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llvm-mirror/lib/Transforms/Scalar/LoopInterchange.cpp
Sebastian Pop 93d67e228d DA: remove uses of GEP, only ask SCEV 
It's been quite some time the Dependence Analysis (DA) is broken,
as it uses the GEP representation to "identify" multi-dimensional arrays.
It even wrongly detects multi-dimensional arrays in single nested loops:

from test/Analysis/DependenceAnalysis/Coupled.ll, example @couple6
;; for (long int i = 0; i < 50; i++) {
;; A[i][3*i - 6] = i;
;; *B++ = A[i][i];

DA used to detect two subscripts, which makes no sense in the LLVM IR
or in C/C++ semantics, as there are no guarantees as in Fortran of
subscripts not overlapping into a next array dimension:

maximum nesting levels = 1
SrcPtrSCEV = %A
DstPtrSCEV = %A
using GEPs
subscript 0
    src = {0,+,1}<nuw><nsw><%for.body>
    dst = {0,+,1}<nuw><nsw><%for.body>
    class = 1
    loops = {1}
subscript 1
    src = {-6,+,3}<nsw><%for.body>
    dst = {0,+,1}<nuw><nsw><%for.body>
    class = 1
    loops = {1}
Separable = {}
Coupled = {1}

With the current patch, DA will correctly work on only one dimension:

maximum nesting levels = 1
SrcSCEV = {(-2424 + %A)<nsw>,+,1212}<%for.body>
DstSCEV = {%A,+,404}<%for.body>
subscript 0
    src = {(-2424 + %A)<nsw>,+,1212}<%for.body>
    dst = {%A,+,404}<%for.body>
    class = 1
    loops = {1}
Separable = {0}
Coupled = {}

This change removes all uses of GEP from DA, and we now only rely
on the SCEV representation.

The patch does not turn on -da-delinearize by default, and so the DA analysis
will be more conservative in the case of multi-dimensional memory accesses in
nested loops.

I disabled some interchange tests, as the DA is not able to disambiguate
the dependence anymore. To make DA stronger, we may need to
compute a bound on the number of iterations based on the access functions
and array dimensions.

The patch cleans up all the CHECKs in test/Transforms/LoopInterchange/*.ll to
avoid checking for snippets of LLVM IR: this form of checking is very hard to
maintain. Instead, we now check for output of the pass that are more meaningful
than dozens of lines of LLVM IR. Some tests now require -debug messages and thus
only enabled with asserts.

Patch written by Sebastian Pop and Aditya Kumar.

Differential Revision: https://reviews.llvm.org/D35430

llvm-svn: 326837
2018-03-06 21:55:59 +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/STLExtras.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/DependenceAnalysis.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/OptimizationRemarkEmitter.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/Analysis/ScalarEvolutionExpressions.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DiagnosticInfo.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/InstrTypes.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Type.h"
#include "llvm/IR/User.h"
#include "llvm/IR/Value.h"
#include "llvm/Pass.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.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 <cassert>
#include <utility>
#include <vector>
using namespace llvm;
#define DEBUG_TYPE "loop-interchange"
static cl::opt<int> LoopInterchangeCostThreshold(
"loop-interchange-threshold", cl::init(0), cl::Hidden,
cl::desc("Interchange if you gain more than this number"));
namespace {
using LoopVector = SmallVector<Loop *, 8>;
// TODO: Check if we can use a sparse matrix here.
using CharMatrix = std::vector<std::vector<char>>;
} // end anonymous namespace
// 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;
#ifdef DUMP_DEP_MATRICIES
static void printDepMatrix(CharMatrix &DepMatrix) {
for (auto &Row : DepMatrix) {
for (auto D : Row)
DEBUG(dbgs() << D << " ");
DEBUG(dbgs() << "\n");
}
}
#endif
static bool populateDependencyMatrix(CharMatrix &DepMatrix, unsigned Level,
Loop *L, DependenceInfo *DI) {
using ValueVector = SmallVector<Value *, 16>;
ValueVector MemInstr;
// For each block.
for (BasicBlock *BB : L->blocks()) {
// Scan the BB and collect legal loads and stores.
for (Instruction &I : *BB) {
if (!isa<Instruction>(I))
return false;
if (auto *Ld = dyn_cast<LoadInst>(&I)) {
if (!Ld->isSimple())
return false;
MemInstr.push_back(&I);
} else if (auto *St = dyn_cast<StoreInst>(&I)) {
if (!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 = cast<Instruction>(*I);
Instruction *Dst = cast<Instruction>(*J);
if (Src == Dst)
continue;
// Ignore Input dependencies.
if (isa<LoadInst>(Src) && isa<LoadInst>(Dst))
continue;
// Track Output, Flow, and Anti dependencies.
if (auto D = DI->depends(Src, Dst, true)) {
assert(D->isOrdered() && "Expected an output, flow or anti dep.");
DEBUG(StringRef DepType =
D->isFlow() ? "flow" : D->isAnti() ? "anti" : "output";
dbgs() << "Found " << DepType
<< " dependency between Src and Dst\n"
<< " Src:" << *Src << "\n Dst:" << *Dst << '\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;
}
}
}
}
return true;
}
// A loop is moved from index 'from' to an index 'to'. Update the Dependence
// matrix by exchanging the two columns.
static void interChangeDependencies(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
// '>'
static 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.
static 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;
}
static 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.
// [Theorem] 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.
static 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;
if (!validDepInterchange(DepMatrix, Row, OuterLoopId, InnerDep, OuterDep))
return false;
}
return true;
}
static void populateWorklist(Loop &L, SmallVector<LoopVector, 8> &V) {
DEBUG(dbgs() << "Calling populateWorklist on Func: "
<< L.getHeader()->getParent()->getName() << " Loop: %"
<< L.getHeader()->getName() << '\n');
LoopVector LoopList;
Loop *CurrentLoop = &L;
const std::vector<Loop *> *Vec = &CurrentLoop->getSubLoops();
while (!Vec->empty()) {
// 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->front();
Vec = &CurrentLoop->getSubLoops();
}
LoopList.push_back(CurrentLoop);
V.push_back(std::move(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);
if (!isa<SCEVConstant>(Step))
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;
}
namespace {
/// LoopInterchangeLegality checks if it is legal to interchange the loop.
class LoopInterchangeLegality {
public:
LoopInterchangeLegality(Loop *Outer, Loop *Inner, ScalarEvolution *SE,
LoopInfo *LI, DominatorTree *DT, bool PreserveLCSSA,
OptimizationRemarkEmitter *ORE)
: OuterLoop(Outer), InnerLoop(Inner), SE(SE), LI(LI), DT(DT),
PreserveLCSSA(PreserveLCSSA), ORE(ORE) {}
/// 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();
bool hasInnerLoopReduction() { return InnerLoopHasReduction; }
private:
bool tightlyNested(Loop *Outer, Loop *Inner);
bool containsUnsafeInstructionsInHeader(BasicBlock *BB);
bool areAllUsesReductions(Instruction *Ins, Loop *L);
bool containsUnsafeInstructionsInLatch(BasicBlock *BB);
bool findInductionAndReductions(Loop *L,
SmallVector<PHINode *, 8> &Inductions,
SmallVector<PHINode *, 8> &Reductions);
Loop *OuterLoop;
Loop *InnerLoop;
ScalarEvolution *SE;
LoopInfo *LI;
DominatorTree *DT;
bool PreserveLCSSA;
/// Interface to emit optimization remarks.
OptimizationRemarkEmitter *ORE;
bool InnerLoopHasReduction = false;
};
/// LoopInterchangeProfitability checks if it is profitable to interchange the
/// loop.
class LoopInterchangeProfitability {
public:
LoopInterchangeProfitability(Loop *Outer, Loop *Inner, ScalarEvolution *SE,
OptimizationRemarkEmitter *ORE)
: OuterLoop(Outer), InnerLoop(Inner), SE(SE), ORE(ORE) {}
/// 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;
/// Interface to emit optimization remarks.
OptimizationRemarkEmitter *ORE;
};
/// LoopInterchangeTransform interchanges the loop.
class LoopInterchangeTransform {
public:
LoopInterchangeTransform(Loop *Outer, Loop *Inner, ScalarEvolution *SE,
LoopInfo *LI, DominatorTree *DT,
BasicBlock *LoopNestExit,
bool InnerLoopContainsReductions)
: OuterLoop(Outer), InnerLoop(Inner), SE(SE), LI(LI), DT(DT),
LoopExit(LoopNestExit),
InnerLoopHasReduction(InnerLoopContainsReductions) {}
/// Interchange OuterLoop and InnerLoop.
bool transform();
void restructureLoops(Loop *InnerLoop, Loop *OuterLoop);
void removeChildLoop(Loop *OuterLoop, Loop *InnerLoop);
private:
void splitInnerLoopLatch(Instruction *);
void splitInnerLoopHeader();
bool adjustLoopLinks();
void adjustLoopPreheaders();
bool adjustLoopBranches();
void updateIncomingBlock(BasicBlock *CurrBlock, BasicBlock *OldPred,
BasicBlock *NewPred);
Loop *OuterLoop;
Loop *InnerLoop;
/// Scev analysis.
ScalarEvolution *SE;
LoopInfo *LI;
DominatorTree *DT;
BasicBlock *LoopExit;
bool InnerLoopHasReduction;
};
// Main LoopInterchange Pass.
struct LoopInterchange : public FunctionPass {
static char ID;
ScalarEvolution *SE = nullptr;
LoopInfo *LI = nullptr;
DependenceInfo *DI = nullptr;
DominatorTree *DT = nullptr;
bool PreserveLCSSA;
/// Interface to emit optimization remarks.
OptimizationRemarkEmitter *ORE;
LoopInterchange() : FunctionPass(ID) {
initializeLoopInterchangePass(*PassRegistry::getPassRegistry());
}
void getAnalysisUsage(AnalysisUsage &AU) const override {
AU.addRequired<ScalarEvolutionWrapperPass>();
AU.addRequired<AAResultsWrapperPass>();
AU.addRequired<DominatorTreeWrapperPass>();
AU.addRequired<LoopInfoWrapperPass>();
AU.addRequired<DependenceAnalysisWrapperPass>();
AU.addRequiredID(LoopSimplifyID);
AU.addRequiredID(LCSSAID);
AU.addRequired<OptimizationRemarkEmitterWrapperPass>();
AU.addPreserved<DominatorTreeWrapperPass>();
}
bool runOnFunction(Function &F) override {
if (skipFunction(F))
return false;
SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE();
LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
DI = &getAnalysis<DependenceAnalysisWrapperPass>().getDI();
DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
ORE = &getAnalysis<OptimizationRemarkEmitterWrapperPass>().getORE();
PreserveLCSSA = mustPreserveAnalysisID(LCSSAID);
// 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, F);
}
return Changed;
}
bool isComputableLoopNest(LoopVector LoopList) {
for (Loop *L : LoopList) {
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(const LoopVector &LoopList) {
// TODO: Add a better heuristic to select the loop to be interchanged based
// on the dependence matrix. Currently we select the innermost loop.
return LoopList.size() - 1;
}
bool processLoopList(LoopVector LoopList, Function &F) {
bool Changed = false;
unsigned LoopNestDepth = LoopList.size();
if (LoopNestDepth < 2) {
DEBUG(dbgs() << "Loop doesn't contain minimum nesting level.\n");
return false;
}
if (LoopNestDepth > MaxLoopNestDepth) {
DEBUG(dbgs() << "Cannot handle loops of depth greater than "
<< MaxLoopNestDepth << "\n");
return false;
}
if (!isComputableLoopNest(LoopList)) {
DEBUG(dbgs() << "Not valid loop candidate for interchange\n");
return false;
}
DEBUG(dbgs() << "Processing LoopList of size = " << LoopNestDepth << "\n");
CharMatrix DependencyMatrix;
Loop *OuterMostLoop = *(LoopList.begin());
if (!populateDependencyMatrix(DependencyMatrix, LoopNestDepth,
OuterMostLoop, DI)) {
DEBUG(dbgs() << "Populating dependency matrix failed\n");
return false;
}
#ifdef DUMP_DEP_MATRICIES
DEBUG(dbgs() << "Dependence before interchange\n");
printDepMatrix(DependencyMatrix);
#endif
BasicBlock *OuterMostLoopLatch = OuterMostLoop->getLoopLatch();
BranchInst *OuterMostLoopLatchBI =
dyn_cast<BranchInst>(OuterMostLoopLatch->getTerminator());
if (!OuterMostLoopLatchBI || OuterMostLoopLatchBI->getNumSuccessors() != 2)
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);
if (isa<PHINode>(LoopNestExit->begin())) {
DEBUG(dbgs() << "PHI Nodes in loop nest exit is not handled for now "
"since on failure all loops branch to loop nest exit.\n");
return false;
}
unsigned SelecLoopId = selectLoopForInterchange(LoopList);
// Move the selected loop outwards to the best possible 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
interChangeDependencies(DependencyMatrix, i, i - 1);
#ifdef DUMP_DEP_MATRICIES
DEBUG(dbgs() << "Dependence after interchange\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 Inner Loop Id = " << InnerLoopId
<< " and OuterLoopId = " << OuterLoopId << "\n");
Loop *InnerLoop = LoopList[InnerLoopId];
Loop *OuterLoop = LoopList[OuterLoopId];
LoopInterchangeLegality LIL(OuterLoop, InnerLoop, SE, LI, DT,
PreserveLCSSA, ORE);
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, ORE);
if (!LIP.isProfitable(InnerLoopId, OuterLoopId, DependencyMatrix)) {
DEBUG(dbgs() << "Interchanging loops not profitable.\n");
return false;
}
ORE->emit([&]() {
return OptimizationRemark(DEBUG_TYPE, "Interchanged",
InnerLoop->getStartLoc(),
InnerLoop->getHeader())
<< "Loop interchanged with enclosing loop.";
});
LoopInterchangeTransform LIT(OuterLoop, InnerLoop, SE, LI, DT,
LoopNestExit, LIL.hasInnerLoopReduction());
LIT.transform();
DEBUG(dbgs() << "Loops interchanged.\n");
return true;
}
};
} // end anonymous namespace
bool LoopInterchangeLegality::areAllUsesReductions(Instruction *Ins, Loop *L) {
return llvm::none_of(Ins->users(), [=](User *U) -> bool {
auto *UserIns = dyn_cast<PHINode>(U);
RecurrenceDescriptor RD;
return !UserIns || !RecurrenceDescriptor::isReductionPHI(UserIns, L, RD);
});
}
bool LoopInterchangeLegality::containsUnsafeInstructionsInHeader(
BasicBlock *BB) {
for (auto I = BB->begin(), E = BB->end(); I != E; ++I) {
// Load corresponding to reduction PHI's are safe while concluding if
// tightly nested.
if (LoadInst *L = dyn_cast<LoadInst>(I)) {
if (!areAllUsesReductions(L, InnerLoop))
return true;
} else if (I->mayHaveSideEffects() || I->mayReadFromMemory())
return true;
}
return false;
}
bool LoopInterchangeLegality::containsUnsafeInstructionsInLatch(
BasicBlock *BB) {
for (auto I = BB->begin(), E = BB->end(); I != E; ++I) {
// Stores corresponding to reductions are safe while concluding if tightly
// nested.
if (StoreInst *L = dyn_cast<StoreInst>(I)) {
if (!isa<PHINode>(L->getOperand(0)))
return true;
} else 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;
for (BasicBlock *Succ : OuterLoopHeaderBI->successors())
if (Succ != InnerLoopPreHeader && Succ != 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 doesn't contain any unsafe instructions.
if (containsUnsafeInstructionsInHeader(OuterLoopHeader) ||
containsUnsafeInstructionsInLatch(OuterLoopLatch))
return false;
DEBUG(dbgs() << "Loops are perfectly nested\n");
// We have a perfect loop nest.
return true;
}
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;
}
bool LoopInterchangeLegality::findInductionAndReductions(
Loop *L, SmallVector<PHINode *, 8> &Inductions,
SmallVector<PHINode *, 8> &Reductions) {
if (!L->getLoopLatch() || !L->getLoopPredecessor())
return false;
for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ++I) {
RecurrenceDescriptor RD;
InductionDescriptor ID;
PHINode *PHI = cast<PHINode>(I);
if (InductionDescriptor::isInductionPHI(PHI, L, SE, ID))
Inductions.push_back(PHI);
else if (RecurrenceDescriptor::isReductionPHI(PHI, L, RD))
Reductions.push_back(PHI);
else {
DEBUG(
dbgs() << "Failed to recognize PHI as an induction or reduction.\n");
return false;
}
}
return true;
}
static bool containsSafePHI(BasicBlock *Block, bool isOuterLoopExitBlock) {
for (auto I = Block->begin(); isa<PHINode>(I); ++I) {
PHINode *PHI = cast<PHINode>(I);
// Reduction lcssa phi will have only 1 incoming block that from loop latch.
if (PHI->getNumIncomingValues() > 1)
return false;
Instruction *Ins = dyn_cast<Instruction>(PHI->getIncomingValue(0));
if (!Ins)
return false;
// Incoming value for lcssa phi's in outer loop exit can only be inner loop
// exits lcssa phi else it would not be tightly nested.
if (!isa<PHINode>(Ins) && isOuterLoopExitBlock)
return false;
}
return true;
}
static BasicBlock *getLoopLatchExitBlock(BasicBlock *LatchBlock,
BasicBlock *LoopHeader) {
if (BranchInst *BI = dyn_cast<BranchInst>(LatchBlock->getTerminator())) {
assert(BI->getNumSuccessors() == 2 &&
"Branch leaving loop latch must have 2 successors");
for (BasicBlock *Succ : BI->successors()) {
if (Succ == LoopHeader)
continue;
return Succ;
}
}
return nullptr;
}
// 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 *InnerLoopLatch = InnerLoop->getLoopLatch();
BasicBlock *OuterLoopLatch = OuterLoop->getLoopLatch();
BasicBlock *OuterLoopHeader = OuterLoop->getHeader();
PHINode *InnerInductionVar;
SmallVector<PHINode *, 8> Inductions;
SmallVector<PHINode *, 8> Reductions;
if (!findInductionAndReductions(InnerLoop, Inductions, Reductions)) {
DEBUG(dbgs() << "Only inner loops with induction or reduction PHI nodes "
<< "are supported currently.\n");
ORE->emit([&]() {
return OptimizationRemarkMissed(DEBUG_TYPE, "UnsupportedPHIInner",
InnerLoop->getStartLoc(),
InnerLoop->getHeader())
<< "Only inner loops with induction or reduction PHI nodes can be"
" interchange currently.";
});
return true;
}
// TODO: Currently we handle only loops with 1 induction variable.
if (Inductions.size() != 1) {
DEBUG(dbgs() << "We currently only support loops with 1 induction variable."
<< "Failed to interchange due to current limitation\n");
ORE->emit([&]() {
return OptimizationRemarkMissed(DEBUG_TYPE, "MultiInductionInner",
InnerLoop->getStartLoc(),
InnerLoop->getHeader())
<< "Only inner loops with 1 induction variable can be "
"interchanged currently.";
});
return true;
}
if (Reductions.size() > 0)
InnerLoopHasReduction = true;
InnerInductionVar = Inductions.pop_back_val();
Reductions.clear();
if (!findInductionAndReductions(OuterLoop, Inductions, Reductions)) {
DEBUG(dbgs() << "Only outer loops with induction or reduction PHI nodes "
<< "are supported currently.\n");
ORE->emit([&]() {
return OptimizationRemarkMissed(DEBUG_TYPE, "UnsupportedPHIOuter",
OuterLoop->getStartLoc(),
OuterLoop->getHeader())
<< "Only outer loops with induction or reduction PHI nodes can be"
" interchanged currently.";
});
return true;
}
// Outer loop cannot have reduction because then loops will not be tightly
// nested.
if (!Reductions.empty()) {
DEBUG(dbgs() << "Outer loops with reductions are not supported "
<< "currently.\n");
ORE->emit([&]() {
return OptimizationRemarkMissed(DEBUG_TYPE, "ReductionsOuter",
OuterLoop->getStartLoc(),
OuterLoop->getHeader())
<< "Outer loops with reductions cannot be interchangeed "
"currently.";
});
return true;
}
// TODO: Currently we handle only loops with 1 induction variable.
if (Inductions.size() != 1) {
DEBUG(dbgs() << "Loops with more than 1 induction variables are not "
<< "supported currently.\n");
ORE->emit([&]() {
return OptimizationRemarkMissed(DEBUG_TYPE, "MultiIndutionOuter",
OuterLoop->getStartLoc(),
OuterLoop->getHeader())
<< "Only outer loops with 1 induction variable can be "
"interchanged currently.";
});
return true;
}
// TODO: Triangular loops are not handled for now.
if (!isLoopStructureUnderstood(InnerInductionVar)) {
DEBUG(dbgs() << "Loop structure not understood by pass\n");
ORE->emit([&]() {
return OptimizationRemarkMissed(DEBUG_TYPE, "UnsupportedStructureInner",
InnerLoop->getStartLoc(),
InnerLoop->getHeader())
<< "Inner loop structure not understood currently.";
});
return true;
}
// TODO: We only handle LCSSA PHI's corresponding to reduction for now.
BasicBlock *LoopExitBlock =
getLoopLatchExitBlock(OuterLoopLatch, OuterLoopHeader);
if (!LoopExitBlock || !containsSafePHI(LoopExitBlock, true)) {
DEBUG(dbgs() << "Can only handle LCSSA PHIs in outer loops currently.\n");
ORE->emit([&]() {
return OptimizationRemarkMissed(DEBUG_TYPE, "NoLCSSAPHIOuter",
OuterLoop->getStartLoc(),
OuterLoop->getHeader())
<< "Only outer loops with LCSSA PHIs can be interchange "
"currently.";
});
return true;
}
LoopExitBlock = getLoopLatchExitBlock(InnerLoopLatch, InnerLoopHeader);
if (!LoopExitBlock || !containsSafePHI(LoopExitBlock, false)) {
DEBUG(dbgs() << "Can only handle LCSSA PHIs in inner loops currently.\n");
ORE->emit([&]() {
return OptimizationRemarkMissed(DEBUG_TYPE, "NoLCSSAPHIOuterInner",
InnerLoop->getStartLoc(),
InnerLoop->getHeader())
<< "Only inner loops with LCSSA PHIs can be interchange "
"currently.";
});
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;
// }
// }
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) {
DEBUG(dbgs() << "Did not find an instruction to increment the induction "
<< "variable.\n");
ORE->emit([&]() {
return OptimizationRemarkMissed(DEBUG_TYPE, "NoIncrementInInner",
InnerLoop->getStartLoc(),
InnerLoop->getHeader())
<< "The inner loop does not increment the induction variable.";
});
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.
bool FoundInduction = false;
for (const Instruction &I : llvm::reverse(*InnerLoopLatch)) {
if (isa<BranchInst>(I) || isa<CmpInst>(I) || isa<TruncInst>(I) ||
isa<ZExtInst>(I))
continue;
// We found an instruction. If this is not induction variable then it is not
// safe to split this loop latch.
if (!I.isIdenticalTo(InnerIndexVarInc)) {
DEBUG(dbgs() << "Found unsupported instructions between induction "
<< "variable increment and branch.\n");
ORE->emit([&]() {
return OptimizationRemarkMissed(
DEBUG_TYPE, "UnsupportedInsBetweenInduction",
InnerLoop->getStartLoc(), InnerLoop->getHeader())
<< "Found unsupported instruction between induction variable "
"increment and branch.";
});
return true;
}
FoundInduction = true;
break;
}
// The loop latch ended and we didn't find the induction variable return as
// current limitation.
if (!FoundInduction) {
DEBUG(dbgs() << "Did not find the induction variable.\n");
ORE->emit([&]() {
return OptimizationRemarkMissed(DEBUG_TYPE, "NoIndutionVariable",
InnerLoop->getStartLoc(),
InnerLoop->getHeader())
<< "Did not find the induction variable.";
});
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");
ORE->emit([&]() {
return OptimizationRemarkMissed(DEBUG_TYPE, "Dependence",
InnerLoop->getStartLoc(),
InnerLoop->getHeader())
<< "Cannot interchange loops due to dependences.";
});
return false;
}
// Check if outer and inner loop contain legal instructions only.
for (auto *BB : OuterLoop->blocks())
for (Instruction &I : *BB)
if (CallInst *CI = dyn_cast<CallInst>(&I)) {
// readnone functions do not prevent interchanging.
if (CI->doesNotReadMemory())
continue;
DEBUG(dbgs() << "Loops with call instructions cannot be interchanged "
<< "safely.");
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, DT, LI, PreserveLCSSA);
}
if (!InnerLoopPreHeader || InnerLoopPreHeader == InnerLoop->getHeader() ||
InnerLoopPreHeader == OuterLoop->getHeader()) {
InnerLoopPreHeader =
InsertPreheaderForLoop(InnerLoop, DT, LI, PreserveLCSSA);
}
// 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;
}
// Check if the loops are tightly nested.
if (!tightlyNested(OuterLoop, InnerLoop)) {
DEBUG(dbgs() << "Loops not tightly nested\n");
ORE->emit([&]() {
return OptimizationRemarkMissed(DEBUG_TYPE, "NotTightlyNested",
InnerLoop->getStartLoc(),
InnerLoop->getHeader())
<< "Cannot interchange loops because they are not tightly "
"nested.";
});
return false;
}
return true;
}
int LoopInterchangeProfitability::getInstrOrderCost() {
unsigned GoodOrder, BadOrder;
BadOrder = GoodOrder = 0;
for (BasicBlock *BB : InnerLoop->blocks()) {
for (Instruction &Ins : *BB) {
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 isProfitableForVectorization(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.
for (auto &Row : DepMatrix) {
if (Row[InnerLoopId] != 'S' && Row[InnerLoopId] != 'I')
return false;
// TODO: We need to improve this heuristic.
if (Row[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 profitability 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 = getInstrOrderCost();
DEBUG(dbgs() << "Cost = " << Cost << "\n");
if (Cost < -LoopInterchangeCostThreshold)
return true;
// It is not profitable as per current cache profitability model. But check if
// we can move this loop outside to improve parallelism.
if (isProfitableForVectorization(InnerLoopId, OuterLoopId, DepMatrix))
return true;
ORE->emit([&]() {
return OptimizationRemarkMissed(DEBUG_TYPE, "InterchangeNotProfitable",
InnerLoop->getStartLoc(),
InnerLoop->getHeader())
<< "Interchanging loops is too costly (cost="
<< ore::NV("Cost", Cost) << ", threshold="
<< ore::NV("Threshold", LoopInterchangeCostThreshold)
<< ") and it does not improve parallelism.";
});
return false;
}
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;
}
}
llvm_unreachable("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);
}
while (!InnerLoop->empty())
OuterLoop->addChildLoop(InnerLoop->removeChildLoop(InnerLoop->begin()));
InnerLoop->addChildLoop(OuterLoop);
}
bool LoopInterchangeTransform::transform() {
bool Transformed = false;
Instruction *InnerIndexVar;
if (InnerLoop->getSubLoops().empty()) {
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));
// Ensure that InductionPHI is the first Phi node as required by
// splitInnerLoopHeader
if (&InductionPHI->getParent()->front() != InductionPHI)
InductionPHI->moveBefore(&InductionPHI->getParent()->front());
// 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 separate 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::splitInnerLoopHeader() {
// Split the inner loop header out. Here make sure that the reduction PHI's
// stay in the innerloop body.
BasicBlock *InnerLoopHeader = InnerLoop->getHeader();
BasicBlock *InnerLoopPreHeader = InnerLoop->getLoopPreheader();
SplitBlock(InnerLoopHeader, InnerLoopHeader->getFirstNonPHI(), DT, LI);
if (InnerLoopHasReduction) {
// Adjust Reduction PHI's in the block. The induction PHI must be the first
// PHI in InnerLoopHeader for this to work.
SmallVector<PHINode *, 8> PHIVec;
for (auto I = std::next(InnerLoopHeader->begin()); isa<PHINode>(I); ++I) {
PHINode *PHI = dyn_cast<PHINode>(I);
Value *V = PHI->getIncomingValueForBlock(InnerLoopPreHeader);
PHI->replaceAllUsesWith(V);
PHIVec.push_back((PHI));
}
for (PHINode *P : PHIVec) {
P->eraseFromParent();
}
}
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->getIterator(), FromList, FromList.begin(),
FromBB->getTerminator()->getIterator());
}
void LoopInterchangeTransform::updateIncomingBlock(BasicBlock *CurrBlock,
BasicBlock *OldPred,
BasicBlock *NewPred) {
for (auto I = CurrBlock->begin(); isa<PHINode>(I); ++I) {
PHINode *PHI = cast<PHINode>(I);
unsigned Num = PHI->getNumIncomingValues();
for (unsigned i = 0; i < Num; ++i) {
if (PHI->getIncomingBlock(i) == OldPred)
PHI->setIncomingBlock(i, NewPred);
}
}
}
/// \brief Update BI to jump to NewBB instead of OldBB. Records updates to
/// the dominator tree in DTUpdates, if DT should be preserved.
static void updateSuccessor(BranchInst *BI, BasicBlock *OldBB,
BasicBlock *NewBB,
std::vector<DominatorTree::UpdateType> &DTUpdates) {
assert(llvm::count_if(BI->successors(),
[OldBB](BasicBlock *BB) { return BB == OldBB; }) < 2 &&
"BI must jump to OldBB at most once.");
for (unsigned i = 0, e = BI->getNumSuccessors(); i < e; ++i) {
if (BI->getSuccessor(i) == OldBB) {
BI->setSuccessor(i, NewBB);
DTUpdates.push_back(
{DominatorTree::UpdateKind::Insert, BI->getParent(), NewBB});
DTUpdates.push_back(
{DominatorTree::UpdateKind::Delete, BI->getParent(), OldBB});
break;
}
}
}
bool LoopInterchangeTransform::adjustLoopBranches() {
DEBUG(dbgs() << "adjustLoopBranches called\n");
std::vector<DominatorTree::UpdateType> DTUpdates;
// 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 *InnerLoopHeaderSuccessor = InnerLoopHeader->getUniqueSuccessor();
if (!InnerLoopHeaderSuccessor)
return false;
// Adjust Loop Preheader and headers
updateSuccessor(OuterLoopPredecessorBI, OuterLoopPreHeader,
InnerLoopPreHeader, DTUpdates);
updateSuccessor(OuterLoopHeaderBI, OuterLoopLatch, LoopExit, DTUpdates);
updateSuccessor(OuterLoopHeaderBI, InnerLoopPreHeader,
InnerLoopHeaderSuccessor, DTUpdates);
// Adjust reduction PHI's now that the incoming block has changed.
updateIncomingBlock(InnerLoopHeaderSuccessor, InnerLoopHeader,
OuterLoopHeader);
updateSuccessor(InnerLoopHeaderBI, InnerLoopHeaderSuccessor,
OuterLoopPreHeader, DTUpdates);
// -------------Adjust loop latches-----------
if (InnerLoopLatchBI->getSuccessor(0) == InnerLoopHeader)
InnerLoopLatchSuccessor = InnerLoopLatchBI->getSuccessor(1);
else
InnerLoopLatchSuccessor = InnerLoopLatchBI->getSuccessor(0);
updateSuccessor(InnerLoopLatchPredecessorBI, InnerLoopLatch,
InnerLoopLatchSuccessor, DTUpdates);
// Adjust PHI nodes in InnerLoopLatchSuccessor. Update all uses of PHI with
// the value and remove this PHI node from inner loop.
SmallVector<PHINode *, 8> LcssaVec;
for (auto I = InnerLoopLatchSuccessor->begin(); isa<PHINode>(I); ++I) {
PHINode *LcssaPhi = cast<PHINode>(I);
LcssaVec.push_back(LcssaPhi);
}
for (PHINode *P : LcssaVec) {
Value *Incoming = P->getIncomingValueForBlock(InnerLoopLatch);
P->replaceAllUsesWith(Incoming);
P->eraseFromParent();
}
if (OuterLoopLatchBI->getSuccessor(0) == OuterLoopHeader)
OuterLoopLatchSuccessor = OuterLoopLatchBI->getSuccessor(1);
else
OuterLoopLatchSuccessor = OuterLoopLatchBI->getSuccessor(0);
updateSuccessor(InnerLoopLatchBI, InnerLoopLatchSuccessor,
OuterLoopLatchSuccessor, DTUpdates);
updateSuccessor(OuterLoopLatchBI, OuterLoopLatchSuccessor, InnerLoopLatch,
DTUpdates);
updateIncomingBlock(OuterLoopLatchSuccessor, OuterLoopLatch, InnerLoopLatch);
DT->applyUpdates(DTUpdates);
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());
// These instructions should now be executed inside the loop.
// Move instruction into a new block after outer header.
moveBBContents(InnerLoopPreHeader, OuterLoopHeader->getTerminator());
// 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_PASS_DEPENDENCY(AAResultsWrapperPass)
INITIALIZE_PASS_DEPENDENCY(DependenceAnalysisWrapperPass)
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)
INITIALIZE_PASS_DEPENDENCY(LoopSimplify)
INITIALIZE_PASS_DEPENDENCY(LCSSAWrapperPass)
INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
INITIALIZE_PASS_DEPENDENCY(OptimizationRemarkEmitterWrapperPass)
INITIALIZE_PASS_END(LoopInterchange, "loop-interchange",
"Interchanges loops for cache reuse", false, false)
Pass *llvm::createLoopInterchangePass() { return new LoopInterchange(); }
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