicBlock *BB = DI->getBlock();
DominanceFrontier::iterator BBDF = DF->find(BB);
DominanceFrontier::DomSetType::iterator DomSetI = BBDF->second.begin();
DominanceFrontier::DomSetType::iterator DomSetE = BBDF->second.end();
while (DomSetI != DomSetE) {
DominanceFrontier::DomSetType::iterator CurrentItr = DomSetI;
++DomSetI;
BasicBlock *DFBB = *CurrentItr;
if (DFBB == ExitingBB || !L->contains(DFBB)) {
BBDF->second.erase(DFBB);
BBDF->second.insert(CondBB);
}
}
}
}
/// updatePHINodes - CFG has been changed.
/// Before
/// - ExitBB's single predecessor was Latch
/// - Latch's second successor was Header
/// Now
/// - ExitBB's single predecessor was Header
/// - Latch's one and only successor was Header
///
/// Update ExitBB PHINodes' to reflect this change.
void LoopIndexSplit::updatePHINodes(BasicBlock *ExitBB, BasicBlock *Latch,
BasicBlock *Header,
PHINode *IV, Instruction *IVIncrement) {
for (BasicBlock::iterator BI = ExitBB->begin(), BE = ExitBB->end();
BI != BE; ++BI) {
PHINode *PN = dyn_cast<PHINode>(BI);
if (!PN)
break;
Value *V = PN->getIncomingValueForBlock(Latch);
if (PHINode *PHV = dyn_cast<PHINode>(V)) {
// PHV is in Latch. PHV has two uses, one use is in ExitBB PHINode
// (i.e. PN :)).
// The second use is in Header and it is new incoming value for PN.
PHINode *U1 = NULL;
PHINode *U2 = NULL;
Value *NewV = NULL;
for (Value::use_iterator UI = PHV->use_begin(), E = PHV->use_end();
UI != E; ++UI) {
if (!U1)
U1 = cast<PHINode>(*UI);
else if (!U2)
U2 = cast<PHINode>(*UI);
else
assert ( 0 && "Unexpected third use of this PHINode");
}
assert (U1 && U2 && "Unable to find two uses");
if (U1->getParent() == Header)
NewV = U1;
else
NewV = U2;
PN->addIncoming(NewV, Header);
} else if (Instruction *PHI = dyn_cast<Instruction>(V)) {
// If this instruction is IVIncrement then IV is new incoming value
// from header otherwise this instruction must be incoming value from
// header because loop is in LCSSA form.
if (PHI == IVIncrement)
PN->addIncoming(IV, Header);
else
PN->addIncoming(V, Header);
} else
// Otherwise this is an incoming value from header because loop is in
// LCSSA form.
PN->addIncoming(V, Header);
// Remove incoming value from Latch.
PN->removeIncomingValue(Latch);
}
}
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