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llvm-6502/lib/Transforms/Scalar/LoopInterchange.cpp
Benjamin Kramer 1bfcd1f675 Re-sort includes with sort-includes.py and insert raw_ostream.h where it's used. 
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@232998 91177308-0d34-0410-b5e6-96231b3b80d8
2015-03-23 19:32:43 +00:00

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//===- LoopInterchange.cpp - Loop interchange pass------------------------===//
//
// The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This Pass handles loop interchange transform.
// This pass interchanges loops to provide a more cache-friendly memory access
// patterns.
//
//===----------------------------------------------------------------------===//
#include "llvm/ADT/SmallVector.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/AliasSetTracker.h"
#include "llvm/Analysis/AssumptionCache.h"
#include "llvm/Analysis/BlockFrequencyInfo.h"
#include "llvm/Analysis/CodeMetrics.h"
#include "llvm/Analysis/DependenceAnalysis.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/LoopIterator.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/Analysis/ScalarEvolutionExpander.h"
#include "llvm/Analysis/ScalarEvolutionExpressions.h"
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/InstIterator.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/Pass.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/LoopUtils.h"
#include "llvm/Transforms/Utils/SSAUpdater.h"
using namespace llvm;
#define DEBUG_TYPE "loop-interchange"
namespace {
typedef SmallVector<Loop *, 8> LoopVector;
// TODO: Check if we can use a sparse matrix here.
typedef std::vector<std::vector<char>> CharMatrix;
// Maximum number of dependencies that can be handled in the dependency matrix.
static const unsigned MaxMemInstrCount = 100;
// Maximum loop depth supported.
static const unsigned MaxLoopNestDepth = 10;
struct LoopInterchange;
#ifdef DUMP_DEP_MATRICIES
void printDepMatrix(CharMatrix &DepMatrix) {
for (auto I = DepMatrix.begin(), E = DepMatrix.end(); I != E; ++I) {
std::vector<char> Vec = *I;
for (auto II = Vec.begin(), EE = Vec.end(); II != EE; ++II)
DEBUG(dbgs() << *II << " ");
DEBUG(dbgs() << "\n");
}
}
#endif
bool populateDependencyMatrix(CharMatrix &DepMatrix, unsigned Level, Loop *L,
DependenceAnalysis *DA) {
typedef SmallVector<Value *, 16> ValueVector;
ValueVector MemInstr;
if (Level > MaxLoopNestDepth) {
DEBUG(dbgs() << "Cannot handle loops of depth greater than "
<< MaxLoopNestDepth << "\n");
return false;
}
// For each block.
for (Loop::block_iterator BB = L->block_begin(), BE = L->block_end();
BB != BE; ++BB) {
// Scan the BB and collect legal loads and stores.
for (BasicBlock::iterator I = (*BB)->begin(), E = (*BB)->end(); I != E;
++I) {
Instruction *Ins = dyn_cast<Instruction>(I);
if (!Ins)
return false;
LoadInst *Ld = dyn_cast<LoadInst>(I);
StoreInst *St = dyn_cast<StoreInst>(I);
if (!St && !Ld)
continue;
if (Ld && !Ld->isSimple())
return false;
if (St && !St->isSimple())
return false;
MemInstr.push_back(I);
}
}
DEBUG(dbgs() << "Found " << MemInstr.size()
<< " Loads and Stores to analyze\n");
ValueVector::iterator I, IE, J, JE;
for (I = MemInstr.begin(), IE = MemInstr.end(); I != IE; ++I) {
for (J = I, JE = MemInstr.end(); J != JE; ++J) {
std::vector<char> Dep;
Instruction *Src = dyn_cast<Instruction>(*I);
Instruction *Des = dyn_cast<Instruction>(*J);
if (Src == Des)
continue;
if (isa<LoadInst>(Src) && isa<LoadInst>(Des))
continue;
if (auto D = DA->depends(Src, Des, true)) {
DEBUG(dbgs() << "Found Dependency between Src=" << Src << " Des=" << Des
<< "\n");
if (D->isFlow()) {
// TODO: Handle Flow dependence.Check if it is sufficient to populate
// the Dependence Matrix with the direction reversed.
DEBUG(dbgs() << "Flow dependence not handled");
return false;
}
if (D->isAnti()) {
DEBUG(dbgs() << "Found Anti dependence \n");
unsigned Levels = D->getLevels();
char Direction;
for (unsigned II = 1; II <= Levels; ++II) {
const SCEV *Distance = D->getDistance(II);
const SCEVConstant *SCEVConst =
dyn_cast_or_null<SCEVConstant>(Distance);
if (SCEVConst) {
const ConstantInt *CI = SCEVConst->getValue();
if (CI->isNegative())
Direction = '<';
else if (CI->isZero())
Direction = '=';
else
Direction = '>';
Dep.push_back(Direction);
} else if (D->isScalar(II)) {
Direction = 'S';
Dep.push_back(Direction);
} else {
unsigned Dir = D->getDirection(II);
if (Dir == Dependence::DVEntry::LT ||
Dir == Dependence::DVEntry::LE)
Direction = '<';
else if (Dir == Dependence::DVEntry::GT ||
Dir == Dependence::DVEntry::GE)
Direction = '>';
else if (Dir == Dependence::DVEntry::EQ)
Direction = '=';
else
Direction = '*';
Dep.push_back(Direction);
}
}
while (Dep.size() != Level) {
Dep.push_back('I');
}
DepMatrix.push_back(Dep);
if (DepMatrix.size() > MaxMemInstrCount) {
DEBUG(dbgs() << "Cannot handle more than " << MaxMemInstrCount
<< " dependencies inside loop\n");
return false;
}
}
}
}
}
// We don't have a DepMatrix to check legality return false
if (DepMatrix.size() == 0)
return false;
return true;
}
// A loop is moved from index 'from' to an index 'to'. Update the Dependence
// matrix by exchanging the two columns.
void interChangeDepedencies(CharMatrix &DepMatrix, unsigned FromIndx,
unsigned ToIndx) {
unsigned numRows = DepMatrix.size();
for (unsigned i = 0; i < numRows; ++i) {
char TmpVal = DepMatrix[i][ToIndx];
DepMatrix[i][ToIndx] = DepMatrix[i][FromIndx];
DepMatrix[i][FromIndx] = TmpVal;
}
}
// Checks if outermost non '=','S'or'I' dependence in the dependence matrix is
// '>'
bool isOuterMostDepPositive(CharMatrix &DepMatrix, unsigned Row,
unsigned Column) {
for (unsigned i = 0; i <= Column; ++i) {
if (DepMatrix[Row][i] == '<')
return false;
if (DepMatrix[Row][i] == '>')
return true;
}
// All dependencies were '=','S' or 'I'
return false;
}
// Checks if no dependence exist in the dependency matrix in Row before Column.
bool containsNoDependence(CharMatrix &DepMatrix, unsigned Row,
unsigned Column) {
for (unsigned i = 0; i < Column; ++i) {
if (DepMatrix[Row][i] != '=' || DepMatrix[Row][i] != 'S' ||
DepMatrix[Row][i] != 'I')
return false;
}
return true;
}
bool validDepInterchange(CharMatrix &DepMatrix, unsigned Row,
unsigned OuterLoopId, char InnerDep, char OuterDep) {
if (isOuterMostDepPositive(DepMatrix, Row, OuterLoopId))
return false;
if (InnerDep == OuterDep)
return true;
// It is legal to interchange if and only if after interchange no row has a
// '>' direction as the leftmost non-'='.
if (InnerDep == '=' || InnerDep == 'S' || InnerDep == 'I')
return true;
if (InnerDep == '<')
return true;
if (InnerDep == '>') {
// If OuterLoopId represents outermost loop then interchanging will make the
// 1st dependency as '>'
if (OuterLoopId == 0)
return false;
// If all dependencies before OuterloopId are '=','S'or 'I'. Then
// interchanging will result in this row having an outermost non '='
// dependency of '>'
if (!containsNoDependence(DepMatrix, Row, OuterLoopId))
return true;
}
return false;
}
// Checks if it is legal to interchange 2 loops.
// [Theorm] A permutation of the loops in a perfect nest is legal if and only if
// the direction matrix, after the same permutation is applied to its columns,
// has no ">" direction as the leftmost non-"=" direction in any row.
bool isLegalToInterChangeLoops(CharMatrix &DepMatrix, unsigned InnerLoopId,
unsigned OuterLoopId) {
unsigned NumRows = DepMatrix.size();
// For each row check if it is valid to interchange.
for (unsigned Row = 0; Row < NumRows; ++Row) {
char InnerDep = DepMatrix[Row][InnerLoopId];
char OuterDep = DepMatrix[Row][OuterLoopId];
if (InnerDep == '*' || OuterDep == '*')
return false;
else if (!validDepInterchange(DepMatrix, Row, OuterLoopId, InnerDep,
OuterDep))
return false;
}
return true;
}
static void populateWorklist(Loop &L, SmallVector<LoopVector, 8> &V) {
DEBUG(dbgs() << "Calling populateWorklist called\n");
LoopVector LoopList;
Loop *CurrentLoop = &L;
std::vector<Loop *> vec = CurrentLoop->getSubLoopsVector();
while (vec.size() != 0) {
// The current loop has multiple subloops in it hence it is not tightly
// nested.
// Discard all loops above it added into Worklist.
if (vec.size() != 1) {
LoopList.clear();
return;
}
LoopList.push_back(CurrentLoop);
CurrentLoop = *(vec.begin());
vec = CurrentLoop->getSubLoopsVector();
}
LoopList.push_back(CurrentLoop);
V.push_back(LoopList);
}
static PHINode *getInductionVariable(Loop *L, ScalarEvolution *SE) {
PHINode *InnerIndexVar = L->getCanonicalInductionVariable();
if (InnerIndexVar)
return InnerIndexVar;
if (L->getLoopLatch() == nullptr || L->getLoopPredecessor() == nullptr)
return nullptr;
for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ++I) {
PHINode *PhiVar = cast<PHINode>(I);
Type *PhiTy = PhiVar->getType();
if (!PhiTy->isIntegerTy() && !PhiTy->isFloatingPointTy() &&
!PhiTy->isPointerTy())
return nullptr;
const SCEVAddRecExpr *AddRec =
dyn_cast<SCEVAddRecExpr>(SE->getSCEV(PhiVar));
if (!AddRec || !AddRec->isAffine())
continue;
const SCEV *Step = AddRec->getStepRecurrence(*SE);
const SCEVConstant *C = dyn_cast<SCEVConstant>(Step);
if (!C)
continue;
// Found the induction variable.
// FIXME: Handle loops with more than one induction variable. Note that,
// currently, legality makes sure we have only one induction variable.
return PhiVar;
}
return nullptr;
}
/// LoopInterchangeLegality checks if it is legal to interchange the loop.
class LoopInterchangeLegality {
public:
LoopInterchangeLegality(Loop *Outer, Loop *Inner, ScalarEvolution *SE,
LoopInterchange *Pass)
: OuterLoop(Outer), InnerLoop(Inner), SE(SE), CurrentPass(Pass) {}
/// Check if the loops can be interchanged.
bool canInterchangeLoops(unsigned InnerLoopId, unsigned OuterLoopId,
CharMatrix &DepMatrix);
/// Check if the loop structure is understood. We do not handle triangular
/// loops for now.
bool isLoopStructureUnderstood(PHINode *InnerInductionVar);
bool currentLimitations();
private:
bool tightlyNested(Loop *Outer, Loop *Inner);
Loop *OuterLoop;
Loop *InnerLoop;
/// Scev analysis.
ScalarEvolution *SE;
LoopInterchange *CurrentPass;
};
/// LoopInterchangeProfitability checks if it is profitable to interchange the
/// loop.
class LoopInterchangeProfitability {
public:
LoopInterchangeProfitability(Loop *Outer, Loop *Inner, ScalarEvolution *SE)
: OuterLoop(Outer), InnerLoop(Inner), SE(SE) {}
/// Check if the loop interchange is profitable
bool isProfitable(unsigned InnerLoopId, unsigned OuterLoopId,
CharMatrix &DepMatrix);
private:
int getInstrOrderCost();
Loop *OuterLoop;
Loop *InnerLoop;
/// Scev analysis.
ScalarEvolution *SE;
};
/// LoopInterchangeTransform interchanges the loop
class LoopInterchangeTransform {
public:
LoopInterchangeTransform(Loop *Outer, Loop *Inner, ScalarEvolution *SE,
LoopInfo *LI, DominatorTree *DT,
LoopInterchange *Pass, BasicBlock *LoopNestExit)
: OuterLoop(Outer), InnerLoop(Inner), SE(SE), LI(LI), DT(DT),
LoopExit(LoopNestExit) {}
/// Interchange OuterLoop and InnerLoop.
bool transform();
void restructureLoops(Loop *InnerLoop, Loop *OuterLoop);
void removeChildLoop(Loop *OuterLoop, Loop *InnerLoop);
private:
void splitInnerLoopLatch(Instruction *);
void splitOuterLoopLatch();
void splitInnerLoopHeader();
bool adjustLoopLinks();
void adjustLoopPreheaders();
void adjustOuterLoopPreheader();
void adjustInnerLoopPreheader();
bool adjustLoopBranches();
Loop *OuterLoop;
Loop *InnerLoop;
/// Scev analysis.
ScalarEvolution *SE;
LoopInfo *LI;
DominatorTree *DT;
BasicBlock *LoopExit;
};
// Main LoopInterchange Pass
struct LoopInterchange : public FunctionPass {
static char ID;
ScalarEvolution *SE;
LoopInfo *LI;
DependenceAnalysis *DA;
DominatorTree *DT;
LoopInterchange()
: FunctionPass(ID), SE(nullptr), LI(nullptr), DA(nullptr), DT(nullptr) {
initializeLoopInterchangePass(*PassRegistry::getPassRegistry());
}
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.addRequired<ScalarEvolution>();
AU.addRequired<AliasAnalysis>();
AU.addRequired<DominatorTreeWrapperPass>();
AU.addRequired<LoopInfoWrapperPass>();
AU.addRequired<DependenceAnalysis>();
AU.addRequiredID(LoopSimplifyID);
AU.addRequiredID(LCSSAID);
}
bool runOnFunction(Function &F) override {
SE = &getAnalysis<ScalarEvolution>();
LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
DA = &getAnalysis<DependenceAnalysis>();
auto *DTWP = getAnalysisIfAvailable<DominatorTreeWrapperPass>();
DT = DTWP ? &DTWP->getDomTree() : nullptr;
// Build up a worklist of loop pairs to analyze.
SmallVector<LoopVector, 8> Worklist;
for (Loop *L : *LI)
populateWorklist(*L, Worklist);
DEBUG(dbgs() << "Worklist size = " << Worklist.size() << "\n");
bool Changed = true;
while (!Worklist.empty()) {
LoopVector LoopList = Worklist.pop_back_val();
Changed = processLoopList(LoopList);
}
return Changed;
}
bool isComputableLoopNest(LoopVector LoopList) {
for (auto I = LoopList.begin(), E = LoopList.end(); I != E; ++I) {
Loop *L = *I;
const SCEV *ExitCountOuter = SE->getBackedgeTakenCount(L);
if (ExitCountOuter == SE->getCouldNotCompute()) {
DEBUG(dbgs() << "Couldn't compute Backedge count\n");
return false;
}
if (L->getNumBackEdges() != 1) {
DEBUG(dbgs() << "NumBackEdges is not equal to 1\n");
return false;
}
if (!L->getExitingBlock()) {
DEBUG(dbgs() << "Loop Doesn't have unique exit block\n");
return false;
}
}
return true;
}
unsigned selectLoopForInterchange(LoopVector LoopList) {
// TODO: Add a better heuristic to select the loop to be interchanged based
// on the dependece matrix. Currently we select the innermost loop.
return LoopList.size() - 1;
}
bool processLoopList(LoopVector LoopList) {
bool Changed = false;
bool containsLCSSAPHI = false;
CharMatrix DependencyMatrix;
if (LoopList.size() < 2) {
DEBUG(dbgs() << "Loop doesn't contain minimum nesting level.\n");
return false;
}
if (!isComputableLoopNest(LoopList)) {
DEBUG(dbgs() << "Not vaild loop candidate for interchange\n");
return false;
}
Loop *OuterMostLoop = *(LoopList.begin());
DEBUG(dbgs() << "Processing LoopList of size = " << LoopList.size()
<< "\n");
if (!populateDependencyMatrix(DependencyMatrix, LoopList.size(),
OuterMostLoop, DA)) {
DEBUG(dbgs() << "Populating Dependency matrix failed\n");
return false;
}
#ifdef DUMP_DEP_MATRICIES
DEBUG(dbgs() << "Dependence before inter change \n");
printDepMatrix(DependencyMatrix);
#endif
BasicBlock *OuterMostLoopLatch = OuterMostLoop->getLoopLatch();
BranchInst *OuterMostLoopLatchBI =
dyn_cast<BranchInst>(OuterMostLoopLatch->getTerminator());
if (!OuterMostLoopLatchBI)
return false;
// Since we currently do not handle LCSSA PHI's any failure in loop
// condition will now branch to LoopNestExit.
// TODO: This should be removed once we handle LCSSA PHI nodes.
// Get the Outermost loop exit.
BasicBlock *LoopNestExit;
if (OuterMostLoopLatchBI->getSuccessor(0) == OuterMostLoop->getHeader())
LoopNestExit = OuterMostLoopLatchBI->getSuccessor(1);
else
LoopNestExit = OuterMostLoopLatchBI->getSuccessor(0);
for (auto I = LoopList.begin(), E = LoopList.end(); I != E; ++I) {
Loop *L = *I;
BasicBlock *Latch = L->getLoopLatch();
BasicBlock *Header = L->getHeader();
if (Latch && Latch != Header && isa<PHINode>(Latch->begin())) {
containsLCSSAPHI = true;
break;
}
}
// TODO: Handle lcssa PHI's. Currently LCSSA PHI's are not handled. Handle
// the same by splitting the loop latch and adjusting loop links
// accordingly.
if (containsLCSSAPHI)
return false;
unsigned SelecLoopId = selectLoopForInterchange(LoopList);
// Move the selected loop outwards to the best posible position.
for (unsigned i = SelecLoopId; i > 0; i--) {
bool Interchanged =
processLoop(LoopList, i, i - 1, LoopNestExit, DependencyMatrix);
if (!Interchanged)
return Changed;
// Loops interchanged reflect the same in LoopList
std::swap(LoopList[i - 1], LoopList[i]);
// Update the DependencyMatrix
interChangeDepedencies(DependencyMatrix, i, i - 1);
#ifdef DUMP_DEP_MATRICIES
DEBUG(dbgs() << "Dependence after inter change \n");
printDepMatrix(DependencyMatrix);
#endif
Changed |= Interchanged;
}
return Changed;
}
bool processLoop(LoopVector LoopList, unsigned InnerLoopId,
unsigned OuterLoopId, BasicBlock *LoopNestExit,
std::vector<std::vector<char>> &DependencyMatrix) {
DEBUG(dbgs() << "Processing Innder Loop Id = " << InnerLoopId
<< " and OuterLoopId = " << OuterLoopId << "\n");
Loop *InnerLoop = LoopList[InnerLoopId];
Loop *OuterLoop = LoopList[OuterLoopId];
LoopInterchangeLegality LIL(OuterLoop, InnerLoop, SE, this);
if (!LIL.canInterchangeLoops(InnerLoopId, OuterLoopId, DependencyMatrix)) {
DEBUG(dbgs() << "Not interchanging Loops. Cannot prove legality\n");
return false;
}
DEBUG(dbgs() << "Loops are legal to interchange\n");
LoopInterchangeProfitability LIP(OuterLoop, InnerLoop, SE);
if (!LIP.isProfitable(InnerLoopId, OuterLoopId, DependencyMatrix)) {
DEBUG(dbgs() << "Interchanging Loops not profitable\n");
return false;
}
LoopInterchangeTransform LIT(OuterLoop, InnerLoop, SE, LI, DT, this,
LoopNestExit);
LIT.transform();
DEBUG(dbgs() << "Loops interchanged\n");
return true;
}
};
} // end of namespace
static bool containsUnsafeInstructions(BasicBlock *BB) {
for (auto I = BB->begin(), E = BB->end(); I != E; ++I) {
if (I->mayHaveSideEffects() || I->mayReadFromMemory())
return true;
}
return false;
}
bool LoopInterchangeLegality::tightlyNested(Loop *OuterLoop, Loop *InnerLoop) {
BasicBlock *OuterLoopHeader = OuterLoop->getHeader();
BasicBlock *InnerLoopPreHeader = InnerLoop->getLoopPreheader();
BasicBlock *OuterLoopLatch = OuterLoop->getLoopLatch();
DEBUG(dbgs() << "Checking if Loops are Tightly Nested\n");
// A perfectly nested loop will not have any branch in between the outer and
// inner block i.e. outer header will branch to either inner preheader and
// outerloop latch.
BranchInst *outerLoopHeaderBI =
dyn_cast<BranchInst>(OuterLoopHeader->getTerminator());
if (!outerLoopHeaderBI)
return false;
unsigned num = outerLoopHeaderBI->getNumSuccessors();
for (unsigned i = 0; i < num; i++) {
if (outerLoopHeaderBI->getSuccessor(i) != InnerLoopPreHeader &&
outerLoopHeaderBI->getSuccessor(i) != OuterLoopLatch)
return false;
}
DEBUG(dbgs() << "Checking instructions in Loop header and Loop latch \n");
// We do not have any basic block in between now make sure the outer header
// and outer loop latch doesnt contain any unsafe instructions.
if (containsUnsafeInstructions(OuterLoopHeader) ||
containsUnsafeInstructions(OuterLoopLatch))
return false;
DEBUG(dbgs() << "Loops are perfectly nested \n");
// We have a perfect loop nest.
return true;
}
static unsigned getPHICount(BasicBlock *BB) {
unsigned PhiCount = 0;
for (auto I = BB->begin(); isa<PHINode>(I); ++I)
PhiCount++;
return PhiCount;
}
bool LoopInterchangeLegality::isLoopStructureUnderstood(
PHINode *InnerInduction) {
unsigned Num = InnerInduction->getNumOperands();
BasicBlock *InnerLoopPreheader = InnerLoop->getLoopPreheader();
for (unsigned i = 0; i < Num; ++i) {
Value *Val = InnerInduction->getOperand(i);
if (isa<Constant>(Val))
continue;
Instruction *I = dyn_cast<Instruction>(Val);
if (!I)
return false;
// TODO: Handle triangular loops.
// e.g. for(int i=0;i<N;i++)
// for(int j=i;j<N;j++)
unsigned IncomBlockIndx = PHINode::getIncomingValueNumForOperand(i);
if (InnerInduction->getIncomingBlock(IncomBlockIndx) ==
InnerLoopPreheader &&
!OuterLoop->isLoopInvariant(I)) {
return false;
}
}
return true;
}
// This function indicates the current limitations in the transform as a result
// of which we do not proceed.
bool LoopInterchangeLegality::currentLimitations() {
BasicBlock *InnerLoopPreHeader = InnerLoop->getLoopPreheader();
BasicBlock *InnerLoopHeader = InnerLoop->getHeader();
BasicBlock *OuterLoopHeader = OuterLoop->getHeader();
BasicBlock *InnerLoopLatch = InnerLoop->getLoopLatch();
BasicBlock *OuterLoopLatch = OuterLoop->getLoopLatch();
PHINode *InnerInductionVar;
PHINode *OuterInductionVar;
// We currently handle only 1 induction variable inside the loop. We also do
// not handle reductions as of now.
if (getPHICount(InnerLoopHeader) > 1)
return true;
if (getPHICount(OuterLoopHeader) > 1)
return true;
InnerInductionVar = getInductionVariable(InnerLoop, SE);
OuterInductionVar = getInductionVariable(OuterLoop, SE);
if (!OuterInductionVar || !InnerInductionVar) {
DEBUG(dbgs() << "Induction variable not found\n");
return true;
}
// TODO: Triangular loops are not handled for now.
if (!isLoopStructureUnderstood(InnerInductionVar)) {
DEBUG(dbgs() << "Loop structure not understood by pass\n");
return true;
}
// TODO: Loops with LCSSA PHI's are currently not handled.
if (isa<PHINode>(OuterLoopLatch->begin())) {
DEBUG(dbgs() << "Found and LCSSA PHI in outer loop latch\n");
return true;
}
if (InnerLoopLatch != InnerLoopHeader &&
isa<PHINode>(InnerLoopLatch->begin())) {
DEBUG(dbgs() << "Found and LCSSA PHI in inner loop latch\n");
return true;
}
// TODO: Current limitation: Since we split the inner loop latch at the point
// were induction variable is incremented (induction.next); We cannot have
// more than 1 user of induction.next since it would result in broken code
// after split.
// e.g.
// for(i=0;i<N;i++) {
// for(j = 0;j<M;j++) {
// A[j+1][i+2] = A[j][i]+k;
// }
// }
bool FoundInduction = false;
Instruction *InnerIndexVarInc = nullptr;
if (InnerInductionVar->getIncomingBlock(0) == InnerLoopPreHeader)
InnerIndexVarInc =
dyn_cast<Instruction>(InnerInductionVar->getIncomingValue(1));
else
InnerIndexVarInc =
dyn_cast<Instruction>(InnerInductionVar->getIncomingValue(0));
if (!InnerIndexVarInc)
return true;
// Since we split the inner loop latch on this induction variable. Make sure
// we do not have any instruction between the induction variable and branch
// instruction.
for (auto I = InnerLoopLatch->rbegin(), E = InnerLoopLatch->rend();
I != E && !FoundInduction; ++I) {
if (isa<BranchInst>(*I) || isa<CmpInst>(*I) || isa<TruncInst>(*I))
continue;
const Instruction &Ins = *I;
// We found an instruction. If this is not induction variable then it is not
// safe to split this loop latch.
if (!Ins.isIdenticalTo(InnerIndexVarInc))
return true;
else
FoundInduction = true;
}
// The loop latch ended and we didnt find the induction variable return as
// current limitation.
if (!FoundInduction)
return true;
return false;
}
bool LoopInterchangeLegality::canInterchangeLoops(unsigned InnerLoopId,
unsigned OuterLoopId,
CharMatrix &DepMatrix) {
if (!isLegalToInterChangeLoops(DepMatrix, InnerLoopId, OuterLoopId)) {
DEBUG(dbgs() << "Failed interchange InnerLoopId = " << InnerLoopId
<< "and OuterLoopId = " << OuterLoopId
<< "due to dependence\n");
return false;
}
// Create unique Preheaders if we already do not have one.
BasicBlock *OuterLoopPreHeader = OuterLoop->getLoopPreheader();
BasicBlock *InnerLoopPreHeader = InnerLoop->getLoopPreheader();
// Create a unique outer preheader -
// 1) If OuterLoop preheader is not present.
// 2) If OuterLoop Preheader is same as OuterLoop Header
// 3) If OuterLoop Preheader is same as Header of the previous loop.
// 4) If OuterLoop Preheader is Entry node.
if (!OuterLoopPreHeader || OuterLoopPreHeader == OuterLoop->getHeader() ||
isa<PHINode>(OuterLoopPreHeader->begin()) ||
!OuterLoopPreHeader->getUniquePredecessor()) {
OuterLoopPreHeader = InsertPreheaderForLoop(OuterLoop, CurrentPass);
}
if (!InnerLoopPreHeader || InnerLoopPreHeader == InnerLoop->getHeader() ||
InnerLoopPreHeader == OuterLoop->getHeader()) {
InnerLoopPreHeader = InsertPreheaderForLoop(InnerLoop, CurrentPass);
}
// Check if the loops are tightly nested.
if (!tightlyNested(OuterLoop, InnerLoop)) {
DEBUG(dbgs() << "Loops not tightly nested\n");
return false;
}
// TODO: The loops could not be interchanged due to current limitations in the
// transform module.
if (currentLimitations()) {
DEBUG(dbgs() << "Not legal because of current transform limitation\n");
return false;
}
return true;
}
int LoopInterchangeProfitability::getInstrOrderCost() {
unsigned GoodOrder, BadOrder;
BadOrder = GoodOrder = 0;
for (auto BI = InnerLoop->block_begin(), BE = InnerLoop->block_end();
BI != BE; ++BI) {
for (auto I = (*BI)->begin(), E = (*BI)->end(); I != E; ++I) {
const Instruction &Ins = *I;
if (const GetElementPtrInst *GEP = dyn_cast<GetElementPtrInst>(&Ins)) {
unsigned NumOp = GEP->getNumOperands();
bool FoundInnerInduction = false;
bool FoundOuterInduction = false;
for (unsigned i = 0; i < NumOp; ++i) {
const SCEV *OperandVal = SE->getSCEV(GEP->getOperand(i));
const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(OperandVal);
if (!AR)
continue;
// If we find the inner induction after an outer induction e.g.
// for(int i=0;i<N;i++)
// for(int j=0;j<N;j++)
// A[i][j] = A[i-1][j-1]+k;
// then it is a good order.
if (AR->getLoop() == InnerLoop) {
// We found an InnerLoop induction after OuterLoop induction. It is
// a good order.
FoundInnerInduction = true;
if (FoundOuterInduction) {
GoodOrder++;
break;
}
}
// If we find the outer induction after an inner induction e.g.
// for(int i=0;i<N;i++)
// for(int j=0;j<N;j++)
// A[j][i] = A[j-1][i-1]+k;
// then it is a bad order.
if (AR->getLoop() == OuterLoop) {
// We found an OuterLoop induction after InnerLoop induction. It is
// a bad order.
FoundOuterInduction = true;
if (FoundInnerInduction) {
BadOrder++;
break;
}
}
}
}
}
}
return GoodOrder - BadOrder;
}
static bool isProfitabileForVectorization(unsigned InnerLoopId,
unsigned OuterLoopId,
CharMatrix &DepMatrix) {
// TODO: Improve this heuristic to catch more cases.
// If the inner loop is loop independent or doesn't carry any dependency it is
// profitable to move this to outer position.
unsigned Row = DepMatrix.size();
for (unsigned i = 0; i < Row; ++i) {
if (DepMatrix[i][InnerLoopId] != 'S' && DepMatrix[i][InnerLoopId] != 'I')
return false;
// TODO: We need to improve this heuristic.
if (DepMatrix[i][OuterLoopId] != '=')
return false;
}
// If outer loop has dependence and inner loop is loop independent then it is
// profitable to interchange to enable parallelism.
return true;
}
bool LoopInterchangeProfitability::isProfitable(unsigned InnerLoopId,
unsigned OuterLoopId,
CharMatrix &DepMatrix) {
// TODO: Add Better Profitibility checks.
// e.g
// 1) Construct dependency matrix and move the one with no loop carried dep
// inside to enable vectorization.
// This is rough cost estimation algorithm. It counts the good and bad order
// of induction variables in the instruction and allows reordering if number
// of bad orders is more than good.
int Cost = 0;
Cost += getInstrOrderCost();
DEBUG(dbgs() << "Cost = " << Cost << "\n");
if (Cost < 0)
return true;
// It is not profitable as per current cache profitibility model. But check if
// we can move this loop outside to improve parallelism.
bool ImprovesPar =
isProfitabileForVectorization(InnerLoopId, OuterLoopId, DepMatrix);
return ImprovesPar;
}
void LoopInterchangeTransform::removeChildLoop(Loop *OuterLoop,
Loop *InnerLoop) {
for (Loop::iterator I = OuterLoop->begin(), E = OuterLoop->end(); I != E;
++I) {
if (*I == InnerLoop) {
OuterLoop->removeChildLoop(I);
return;
}
}
assert(false && "Couldn't find loop");
}
void LoopInterchangeTransform::restructureLoops(Loop *InnerLoop,
Loop *OuterLoop) {
Loop *OuterLoopParent = OuterLoop->getParentLoop();
if (OuterLoopParent) {
// Remove the loop from its parent loop.
removeChildLoop(OuterLoopParent, OuterLoop);
removeChildLoop(OuterLoop, InnerLoop);
OuterLoopParent->addChildLoop(InnerLoop);
} else {
removeChildLoop(OuterLoop, InnerLoop);
LI->changeTopLevelLoop(OuterLoop, InnerLoop);
}
for (Loop::iterator I = InnerLoop->begin(), E = InnerLoop->end(); I != E; ++I)
OuterLoop->addChildLoop(InnerLoop->removeChildLoop(I));
InnerLoop->addChildLoop(OuterLoop);
}
bool LoopInterchangeTransform::transform() {
DEBUG(dbgs() << "transform\n");
bool Transformed = false;
Instruction *InnerIndexVar;
if (InnerLoop->getSubLoops().size() == 0) {
BasicBlock *InnerLoopPreHeader = InnerLoop->getLoopPreheader();
DEBUG(dbgs() << "Calling Split Inner Loop\n");
PHINode *InductionPHI = getInductionVariable(InnerLoop, SE);
if (!InductionPHI) {
DEBUG(dbgs() << "Failed to find the point to split loop latch \n");
return false;
}
if (InductionPHI->getIncomingBlock(0) == InnerLoopPreHeader)
InnerIndexVar = dyn_cast<Instruction>(InductionPHI->getIncomingValue(1));
else
InnerIndexVar = dyn_cast<Instruction>(InductionPHI->getIncomingValue(0));
//
// Split at the place were the induction variable is
// incremented/decremented.
// TODO: This splitting logic may not work always. Fix this.
splitInnerLoopLatch(InnerIndexVar);
DEBUG(dbgs() << "splitInnerLoopLatch Done\n");
// Splits the inner loops phi nodes out into a seperate basic block.
splitInnerLoopHeader();
DEBUG(dbgs() << "splitInnerLoopHeader Done\n");
}
Transformed |= adjustLoopLinks();
if (!Transformed) {
DEBUG(dbgs() << "adjustLoopLinks Failed\n");
return false;
}
restructureLoops(InnerLoop, OuterLoop);
return true;
}
void LoopInterchangeTransform::splitInnerLoopLatch(Instruction *Inc) {
BasicBlock *InnerLoopLatch = InnerLoop->getLoopLatch();
BasicBlock *InnerLoopLatchPred = InnerLoopLatch;
InnerLoopLatch = SplitBlock(InnerLoopLatchPred, Inc, DT, LI);
}
void LoopInterchangeTransform::splitOuterLoopLatch() {
BasicBlock *OuterLoopLatch = OuterLoop->getLoopLatch();
BasicBlock *OuterLatchLcssaPhiBlock = OuterLoopLatch;
OuterLoopLatch = SplitBlock(OuterLatchLcssaPhiBlock,
OuterLoopLatch->getFirstNonPHI(), DT, LI);
}
void LoopInterchangeTransform::splitInnerLoopHeader() {
// Split the inner loop header out.
BasicBlock *InnerLoopHeader = InnerLoop->getHeader();
SplitBlock(InnerLoopHeader, InnerLoopHeader->getFirstNonPHI(), DT, LI);
DEBUG(dbgs() << "Output of splitInnerLoopHeader InnerLoopHeaderSucc & "
"InnerLoopHeader \n");
}
/// \brief Move all instructions except the terminator from FromBB right before
/// InsertBefore
static void moveBBContents(BasicBlock *FromBB, Instruction *InsertBefore) {
auto &ToList = InsertBefore->getParent()->getInstList();
auto &FromList = FromBB->getInstList();
ToList.splice(InsertBefore, FromList, FromList.begin(),
FromBB->getTerminator());
}
void LoopInterchangeTransform::adjustOuterLoopPreheader() {
BasicBlock *OuterLoopPreHeader = OuterLoop->getLoopPreheader();
BasicBlock *InnerPreHeader = InnerLoop->getLoopPreheader();
moveBBContents(OuterLoopPreHeader, InnerPreHeader->getTerminator());
}
void LoopInterchangeTransform::adjustInnerLoopPreheader() {
BasicBlock *InnerLoopPreHeader = InnerLoop->getLoopPreheader();
BasicBlock *OuterHeader = OuterLoop->getHeader();
moveBBContents(InnerLoopPreHeader, OuterHeader->getTerminator());
}
bool LoopInterchangeTransform::adjustLoopBranches() {
DEBUG(dbgs() << "adjustLoopBranches called\n");
// Adjust the loop preheader
BasicBlock *InnerLoopHeader = InnerLoop->getHeader();
BasicBlock *OuterLoopHeader = OuterLoop->getHeader();
BasicBlock *InnerLoopLatch = InnerLoop->getLoopLatch();
BasicBlock *OuterLoopLatch = OuterLoop->getLoopLatch();
BasicBlock *OuterLoopPreHeader = OuterLoop->getLoopPreheader();
BasicBlock *InnerLoopPreHeader = InnerLoop->getLoopPreheader();
BasicBlock *OuterLoopPredecessor = OuterLoopPreHeader->getUniquePredecessor();
BasicBlock *InnerLoopLatchPredecessor =
InnerLoopLatch->getUniquePredecessor();
BasicBlock *InnerLoopLatchSuccessor;
BasicBlock *OuterLoopLatchSuccessor;
BranchInst *OuterLoopLatchBI =
dyn_cast<BranchInst>(OuterLoopLatch->getTerminator());
BranchInst *InnerLoopLatchBI =
dyn_cast<BranchInst>(InnerLoopLatch->getTerminator());
BranchInst *OuterLoopHeaderBI =
dyn_cast<BranchInst>(OuterLoopHeader->getTerminator());
BranchInst *InnerLoopHeaderBI =
dyn_cast<BranchInst>(InnerLoopHeader->getTerminator());
if (!OuterLoopPredecessor || !InnerLoopLatchPredecessor ||
!OuterLoopLatchBI || !InnerLoopLatchBI || !OuterLoopHeaderBI ||
!InnerLoopHeaderBI)
return false;
BranchInst *InnerLoopLatchPredecessorBI =
dyn_cast<BranchInst>(InnerLoopLatchPredecessor->getTerminator());
BranchInst *OuterLoopPredecessorBI =
dyn_cast<BranchInst>(OuterLoopPredecessor->getTerminator());
if (!OuterLoopPredecessorBI || !InnerLoopLatchPredecessorBI)
return false;
BasicBlock *InnerLoopHeaderSucessor = InnerLoopHeader->getUniqueSuccessor();
if (!InnerLoopHeaderSucessor)
return false;
// Adjust Loop Preheader and headers
unsigned NumSucc = OuterLoopPredecessorBI->getNumSuccessors();
for (unsigned i = 0; i < NumSucc; ++i) {
if (OuterLoopPredecessorBI->getSuccessor(i) == OuterLoopPreHeader)
OuterLoopPredecessorBI->setSuccessor(i, InnerLoopPreHeader);
}
NumSucc = OuterLoopHeaderBI->getNumSuccessors();
for (unsigned i = 0; i < NumSucc; ++i) {
if (OuterLoopHeaderBI->getSuccessor(i) == OuterLoopLatch)
OuterLoopHeaderBI->setSuccessor(i, LoopExit);
else if (OuterLoopHeaderBI->getSuccessor(i) == InnerLoopPreHeader)
OuterLoopHeaderBI->setSuccessor(i, InnerLoopHeaderSucessor);
}
BranchInst::Create(OuterLoopPreHeader, InnerLoopHeaderBI);
InnerLoopHeaderBI->eraseFromParent();
// -------------Adjust loop latches-----------
if (InnerLoopLatchBI->getSuccessor(0) == InnerLoopHeader)
InnerLoopLatchSuccessor = InnerLoopLatchBI->getSuccessor(1);
else
InnerLoopLatchSuccessor = InnerLoopLatchBI->getSuccessor(0);
NumSucc = InnerLoopLatchPredecessorBI->getNumSuccessors();
for (unsigned i = 0; i < NumSucc; ++i) {
if (InnerLoopLatchPredecessorBI->getSuccessor(i) == InnerLoopLatch)
InnerLoopLatchPredecessorBI->setSuccessor(i, InnerLoopLatchSuccessor);
}
if (OuterLoopLatchBI->getSuccessor(0) == OuterLoopHeader)
OuterLoopLatchSuccessor = OuterLoopLatchBI->getSuccessor(1);
else
OuterLoopLatchSuccessor = OuterLoopLatchBI->getSuccessor(0);
if (InnerLoopLatchBI->getSuccessor(1) == InnerLoopLatchSuccessor)
InnerLoopLatchBI->setSuccessor(1, OuterLoopLatchSuccessor);
else
InnerLoopLatchBI->setSuccessor(0, OuterLoopLatchSuccessor);
if (OuterLoopLatchBI->getSuccessor(0) == OuterLoopLatchSuccessor) {
OuterLoopLatchBI->setSuccessor(0, InnerLoopLatch);
} else {
OuterLoopLatchBI->setSuccessor(1, InnerLoopLatch);
}
return true;
}
void LoopInterchangeTransform::adjustLoopPreheaders() {
// We have interchanged the preheaders so we need to interchange the data in
// the preheader as well.
// This is because the content of inner preheader was previously executed
// inside the outer loop.
BasicBlock *OuterLoopPreHeader = OuterLoop->getLoopPreheader();
BasicBlock *InnerLoopPreHeader = InnerLoop->getLoopPreheader();
BasicBlock *OuterLoopHeader = OuterLoop->getHeader();
BranchInst *InnerTermBI =
cast<BranchInst>(InnerLoopPreHeader->getTerminator());
BasicBlock *HeaderSplit =
SplitBlock(OuterLoopHeader, OuterLoopHeader->getTerminator(), DT, LI);
Instruction *InsPoint = HeaderSplit->getFirstNonPHI();
// These instructions should now be executed inside the loop.
// Move instruction into a new block after outer header.
moveBBContents(InnerLoopPreHeader, InsPoint);
// These instructions were not executed previously in the loop so move them to
// the older inner loop preheader.
moveBBContents(OuterLoopPreHeader, InnerTermBI);
}
bool LoopInterchangeTransform::adjustLoopLinks() {
// Adjust all branches in the inner and outer loop.
bool Changed = adjustLoopBranches();
if (Changed)
adjustLoopPreheaders();
return Changed;
}
char LoopInterchange::ID = 0;
INITIALIZE_PASS_BEGIN(LoopInterchange, "loop-interchange",
"Interchanges loops for cache reuse", false, false)
INITIALIZE_AG_DEPENDENCY(AliasAnalysis)
INITIALIZE_PASS_DEPENDENCY(DependenceAnalysis)
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
INITIALIZE_PASS_DEPENDENCY(ScalarEvolution)
INITIALIZE_PASS_DEPENDENCY(LoopSimplify)
INITIALIZE_PASS_DEPENDENCY(LCSSA)
INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
INITIALIZE_PASS_END(LoopInterchange, "loop-interchange",
"Interchanges loops for cache reuse", false, false)
Pass *llvm::createLoopInterchangePass() { return new LoopInterchange(); }
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