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llvm-mirror/lib/Transforms/Scalar/LoopIndexSplit.cpp
Dan Gohman 0f8f7f179d Add Loop contains utility methods for testing whether a loop 
contains another loop, or an instruction. The loop form is
substantially more efficient on large loops than the typical
code it replaces.

llvm-svn: 91654
2009-12-18 01:24:09 +00:00

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//===- LoopIndexSplit.cpp - Loop Index Splitting Pass ---------------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements Loop Index Splitting Pass. This pass handles three
// kinds of loops.
//
// [1] A loop may be eliminated if the body is executed exactly once.
// For example,
//
// for (i = 0; i < N; ++i) {
// if (i == X) {
// body;
// }
// }
//
// is transformed to
//
// i = X;
// body;
//
// [2] A loop's iteration space may be shrunk if the loop body is executed
// for a proper sub-range of the loop's iteration space. For example,
//
// for (i = 0; i < N; ++i) {
// if (i > A && i < B) {
// ...
// }
// }
//
// is transformed to iterators from A to B, if A > 0 and B < N.
//
// [3] A loop may be split if the loop body is dominated by a branch.
// For example,
//
// 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;
//
//===----------------------------------------------------------------------===//
#define DEBUG_TYPE "loop-index-split"
#include "llvm/Transforms/Scalar.h"
#include "llvm/IntrinsicInst.h"
#include "llvm/LLVMContext.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/Analysis/Dominators.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/Cloning.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/ADT/DepthFirstIterator.h"
#include "llvm/ADT/Statistic.h"
using namespace llvm;
STATISTIC(NumIndexSplit, "Number of loop index split");
STATISTIC(NumIndexSplitRemoved, "Number of loops eliminated by loop index split");
STATISTIC(NumRestrictBounds, "Number of loop iteration space restricted");
namespace {
class LoopIndexSplit : public LoopPass {
public:
static char ID; // Pass ID, replacement for typeid
LoopIndexSplit() : LoopPass(&ID) {}
// Index split Loop L. Return true if loop is split.
bool runOnLoop(Loop *L, LPPassManager &LPM);
void getAnalysisUsage(AnalysisUsage &AU) const {
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:
/// processOneIterationLoop -- Eliminate loop if loop body is executed
/// only once. For example,
/// for (i = 0; i < N; ++i) {
/// if ( i == X) {
/// ...
/// }
/// }
///
bool processOneIterationLoop();
// -- Routines used by updateLoopIterationSpace();
/// updateLoopIterationSpace -- Update loop's iteration space if loop
/// body is executed for certain IV range only. For example,
///
/// for (i = 0; i < N; ++i) {
/// if ( i > A && i < B) {
/// ...
/// }
/// }
/// is transformed to iterators from A to B, if A > 0 and B < N.
///
bool updateLoopIterationSpace();
/// restrictLoopBound - Op dominates loop body. Op compares an IV based value
/// with a loop invariant value. Update loop's lower and upper bound based on
/// the loop invariant value.
bool restrictLoopBound(ICmpInst &Op);
// --- Routines used by splitLoop(). --- /
bool splitLoop();
/// 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);
/// moveExitCondition - Move exit condition EC into split condition block.
void moveExitCondition(BasicBlock *CondBB, BasicBlock *ActiveBB,
BasicBlock *ExitBB, ICmpInst *EC, ICmpInst *SC,
PHINode *IV, Instruction *IVAdd, Loop *LP,
unsigned);
/// 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, Loop *LP);
// --- Utility routines --- /
/// cleanBlock - A block is considered clean if all non terminal
/// instructions are either PHINodes or IV based values.
bool cleanBlock(BasicBlock *BB);
/// IVisLT - If Op is comparing IV based value with an loop invariant and
/// IV based value is less than the loop invariant then return the loop
/// invariant. Otherwise return NULL.
Value * IVisLT(ICmpInst &Op);
/// IVisLE - If Op is comparing IV based value with an loop invariant and
/// IV based value is less than or equal to the loop invariant then
/// return the loop invariant. Otherwise return NULL.
Value * IVisLE(ICmpInst &Op);
/// IVisGT - If Op is comparing IV based value with an loop invariant and
/// IV based value is greater than the loop invariant then return the loop
/// invariant. Otherwise return NULL.
Value * IVisGT(ICmpInst &Op);
/// IVisGE - If Op is comparing IV based value with an loop invariant and
/// IV based value is greater than or equal to the loop invariant then
/// return the loop invariant. Otherwise return NULL.
Value * IVisGE(ICmpInst &Op);
private:
// Current Loop information.
Loop *L;
LPPassManager *LPM;
LoopInfo *LI;
DominatorTree *DT;
DominanceFrontier *DF;
PHINode *IndVar;
ICmpInst *ExitCondition;
ICmpInst *SplitCondition;
Value *IVStartValue;
Value *IVExitValue;
Instruction *IVIncrement;
SmallPtrSet<Value *, 4> IVBasedValues;
};
}
char LoopIndexSplit::ID = 0;
static RegisterPass<LoopIndexSplit>
X("loop-index-split", "Index Split Loops");
Pass *llvm::createLoopIndexSplitPass() {
return new LoopIndexSplit();
}
// Index split Loop L. Return true if loop is split.
bool LoopIndexSplit::runOnLoop(Loop *IncomingLoop, LPPassManager &LPM_Ref) {
L = IncomingLoop;
LPM = &LPM_Ref;
// If LoopSimplify form is not available, stay out of trouble.
if (!L->isLoopSimplifyForm())
return false;
// FIXME - Nested loops make dominator info updates tricky.
if (!L->getSubLoops().empty())
return false;
DT = &getAnalysis<DominatorTree>();
LI = &getAnalysis<LoopInfo>();
DF = &getAnalysis<DominanceFrontier>();
// Initialize loop data.
IndVar = L->getCanonicalInductionVariable();
if (!IndVar) return false;
bool P1InLoop = L->contains(IndVar->getIncomingBlock(1));
IVStartValue = IndVar->getIncomingValue(!P1InLoop);
IVIncrement = dyn_cast<Instruction>(IndVar->getIncomingValue(P1InLoop));
if (!IVIncrement) return false;
IVBasedValues.clear();
IVBasedValues.insert(IndVar);
IVBasedValues.insert(IVIncrement);
for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
I != E; ++I)
for(BasicBlock::iterator BI = (*I)->begin(), BE = (*I)->end();
BI != BE; ++BI) {
if (BinaryOperator *BO = dyn_cast<BinaryOperator>(BI))
if (BO != IVIncrement
&& (BO->getOpcode() == Instruction::Add
|| BO->getOpcode() == Instruction::Sub))
if (IVBasedValues.count(BO->getOperand(0))
&& L->isLoopInvariant(BO->getOperand(1)))
IVBasedValues.insert(BO);
}
// Reject loop if loop exit condition is not suitable.
BasicBlock *ExitingBlock = L->getExitingBlock();
if (!ExitingBlock)
return false;
BranchInst *EBR = dyn_cast<BranchInst>(ExitingBlock->getTerminator());
if (!EBR) return false;
ExitCondition = dyn_cast<ICmpInst>(EBR->getCondition());
if (!ExitCondition) return false;
if (ExitingBlock != L->getLoopLatch()) return false;
IVExitValue = ExitCondition->getOperand(1);
if (!L->isLoopInvariant(IVExitValue))
IVExitValue = ExitCondition->getOperand(0);
if (!L->isLoopInvariant(IVExitValue))
return false;
if (!IVBasedValues.count(
ExitCondition->getOperand(IVExitValue == ExitCondition->getOperand(0))))
return false;
// If start value is more then exit value where induction variable
// increments by 1 then we are potentially dealing with an infinite loop.
// Do not index split this loop.
if (ConstantInt *SV = dyn_cast<ConstantInt>(IVStartValue))
if (ConstantInt *EV = dyn_cast<ConstantInt>(IVExitValue))
if (SV->getSExtValue() > EV->getSExtValue())
return false;
if (processOneIterationLoop())
return true;
if (updateLoopIterationSpace())
return true;
if (splitLoop())
return true;
return false;
}
// --- Helper routines ---
// isUsedOutsideLoop - Returns true iff V is used outside the loop L.
static bool isUsedOutsideLoop(Value *V, Loop *L) {
for(Value::use_iterator UI = V->use_begin(), E = V->use_end(); UI != E; ++UI)
if (!L->contains(cast<Instruction>(*UI)))
return true;
return false;
}
// Return V+1
static Value *getPlusOne(Value *V, bool Sign, Instruction *InsertPt,
LLVMContext &Context) {
Constant *One = ConstantInt::get(V->getType(), 1, Sign);
return BinaryOperator::CreateAdd(V, One, "lsp", InsertPt);
}
// Return V-1
static Value *getMinusOne(Value *V, bool Sign, Instruction *InsertPt,
LLVMContext &Context) {
Constant *One = ConstantInt::get(V->getType(), 1, Sign);
return BinaryOperator::CreateSub(V, One, "lsp", InsertPt);
}
// Return min(V1, V1)
static Value *getMin(Value *V1, Value *V2, bool Sign, Instruction *InsertPt) {
Value *C = new ICmpInst(InsertPt,
Sign ? ICmpInst::ICMP_SLT : ICmpInst::ICMP_ULT,
V1, V2, "lsp");
return SelectInst::Create(C, V1, V2, "lsp", InsertPt);
}
// Return max(V1, V2)
static Value *getMax(Value *V1, Value *V2, bool Sign, Instruction *InsertPt) {
Value *C = new ICmpInst(InsertPt,
Sign ? ICmpInst::ICMP_SLT : ICmpInst::ICMP_ULT,
V1, V2, "lsp");
return SelectInst::Create(C, V2, V1, "lsp", InsertPt);
}
/// processOneIterationLoop -- Eliminate loop if loop body is executed
/// only once. For example,
/// for (i = 0; i < N; ++i) {
/// if ( i == X) {
/// ...
/// }
/// }
///
bool LoopIndexSplit::processOneIterationLoop() {
SplitCondition = NULL;
BasicBlock *Latch = L->getLoopLatch();
BasicBlock *Header = L->getHeader();
BranchInst *BR = dyn_cast<BranchInst>(Header->getTerminator());
if (!BR) return false;
if (!isa<BranchInst>(Latch->getTerminator())) return false;
if (BR->isUnconditional()) return false;
SplitCondition = dyn_cast<ICmpInst>(BR->getCondition());
if (!SplitCondition) return false;
if (SplitCondition == ExitCondition) return false;
if (SplitCondition->getPredicate() != ICmpInst::ICMP_EQ) return false;
if (BR->getOperand(1) != Latch) return false;
if (!IVBasedValues.count(SplitCondition->getOperand(0))
&& !IVBasedValues.count(SplitCondition->getOperand(1)))
return false;
// If IV is used outside the loop then this loop traversal is required.
// FIXME: Calculate and use last IV value.
if (isUsedOutsideLoop(IVIncrement, L))
return false;
// If BR operands are not IV or not loop invariants then skip this loop.
Value *OPV = SplitCondition->getOperand(0);
Value *SplitValue = SplitCondition->getOperand(1);
if (!L->isLoopInvariant(SplitValue))
std::swap(OPV, SplitValue);
if (!L->isLoopInvariant(SplitValue))
return false;
Instruction *OPI = dyn_cast<Instruction>(OPV);
if (!OPI)
return false;
if (OPI->getParent() != Header || isUsedOutsideLoop(OPI, L))
return false;
Value *StartValue = IVStartValue;
Value *ExitValue = IVExitValue;;
if (OPV != IndVar) {
// If BR operand is IV based then use this operand to calculate
// effective conditions for loop body.
BinaryOperator *BOPV = dyn_cast<BinaryOperator>(OPV);
if (!BOPV)
return false;
if (BOPV->getOpcode() != Instruction::Add)
return false;
StartValue = BinaryOperator::CreateAdd(OPV, StartValue, "" , BR);
ExitValue = BinaryOperator::CreateAdd(OPV, ExitValue, "" , BR);
}
if (!cleanBlock(Header))
return false;
if (!cleanBlock(Latch))
return false;
// If the merge point for BR is not loop latch then skip this loop.
if (BR->getSuccessor(0) != Latch) {
DominanceFrontier::iterator DF0 = DF->find(BR->getSuccessor(0));
assert (DF0 != DF->end() && "Unable to find dominance frontier");
if (!DF0->second.count(Latch))
return false;
}
if (BR->getSuccessor(1) != Latch) {
DominanceFrontier::iterator DF1 = DF->find(BR->getSuccessor(1));
assert (DF1 != DF->end() && "Unable to find dominance frontier");
if (!DF1->second.count(Latch))
return false;
}
// Now, Current loop L contains compare instruction
// that compares induction variable, IndVar, against loop invariant. And
// entire (i.e. meaningful) loop body is dominated by this compare
// instruction. 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
// }
// Replace index variable with split value in loop body. Loop body is executed
// only when index variable is equal to split value.
IndVar->replaceAllUsesWith(SplitValue);
// 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
Instruction *C1 = new ICmpInst(BR, ExitCondition->isSigned() ?
ICmpInst::ICMP_SGE : ICmpInst::ICMP_UGE,
SplitValue, StartValue, "lisplit");
CmpInst::Predicate C2P = ExitCondition->getPredicate();
BranchInst *LatchBR = cast<BranchInst>(Latch->getTerminator());
if (LatchBR->getOperand(1) != Header)
C2P = CmpInst::getInversePredicate(C2P);
Instruction *C2 = new ICmpInst(BR, C2P, SplitValue, ExitValue, "lisplit");
Instruction *NSplitCond = BinaryOperator::CreateAnd(C1, C2, "lisplit", BR);
SplitCondition->replaceAllUsesWith(NSplitCond);
SplitCondition->eraseFromParent();
// 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;
}
// Clean up latch block.
Value *LatchBRCond = LatchBR->getCondition();
LatchBR->setUnconditionalDest(LatchSucc);
RecursivelyDeleteTriviallyDeadInstructions(LatchBRCond);
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);
}
++NumIndexSplitRemoved;
return true;
}
/// restrictLoopBound - Op dominates loop body. Op compares an IV based value
/// with a loop invariant value. Update loop's lower and upper bound based on
/// the loop invariant value.
bool LoopIndexSplit::restrictLoopBound(ICmpInst &Op) {
bool Sign = Op.isSigned();
Instruction *PHTerm = L->getLoopPreheader()->getTerminator();
if (IVisGT(*ExitCondition) || IVisGE(*ExitCondition)) {
BranchInst *EBR =
cast<BranchInst>(ExitCondition->getParent()->getTerminator());
ExitCondition->setPredicate(ExitCondition->getInversePredicate());
BasicBlock *T = EBR->getSuccessor(0);
EBR->setSuccessor(0, EBR->getSuccessor(1));
EBR->setSuccessor(1, T);
}
LLVMContext &Context = Op.getContext();
// New upper and lower bounds.
Value *NLB = NULL;
Value *NUB = NULL;
if (Value *V = IVisLT(Op)) {
// Restrict upper bound.
if (IVisLE(*ExitCondition))
V = getMinusOne(V, Sign, PHTerm, Context);
NUB = getMin(V, IVExitValue, Sign, PHTerm);
} else if (Value *V = IVisLE(Op)) {
// Restrict upper bound.
if (IVisLT(*ExitCondition))
V = getPlusOne(V, Sign, PHTerm, Context);
NUB = getMin(V, IVExitValue, Sign, PHTerm);
} else if (Value *V = IVisGT(Op)) {
// Restrict lower bound.
V = getPlusOne(V, Sign, PHTerm, Context);
NLB = getMax(V, IVStartValue, Sign, PHTerm);
} else if (Value *V = IVisGE(Op))
// Restrict lower bound.
NLB = getMax(V, IVStartValue, Sign, PHTerm);
if (!NLB && !NUB)
return false;
if (NLB) {
unsigned i = IndVar->getBasicBlockIndex(L->getLoopPreheader());
IndVar->setIncomingValue(i, NLB);
}
if (NUB) {
unsigned i = (ExitCondition->getOperand(0) != IVExitValue);
ExitCondition->setOperand(i, NUB);
}
return true;
}
/// updateLoopIterationSpace -- Update loop's iteration space if loop
/// body is executed for certain IV range only. For example,
///
/// for (i = 0; i < N; ++i) {
/// if ( i > A && i < B) {
/// ...
/// }
/// }
/// is transformed to iterators from A to B, if A > 0 and B < N.
///
bool LoopIndexSplit::updateLoopIterationSpace() {
SplitCondition = NULL;
if (ExitCondition->getPredicate() == ICmpInst::ICMP_NE
|| ExitCondition->getPredicate() == ICmpInst::ICMP_EQ)
return false;
BasicBlock *Latch = L->getLoopLatch();
BasicBlock *Header = L->getHeader();
BranchInst *BR = dyn_cast<BranchInst>(Header->getTerminator());
if (!BR) return false;
if (!isa<BranchInst>(Latch->getTerminator())) return false;
if (BR->isUnconditional()) return false;
BinaryOperator *AND = dyn_cast<BinaryOperator>(BR->getCondition());
if (!AND) return false;
if (AND->getOpcode() != Instruction::And) return false;
ICmpInst *Op0 = dyn_cast<ICmpInst>(AND->getOperand(0));
ICmpInst *Op1 = dyn_cast<ICmpInst>(AND->getOperand(1));
if (!Op0 || !Op1)
return false;
IVBasedValues.insert(AND);
IVBasedValues.insert(Op0);
IVBasedValues.insert(Op1);
if (!cleanBlock(Header)) return false;
BasicBlock *ExitingBlock = ExitCondition->getParent();
if (!cleanBlock(ExitingBlock)) return false;
// If the merge point for BR is not loop latch then skip this loop.
if (BR->getSuccessor(0) != Latch) {
DominanceFrontier::iterator DF0 = DF->find(BR->getSuccessor(0));
assert (DF0 != DF->end() && "Unable to find dominance frontier");
if (!DF0->second.count(Latch))
return false;
}
if (BR->getSuccessor(1) != Latch) {
DominanceFrontier::iterator DF1 = DF->find(BR->getSuccessor(1));
assert (DF1 != DF->end() && "Unable to find dominance frontier");
if (!DF1->second.count(Latch))
return false;
}
// Verify that loop exiting block has only two predecessor, where one pred
// is split condition block. The other predecessor will become exiting block's
// dominator after CFG is updated. TODO : Handle CFG's where exiting block has
// more then two predecessors. This requires extra work in updating dominator
// information.
BasicBlock *ExitingBBPred = NULL;
for (pred_iterator PI = pred_begin(ExitingBlock), PE = pred_end(ExitingBlock);
PI != PE; ++PI) {
BasicBlock *BB = *PI;
if (Header == BB)
continue;
if (ExitingBBPred)
return false;
else
ExitingBBPred = BB;
}
if (!restrictLoopBound(*Op0))
return false;
if (!restrictLoopBound(*Op1))
return false;
// Update CFG.
if (BR->getSuccessor(0) == ExitingBlock)
BR->setUnconditionalDest(BR->getSuccessor(1));
else
BR->setUnconditionalDest(BR->getSuccessor(0));
AND->eraseFromParent();
if (Op0->use_empty())
Op0->eraseFromParent();
if (Op1->use_empty())
Op1->eraseFromParent();
// Update domiantor info. Now, ExitingBlock has only one predecessor,
// ExitingBBPred, and it is ExitingBlock's immediate domiantor.
DT->changeImmediateDominator(ExitingBlock, ExitingBBPred);
BasicBlock *ExitBlock = ExitingBlock->getTerminator()->getSuccessor(1);
if (L->contains(ExitBlock))
ExitBlock = ExitingBlock->getTerminator()->getSuccessor(0);
// If ExitingBlock is a member of the loop basic blocks' DF list then
// replace ExitingBlock with header and exit block in the DF list
DominanceFrontier::iterator ExitingBlockDF = DF->find(ExitingBlock);
for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
I != E; ++I) {
BasicBlock *BB = *I;
if (BB == Header || BB == ExitingBlock)
continue;
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 == ExitingBlock) {
BBDF->second.erase(DFBB);
for (DominanceFrontier::DomSetType::iterator
EBI = ExitingBlockDF->second.begin(),
EBE = ExitingBlockDF->second.end(); EBI != EBE; ++EBI)
BBDF->second.insert(*EBI);
}
}
}
NumRestrictBounds++;
return true;
}
/// 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::getLabelTy(DeadBB->getContext())));
}
while (!WorkList.empty()) {
BasicBlock *BB = WorkList.back(); WorkList.pop_back();
LPM->deleteSimpleAnalysisValue(BB, LP);
for(BasicBlock::iterator BBI = BB->begin(), BBE = BB->end();
BBI != BBE; ) {
Instruction *I = BBI;
++BBI;
I->replaceAllUsesWith(UndefValue::get(I->getType()));
LPM->deleteSimpleAnalysisValue(I, LP);
I->eraseFromParent();
}
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);
}
}
// 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,
unsigned ExitValueNum) {
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);
BasicBlock *OrigDestBB = NULL;
if (ExitingBR->getSuccessor(0) == ExitBB) {
OrigDestBB = ExitingBR->getSuccessor(1);
ExitingBR->setSuccessor(1, ActiveBB);
}
else {
OrigDestBB = ExitingBR->getSuccessor(0);
ExitingBR->setSuccessor(0, ActiveBB);
}
// Remove split condition and current split condition branch.
SC->eraseFromParent();
CurrentBR->eraseFromParent();
// Connect exiting block to original destination.
BranchInst::Create(OrigDestBB, ExitingBB);
// Update PHINodes
updatePHINodes(ExitBB, ExitingBB, CondBB, IV, IVAdd, LP);
// Fix dominator info.
// ExitBB is now dominated by CondBB
DT->changeImmediateDominator(ExitBB, CondBB);
DF->changeImmediateDominator(ExitBB, CondBB, DT);
// Blocks outside the loop may have been in the dominance frontier of blocks
// inside the condition; this is now impossible because the blocks inside the
// condition no loger dominate the exit. Remove the relevant blocks from
// the dominance frontiers.
for (Loop::block_iterator I = LP->block_begin(), E = LP->block_end();
I != E; ++I) {
if (*I == CondBB || !DT->dominates(CondBB, *I)) continue;
DominanceFrontier::iterator BBDF = DF->find(*I);
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 (!LP->contains(DFBB))
BBDF->second.erase(DFBB);
}
}
}
/// updatePHINodes - CFG has been changed.
/// Before
/// - ExitBB's single predecessor was Latch
/// - Latch's second successor was Header
/// Now
/// - ExitBB's single predecessor is Header
/// - Latch's one and only successor is Header
///
/// Update ExitBB PHINodes' to reflect this change.
void LoopIndexSplit::updatePHINodes(BasicBlock *ExitBB, BasicBlock *Latch,
BasicBlock *Header,
PHINode *IV, Instruction *IVIncrement,
Loop *LP) {
for (BasicBlock::iterator BI = ExitBB->begin(), BE = ExitBB->end();
BI != BE; ) {
PHINode *PN = dyn_cast<PHINode>(BI);
++BI;
if (!PN)
break;
Value *V = PN->getIncomingValueForBlock(Latch);
if (PHINode *PHV = dyn_cast<PHINode>(V)) {
// PHV is in Latch. PHV has one use is in ExitBB PHINode. And one use
// in Header which is new incoming value for PN.
Value *NewV = NULL;
for (Value::use_iterator UI = PHV->use_begin(), E = PHV->use_end();
UI != E; ++UI)
if (PHINode *U = dyn_cast<PHINode>(*UI))
if (LP->contains(U)) {
NewV = U;
break;
}
// Add incoming value from header only if PN has any use inside the loop.
if (NewV)
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);
}
}
bool LoopIndexSplit::splitLoop() {
SplitCondition = NULL;
if (ExitCondition->getPredicate() == ICmpInst::ICMP_NE
|| ExitCondition->getPredicate() == ICmpInst::ICMP_EQ)
return false;
BasicBlock *Header = L->getHeader();
BasicBlock *Latch = L->getLoopLatch();
BranchInst *SBR = NULL; // Split Condition Branch
BranchInst *EBR = cast<BranchInst>(ExitCondition->getParent()->getTerminator());
// If Exiting block includes loop variant instructions then this
// loop may not be split safely.
BasicBlock *ExitingBlock = ExitCondition->getParent();
if (!cleanBlock(ExitingBlock)) return false;
LLVMContext &Context = Header->getContext();
for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
I != E; ++I) {
BranchInst *BR = dyn_cast<BranchInst>((*I)->getTerminator());
if (!BR || BR->isUnconditional()) continue;
ICmpInst *CI = dyn_cast<ICmpInst>(BR->getCondition());
if (!CI || CI == ExitCondition
|| CI->getPredicate() == ICmpInst::ICMP_NE
|| CI->getPredicate() == ICmpInst::ICMP_EQ)
continue;
// Unable to handle triangle 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().
if (Header == (*I)
&& (Latch == BR->getSuccessor(0) || Latch == BR->getSuccessor(1)))
continue;
// If the block does not dominate the latch then this is not a diamond.
// Such loop may not benefit from index split.
if (!DT->dominates((*I), Latch))
continue;
// If split condition branches heads do not have single predecessor,
// SplitCondBlock, then is not possible to remove inactive branch.
if (!BR->getSuccessor(0)->getSinglePredecessor()
|| !BR->getSuccessor(1)->getSinglePredecessor())
return false;
// If the merge point for BR is not loop latch then skip this condition.
if (BR->getSuccessor(0) != Latch) {
DominanceFrontier::iterator DF0 = DF->find(BR->getSuccessor(0));
assert (DF0 != DF->end() && "Unable to find dominance frontier");
if (!DF0->second.count(Latch))
continue;
}
if (BR->getSuccessor(1) != Latch) {
DominanceFrontier::iterator DF1 = DF->find(BR->getSuccessor(1));
assert (DF1 != DF->end() && "Unable to find dominance frontier");
if (!DF1->second.count(Latch))
continue;
}
SplitCondition = CI;
SBR = BR;
break;
}
if (!SplitCondition)
return false;
// If the predicate sign does not match then skip.
if (ExitCondition->isSigned() != SplitCondition->isSigned())
return false;
unsigned EVOpNum = (ExitCondition->getOperand(1) == IVExitValue);
unsigned SVOpNum = IVBasedValues.count(SplitCondition->getOperand(0));
Value *SplitValue = SplitCondition->getOperand(SVOpNum);
if (!L->isLoopInvariant(SplitValue))
return false;
if (!IVBasedValues.count(SplitCondition->getOperand(!SVOpNum)))
return false;
// Normalize loop conditions so that it is easier to calculate new loop
// bounds.
if (IVisGT(*ExitCondition) || IVisGE(*ExitCondition)) {
ExitCondition->setPredicate(ExitCondition->getInversePredicate());
BasicBlock *T = EBR->getSuccessor(0);
EBR->setSuccessor(0, EBR->getSuccessor(1));
EBR->setSuccessor(1, T);
}
if (IVisGT(*SplitCondition) || IVisGE(*SplitCondition)) {
SplitCondition->setPredicate(SplitCondition->getInversePredicate());
BasicBlock *T = SBR->getSuccessor(0);
SBR->setSuccessor(0, SBR->getSuccessor(1));
SBR->setSuccessor(1, T);
}
//[*] Calculate new loop bounds.
Value *AEV = SplitValue;
Value *BSV = SplitValue;
bool Sign = SplitCondition->isSigned();
Instruction *PHTerm = L->getLoopPreheader()->getTerminator();
if (IVisLT(*ExitCondition)) {
if (IVisLT(*SplitCondition)) {
/* Do nothing */
}
else if (IVisLE(*SplitCondition)) {
AEV = getPlusOne(SplitValue, Sign, PHTerm, Context);
BSV = getPlusOne(SplitValue, Sign, PHTerm, Context);
} else {
assert (0 && "Unexpected split condition!");
}
}
else if (IVisLE(*ExitCondition)) {
if (IVisLT(*SplitCondition)) {
AEV = getMinusOne(SplitValue, Sign, PHTerm, Context);
}
else if (IVisLE(*SplitCondition)) {
BSV = getPlusOne(SplitValue, Sign, PHTerm, Context);
} else {
assert (0 && "Unexpected split condition!");
}
} else {
assert (0 && "Unexpected exit condition!");
}
AEV = getMin(AEV, IVExitValue, Sign, PHTerm);
BSV = getMax(BSV, IVStartValue, Sign, PHTerm);
// [*] Clone Loop
DenseMap<const Value *, Value *> ValueMap;
Loop *BLoop = CloneLoop(L, LPM, LI, ValueMap, this);
Loop *ALoop = L;
// [*] ALoop's exiting edge enters 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);
BasicBlock *B_Header = BLoop->getHeader();
if (ALoop->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(EVOpNum, AEV);
// [*] 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 = ALoop->getHeader()->begin(),
BE = ALoop->getHeader()->end(); BI != BE; ++BI) {
if (PHINode *PN = dyn_cast<PHINode>(BI)) {
PHINode *PNClone = cast<PHINode>(ValueMap[PN]);
InverseMap[PNClone] = PN;
} else
break;
}
BasicBlock *A_Preheader = ALoop->getLoopPreheader();
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(A_Preheader);
// Add incoming value from A_ExitingBlock.
if (PN == B_IndVar)
PN->addIncoming(BSV, A_ExitingBlock);
else {
PHINode *OrigPN = cast<PHINode>(InverseMap[PN]);
Value *V2 = NULL;
// If loop header is also loop exiting block then
// OrigPN is incoming value for B loop header.
if (A_ExitingBlock == ALoop->getHeader())
V2 = OrigPN;
else
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 = PHINode::Create(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 = SplitCondition->getParent();
BranchInst *A_BR = cast<BranchInst>(A_SplitCondBlock->getTerminator());
BasicBlock *A_InactiveBranch = NULL;
BasicBlock *A_ActiveBranch = NULL;
A_ActiveBranch = A_BR->getSuccessor(0);
A_InactiveBranch = A_BR->getSuccessor(1);
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;
B_ActiveBranch = B_BR->getSuccessor(1);
B_InactiveBranch = B_BR->getSuccessor(0);
B_BR->setUnconditionalDest(B_ActiveBranch);
removeBlocks(B_InactiveBranch, BLoop, B_ActiveBranch);
BasicBlock *A_Header = ALoop->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[IVIncrement]);
ICmpInst *B_SplitCondition = cast<ICmpInst>(ValueMap[SplitCondition]);
moveExitCondition(A_SplitCondBlock, A_ActiveBranch, A_ExitBlock, ExitCondition,
cast<ICmpInst>(SplitCondition), IndVar, IVIncrement,
ALoop, EVOpNum);
moveExitCondition(B_SplitCondBlock, B_ActiveBranch,
B_ExitBlock, B_ExitCondition,
B_SplitCondition, B_IndVar, B_IndVarIncrement,
BLoop, EVOpNum);
NumIndexSplit++;
return true;
}
/// cleanBlock - A block is considered clean if all non terminal instructions
/// are either, PHINodes, IV based.
bool LoopIndexSplit::cleanBlock(BasicBlock *BB) {
Instruction *Terminator = BB->getTerminator();
for(BasicBlock::iterator BI = BB->begin(), BE = BB->end();
BI != BE; ++BI) {
Instruction *I = BI;
if (isa<PHINode>(I) || I == Terminator || I == ExitCondition
|| I == SplitCondition || IVBasedValues.count(I)
|| isa<DbgInfoIntrinsic>(I))
continue;
if (I->mayHaveSideEffects())
return false;
// I is used only inside this block then it is OK.
bool usedOutsideBB = false;
for (Value::use_iterator UI = I->use_begin(), UE = I->use_end();
UI != UE; ++UI) {
Instruction *U = cast<Instruction>(UI);
if (U->getParent() != BB)
usedOutsideBB = true;
}
if (!usedOutsideBB)
continue;
// Otherwise we have a instruction that may not allow loop spliting.
return false;
}
return true;
}
/// IVisLT - If Op is comparing IV based value with an loop invariant and
/// IV based value is less than the loop invariant then return the loop
/// invariant. Otherwise return NULL.
Value * LoopIndexSplit::IVisLT(ICmpInst &Op) {
ICmpInst::Predicate P = Op.getPredicate();
if ((P == ICmpInst::ICMP_SLT || P == ICmpInst::ICMP_ULT)
&& IVBasedValues.count(Op.getOperand(0))
&& L->isLoopInvariant(Op.getOperand(1)))
return Op.getOperand(1);
if ((P == ICmpInst::ICMP_SGT || P == ICmpInst::ICMP_UGT)
&& IVBasedValues.count(Op.getOperand(1))
&& L->isLoopInvariant(Op.getOperand(0)))
return Op.getOperand(0);
return NULL;
}
/// IVisLE - If Op is comparing IV based value with an loop invariant and
/// IV based value is less than or equal to the loop invariant then
/// return the loop invariant. Otherwise return NULL.
Value * LoopIndexSplit::IVisLE(ICmpInst &Op) {
ICmpInst::Predicate P = Op.getPredicate();
if ((P == ICmpInst::ICMP_SLE || P == ICmpInst::ICMP_ULE)
&& IVBasedValues.count(Op.getOperand(0))
&& L->isLoopInvariant(Op.getOperand(1)))
return Op.getOperand(1);
if ((P == ICmpInst::ICMP_SGE || P == ICmpInst::ICMP_UGE)
&& IVBasedValues.count(Op.getOperand(1))
&& L->isLoopInvariant(Op.getOperand(0)))
return Op.getOperand(0);
return NULL;
}
/// IVisGT - If Op is comparing IV based value with an loop invariant and
/// IV based value is greater than the loop invariant then return the loop
/// invariant. Otherwise return NULL.
Value * LoopIndexSplit::IVisGT(ICmpInst &Op) {
ICmpInst::Predicate P = Op.getPredicate();
if ((P == ICmpInst::ICMP_SGT || P == ICmpInst::ICMP_UGT)
&& IVBasedValues.count(Op.getOperand(0))
&& L->isLoopInvariant(Op.getOperand(1)))
return Op.getOperand(1);
if ((P == ICmpInst::ICMP_SLT || P == ICmpInst::ICMP_ULT)
&& IVBasedValues.count(Op.getOperand(1))
&& L->isLoopInvariant(Op.getOperand(0)))
return Op.getOperand(0);
return NULL;
}
/// IVisGE - If Op is comparing IV based value with an loop invariant and
/// IV based value is greater than or equal to the loop invariant then
/// return the loop invariant. Otherwise return NULL.
Value * LoopIndexSplit::IVisGE(ICmpInst &Op) {
ICmpInst::Predicate P = Op.getPredicate();
if ((P == ICmpInst::ICMP_SGE || P == ICmpInst::ICMP_UGE)
&& IVBasedValues.count(Op.getOperand(0))
&& L->isLoopInvariant(Op.getOperand(1)))
return Op.getOperand(1);
if ((P == ICmpInst::ICMP_SLE || P == ICmpInst::ICMP_ULE)
&& IVBasedValues.count(Op.getOperand(1))
&& L->isLoopInvariant(Op.getOperand(0)))
return Op.getOperand(0);
return NULL;
}
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