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//===- LoopDistribute.cpp - Loop Distribution Pass ------------------------===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// This file implements the Loop Distribution Pass. Its main focus is to
// distribute loops that cannot be vectorized due to dependence cycles. It
// tries to isolate the offending dependences into a new loop allowing
// vectorization of the remaining parts.
// For dependence analysis, the pass uses the LoopVectorizer's
// LoopAccessAnalysis. Because this analysis presumes no change in the order of
// memory operations, special care is taken to preserve the lexical order of
// these operations.
// Similarly to the Vectorizer, the pass also supports loop versioning to
// run-time disambiguate potentially overlapping arrays.
#include "llvm/Transforms/Scalar/LoopDistribute.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/DepthFirstIterator.h"
#include "llvm/ADT/EquivalenceClasses.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/ADT/Twine.h"
#include "llvm/ADT/iterator_range.h"
#include "llvm/Analysis/AssumptionCache.h"
#include "llvm/Analysis/GlobalsModRef.h"
#include "llvm/Analysis/LoopAccessAnalysis.h"
#include "llvm/Analysis/LoopAnalysisManager.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/OptimizationRemarkEmitter.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/Analysis/TargetTransformInfo.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/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/Metadata.h"
#include "llvm/IR/PassManager.h"
#include "llvm/IR/Value.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/CommandLine.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/Cloning.h"
#include "llvm/Transforms/Utils/LoopUtils.h"
#include "llvm/Transforms/Utils/LoopVersioning.h"
#include "llvm/Transforms/Utils/ValueMapper.h"
#include <cassert>
#include <functional>
#include <list>
#include <tuple>
#include <utility>
using namespace llvm;
#define LDIST_NAME "loop-distribute"
/// @{
/// Metadata attribute names
static const char *const LLVMLoopDistributeFollowupAll =
static const char *const LLVMLoopDistributeFollowupCoincident =
static const char *const LLVMLoopDistributeFollowupSequential =
static const char *const LLVMLoopDistributeFollowupFallback =
/// @}
static cl::opt<bool>
LDistVerify("loop-distribute-verify", cl::Hidden,
cl::desc("Turn on DominatorTree and LoopInfo verification "
"after Loop Distribution"),
static cl::opt<bool> DistributeNonIfConvertible(
"loop-distribute-non-if-convertible", cl::Hidden,
cl::desc("Whether to distribute into a loop that may not be "
"if-convertible by the loop vectorizer"),
static cl::opt<unsigned> DistributeSCEVCheckThreshold(
"loop-distribute-scev-check-threshold", cl::init(8), cl::Hidden,
cl::desc("The maximum number of SCEV checks allowed for Loop "
static cl::opt<unsigned> PragmaDistributeSCEVCheckThreshold(
"loop-distribute-scev-check-threshold-with-pragma", cl::init(128),
"The maximum number of SCEV checks allowed for Loop "
"Distribution for loop marked with #pragma loop distribute(enable)"));
static cl::opt<bool> EnableLoopDistribute(
"enable-loop-distribute", cl::Hidden,
cl::desc("Enable the new, experimental LoopDistribution Pass"),
STATISTIC(NumLoopsDistributed, "Number of loops distributed");
namespace {
/// Maintains the set of instructions of the loop for a partition before
/// cloning. After cloning, it hosts the new loop.
class InstPartition {
using InstructionSet = SmallPtrSet<Instruction *, 8>;
InstPartition(Instruction *I, Loop *L, bool DepCycle = false)
: DepCycle(DepCycle), OrigLoop(L) {
/// Returns whether this partition contains a dependence cycle.
bool hasDepCycle() const { return DepCycle; }
/// Adds an instruction to this partition.
void add(Instruction *I) { Set.insert(I); }
/// Collection accessors.
InstructionSet::iterator begin() { return Set.begin(); }
InstructionSet::iterator end() { return Set.end(); }
InstructionSet::const_iterator begin() const { return Set.begin(); }
InstructionSet::const_iterator end() const { return Set.end(); }
bool empty() const { return Set.empty(); }
/// Moves this partition into \p Other. This partition becomes empty
/// after this.
void moveTo(InstPartition &Other) {
Other.Set.insert(Set.begin(), Set.end());
Other.DepCycle |= DepCycle;
/// Populates the partition with a transitive closure of all the
/// instructions that the seeded instructions dependent on.
void populateUsedSet() {
// FIXME: We currently don't use control-dependence but simply include all
// blocks (possibly empty at the end) and let simplifycfg mostly clean this
// up.
for (auto *B : OrigLoop->getBlocks())
// Follow the use-def chains to form a transitive closure of all the
// instructions that the originally seeded instructions depend on.
SmallVector<Instruction *, 8> Worklist(Set.begin(), Set.end());
while (!Worklist.empty()) {
Instruction *I = Worklist.pop_back_val();
// Insert instructions from the loop that we depend on.
for (Value *V : I->operand_values()) {
auto *I = dyn_cast<Instruction>(V);
if (I && OrigLoop->contains(I->getParent()) && Set.insert(I).second)
/// Clones the original loop.
/// Updates LoopInfo and DominatorTree using the information that block \p
/// LoopDomBB dominates the loop.
Loop *cloneLoopWithPreheader(BasicBlock *InsertBefore, BasicBlock *LoopDomBB,
unsigned Index, LoopInfo *LI,
DominatorTree *DT) {
ClonedLoop = ::cloneLoopWithPreheader(InsertBefore, LoopDomBB, OrigLoop,
VMap, Twine(".ldist") + Twine(Index),
LI, DT, ClonedLoopBlocks);
return ClonedLoop;
/// The cloned loop. If this partition is mapped to the original loop,
/// this is null.
const Loop *getClonedLoop() const { return ClonedLoop; }
/// Returns the loop where this partition ends up after distribution.
/// If this partition is mapped to the original loop then use the block from
/// the loop.
Loop *getDistributedLoop() const {
return ClonedLoop ? ClonedLoop : OrigLoop;
/// The VMap that is populated by cloning and then used in
/// remapinstruction to remap the cloned instructions.
ValueToValueMapTy &getVMap() { return VMap; }
/// Remaps the cloned instructions using VMap.
void remapInstructions() {
remapInstructionsInBlocks(ClonedLoopBlocks, VMap);
/// Based on the set of instructions selected for this partition,
/// removes the unnecessary ones.
void removeUnusedInsts() {
SmallVector<Instruction *, 8> Unused;
for (auto *Block : OrigLoop->getBlocks())
for (auto &Inst : *Block)
if (!Set.count(&Inst)) {
Instruction *NewInst = &Inst;
if (!VMap.empty())
NewInst = cast<Instruction>(VMap[NewInst]);
assert(!isa<BranchInst>(NewInst) &&
"Branches are marked used early on");
// Delete the instructions backwards, as it has a reduced likelihood of
// having to update as many def-use and use-def chains.
for (auto *Inst : reverse(Unused)) {
if (!Inst->use_empty())
void print() const {
if (DepCycle)
dbgs() << " (cycle)\n";
for (auto *I : Set)
// Prefix with the block name.
dbgs() << " " << I->getParent()->getName() << ":" << *I << "\n";
void printBlocks() const {
for (auto *BB : getDistributedLoop()->getBlocks())
dbgs() << *BB;
/// Instructions from OrigLoop selected for this partition.
InstructionSet Set;
/// Whether this partition contains a dependence cycle.
bool DepCycle;
/// The original loop.
Loop *OrigLoop;
/// The cloned loop. If this partition is mapped to the original loop,
/// this is null.
Loop *ClonedLoop = nullptr;
/// The blocks of ClonedLoop including the preheader. If this
/// partition is mapped to the original loop, this is empty.
SmallVector<BasicBlock *, 8> ClonedLoopBlocks;
/// These gets populated once the set of instructions have been
/// finalized. If this partition is mapped to the original loop, these are not
/// set.
ValueToValueMapTy VMap;
/// Holds the set of Partitions. It populates them, merges them and then
/// clones the loops.
class InstPartitionContainer {
using InstToPartitionIdT = DenseMap<Instruction *, int>;
InstPartitionContainer(Loop *L, LoopInfo *LI, DominatorTree *DT)
: L(L), LI(LI), DT(DT) {}
/// Returns the number of partitions.
unsigned getSize() const { return PartitionContainer.size(); }
/// Adds \p Inst into the current partition if that is marked to
/// contain cycles. Otherwise start a new partition for it.
void addToCyclicPartition(Instruction *Inst) {
// If the current partition is non-cyclic. Start a new one.
if (PartitionContainer.empty() || !PartitionContainer.back().hasDepCycle())
PartitionContainer.emplace_back(Inst, L, /*DepCycle=*/true);
/// Adds \p Inst into a partition that is not marked to contain
/// dependence cycles.
// Initially we isolate memory instructions into as many partitions as
// possible, then later we may merge them back together.
void addToNewNonCyclicPartition(Instruction *Inst) {
PartitionContainer.emplace_back(Inst, L);
/// Merges adjacent non-cyclic partitions.
/// The idea is that we currently only want to isolate the non-vectorizable
/// partition. We could later allow more distribution among these partition
/// too.
void mergeAdjacentNonCyclic() {
[](const InstPartition *P) { return !P->hasDepCycle(); });
/// If a partition contains only conditional stores, we won't vectorize
/// it. Try to merge it with a previous cyclic partition.
void mergeNonIfConvertible() {
mergeAdjacentPartitionsIf([&](const InstPartition *Partition) {
if (Partition->hasDepCycle())
return true;
// Now, check if all stores are conditional in this partition.
bool seenStore = false;
for (auto *Inst : *Partition)
if (isa<StoreInst>(Inst)) {
seenStore = true;
if (!LoopAccessInfo::blockNeedsPredication(Inst->getParent(), L, DT))
return false;
return seenStore;
/// Merges the partitions according to various heuristics.
void mergeBeforePopulating() {
if (!DistributeNonIfConvertible)
/// Merges partitions in order to ensure that no loads are duplicated.
/// We can't duplicate loads because that could potentially reorder them.
/// LoopAccessAnalysis provides dependency information with the context that
/// the order of memory operation is preserved.
/// Return if any partitions were merged.
bool mergeToAvoidDuplicatedLoads() {
using LoadToPartitionT = DenseMap<Instruction *, InstPartition *>;
using ToBeMergedT = EquivalenceClasses<InstPartition *>;
LoadToPartitionT LoadToPartition;
ToBeMergedT ToBeMerged;
// Step through the partitions and create equivalence between partitions
// that contain the same load. Also put partitions in between them in the
// same equivalence class to avoid reordering of memory operations.
for (PartitionContainerT::iterator I = PartitionContainer.begin(),
E = PartitionContainer.end();
I != E; ++I) {
auto *PartI = &*I;
// If a load occurs in two partitions PartI and PartJ, merge all
// partitions (PartI, PartJ] into PartI.
for (Instruction *Inst : *PartI)
if (isa<LoadInst>(Inst)) {
bool NewElt;
LoadToPartitionT::iterator LoadToPart;
std::tie(LoadToPart, NewElt) =
LoadToPartition.insert(std::make_pair(Inst, PartI));
if (!NewElt) {
<< "Merging partitions due to this load in multiple "
<< "partitions: " << PartI << ", " << LoadToPart->second
<< "\n"
<< *Inst << "\n");
auto PartJ = I;
do {
ToBeMerged.unionSets(PartI, &*PartJ);
} while (&*PartJ != LoadToPart->second);
if (ToBeMerged.empty())
return false;
// Merge the member of an equivalence class into its class leader. This
// makes the members empty.
for (ToBeMergedT::iterator I = ToBeMerged.begin(), E = ToBeMerged.end();
I != E; ++I) {
if (!I->isLeader())
auto PartI = I->getData();
for (auto *PartJ : make_range(std::next(ToBeMerged.member_begin(I)),
ToBeMerged.member_end())) {
// Remove the empty partitions.
[](const InstPartition &P) { return P.empty(); });
return true;
/// Sets up the mapping between instructions to partitions. If the
/// instruction is duplicated across multiple partitions, set the entry to -1.
void setupPartitionIdOnInstructions() {
int PartitionID = 0;
for (const auto &Partition : PartitionContainer) {
for (Instruction *Inst : Partition) {
bool NewElt;
InstToPartitionIdT::iterator Iter;
std::tie(Iter, NewElt) =
InstToPartitionId.insert(std::make_pair(Inst, PartitionID));
if (!NewElt)
Iter->second = -1;
/// Populates the partition with everything that the seeding
/// instructions require.
void populateUsedSet() {
for (auto &P : PartitionContainer)
/// This performs the main chunk of the work of cloning the loops for
/// the partitions.
void cloneLoops() {
BasicBlock *OrigPH = L->getLoopPreheader();
// At this point the predecessor of the preheader is either the memcheck
// block or the top part of the original preheader.
BasicBlock *Pred = OrigPH->getSinglePredecessor();
assert(Pred && "Preheader does not have a single predecessor");
BasicBlock *ExitBlock = L->getExitBlock();
assert(ExitBlock && "No single exit block");
Loop *NewLoop;
assert(!PartitionContainer.empty() && "at least two partitions expected");
// We're cloning the preheader along with the loop so we already made sure
// it was empty.
assert(&*OrigPH->begin() == OrigPH->getTerminator() &&
"preheader not empty");
// Preserve the original loop ID for use after the transformation.
MDNode *OrigLoopID = L->getLoopID();
// Create a loop for each partition except the last. Clone the original
// loop before PH along with adding a preheader for the cloned loop. Then
// update PH to point to the newly added preheader.
BasicBlock *TopPH = OrigPH;
unsigned Index = getSize() - 1;
for (auto &Part : llvm::drop_begin(llvm::reverse(PartitionContainer))) {
NewLoop = Part.cloneLoopWithPreheader(TopPH, Pred, Index, LI, DT);
Part.getVMap()[ExitBlock] = TopPH;
setNewLoopID(OrigLoopID, &Part);
TopPH = NewLoop->getLoopPreheader();
Pred->getTerminator()->replaceUsesOfWith(OrigPH, TopPH);
// Also set a new loop ID for the last loop.
setNewLoopID(OrigLoopID, &PartitionContainer.back());
// Now go in forward order and update the immediate dominator for the
// preheaders with the exiting block of the previous loop. Dominance
// within the loop is updated in cloneLoopWithPreheader.
for (auto Curr = PartitionContainer.cbegin(),
Next = std::next(PartitionContainer.cbegin()),
E = PartitionContainer.cend();
Next != E; ++Curr, ++Next)
/// Removes the dead instructions from the cloned loops.
void removeUnusedInsts() {
for (auto &Partition : PartitionContainer)
/// For each memory pointer, it computes the partitionId the pointer is
/// used in.
/// This returns an array of int where the I-th entry corresponds to I-th
/// entry in LAI.getRuntimePointerCheck(). If the pointer is used in multiple
/// partitions its entry is set to -1.
SmallVector<int, 8>
computePartitionSetForPointers(const LoopAccessInfo &LAI) {
const RuntimePointerChecking *RtPtrCheck = LAI.getRuntimePointerChecking();
unsigned N = RtPtrCheck->Pointers.size();
SmallVector<int, 8> PtrToPartitions(N);
for (unsigned I = 0; I < N; ++I) {
Value *Ptr = RtPtrCheck->Pointers[I].PointerValue;
auto Instructions =
LAI.getInstructionsForAccess(Ptr, RtPtrCheck->Pointers[I].IsWritePtr);
int &Partition = PtrToPartitions[I];
// First set it to uninitialized.
Partition = -2;
for (Instruction *Inst : Instructions) {
// Note that this could be -1 if Inst is duplicated across multiple
// partitions.
int ThisPartition = this->InstToPartitionId[Inst];
if (Partition == -2)
Partition = ThisPartition;
// -1 means belonging to multiple partitions.
else if (Partition == -1)
else if (Partition != (int)ThisPartition)
Partition = -1;
assert(Partition != -2 && "Pointer not belonging to any partition");
return PtrToPartitions;
void print(raw_ostream &OS) const {
unsigned Index = 0;
for (const auto &P : PartitionContainer) {
OS << "Partition " << Index++ << " (" << &P << "):\n";
void dump() const { print(dbgs()); }
#ifndef NDEBUG
friend raw_ostream &operator<<(raw_ostream &OS,
const InstPartitionContainer &Partitions) {
return OS;
void printBlocks() const {
unsigned Index = 0;
for (const auto &P : PartitionContainer) {
dbgs() << "\nPartition " << Index++ << " (" << &P << "):\n";
using PartitionContainerT = std::list<InstPartition>;
/// List of partitions.
PartitionContainerT PartitionContainer;
/// Mapping from Instruction to partition Id. If the instruction
/// belongs to multiple partitions the entry contains -1.
InstToPartitionIdT InstToPartitionId;
Loop *L;
LoopInfo *LI;
DominatorTree *DT;
/// The control structure to merge adjacent partitions if both satisfy
/// the \p Predicate.
template <class UnaryPredicate>
void mergeAdjacentPartitionsIf(UnaryPredicate Predicate) {
InstPartition *PrevMatch = nullptr;
for (auto I = PartitionContainer.begin(); I != PartitionContainer.end();) {
auto DoesMatch = Predicate(&*I);
if (PrevMatch == nullptr && DoesMatch) {
PrevMatch = &*I;
} else if (PrevMatch != nullptr && DoesMatch) {
I = PartitionContainer.erase(I);
} else {
PrevMatch = nullptr;
/// Assign new LoopIDs for the partition's cloned loop.
void setNewLoopID(MDNode *OrigLoopID, InstPartition *Part) {
std::optional<MDNode *> PartitionID = makeFollowupLoopID(
Part->hasDepCycle() ? LLVMLoopDistributeFollowupSequential
: LLVMLoopDistributeFollowupCoincident});
if (PartitionID) {
Loop *NewLoop = Part->getDistributedLoop();
/// For each memory instruction, this class maintains difference of the
/// number of unsafe dependences that start out from this instruction minus
/// those that end here.
/// By traversing the memory instructions in program order and accumulating this
/// number, we know whether any unsafe dependence crosses over a program point.
class MemoryInstructionDependences {
using Dependence = MemoryDepChecker::Dependence;
struct Entry {
Instruction *Inst;
unsigned NumUnsafeDependencesStartOrEnd = 0;
Entry(Instruction *Inst) : Inst(Inst) {}
using AccessesType = SmallVector<Entry, 8>;
AccessesType::const_iterator begin() const { return Accesses.begin(); }
AccessesType::const_iterator end() const { return Accesses.end(); }
const SmallVectorImpl<Instruction *> &Instructions,
const SmallVectorImpl<Dependence> &Dependences) {
Accesses.append(Instructions.begin(), Instructions.end());
LLVM_DEBUG(dbgs() << "Backward dependences:\n");
for (const auto &Dep : Dependences)
if (Dep.isPossiblyBackward()) {
// Note that the designations source and destination follow the program
// order, i.e. source is always first. (The direction is given by the
// DepType.)
LLVM_DEBUG(Dep.print(dbgs(), 2, Instructions));
AccessesType Accesses;
/// The actual class performing the per-loop work.
class LoopDistributeForLoop {
LoopDistributeForLoop(Loop *L, Function *F, LoopInfo *LI, DominatorTree *DT,
ScalarEvolution *SE, LoopAccessInfoManager &LAIs,
OptimizationRemarkEmitter *ORE)
: L(L), F(F), LI(LI), DT(DT), SE(SE), LAIs(LAIs), ORE(ORE) {
/// Try to distribute an inner-most loop.
bool processLoop() {
assert(L->isInnermost() && "Only process inner loops.");
LLVM_DEBUG(dbgs() << "\nLDist: In \""
<< L->getHeader()->getParent()->getName()
<< "\" checking " << *L << "\n");
// Having a single exit block implies there's also one exiting block.
if (!L->getExitBlock())
return fail("MultipleExitBlocks", "multiple exit blocks");
if (!L->isLoopSimplifyForm())
return fail("NotLoopSimplifyForm",
"loop is not in loop-simplify form");
if (!L->isRotatedForm())
return fail("NotBottomTested", "loop is not bottom tested");
BasicBlock *PH = L->getLoopPreheader();
LAI = &LAIs.getInfo(*L);
// Currently, we only distribute to isolate the part of the loop with
// dependence cycles to enable partial vectorization.
if (LAI->canVectorizeMemory())
return fail("MemOpsCanBeVectorized",
"memory operations are safe for vectorization");
auto *Dependences = LAI->getDepChecker().getDependences();
if (!Dependences || Dependences->empty())
return fail("NoUnsafeDeps", "no unsafe dependences to isolate");
InstPartitionContainer Partitions(L, LI, DT);
// First, go through each memory operation and assign them to consecutive
// partitions (the order of partitions follows program order). Put those
// with unsafe dependences into "cyclic" partition otherwise put each store
// in its own "non-cyclic" partition (we'll merge these later).
// Note that a memory operation (e.g. Load2 below) at a program point that
// has an unsafe dependence (Store3->Load1) spanning over it must be
// included in the same cyclic partition as the dependent operations. This
// is to preserve the original program order after distribution. E.g.:
// NumUnsafeDependencesStartOrEnd NumUnsafeDependencesActive
// Load1 -. 1 0->1
// Load2 | /Unsafe/ 0 1
// Store3 -' -1 1->0
// Load4 0 0
// NumUnsafeDependencesActive > 0 indicates this situation and in this case
// we just keep assigning to the same cyclic partition until
// NumUnsafeDependencesActive reaches 0.
const MemoryDepChecker &DepChecker = LAI->getDepChecker();
MemoryInstructionDependences MID(DepChecker.getMemoryInstructions(),
int NumUnsafeDependencesActive = 0;
for (const auto &InstDep : MID) {
Instruction *I = InstDep.Inst;
// We update NumUnsafeDependencesActive post-instruction, catch the
// start of a dependence directly via NumUnsafeDependencesStartOrEnd.
if (NumUnsafeDependencesActive ||
InstDep.NumUnsafeDependencesStartOrEnd > 0)
NumUnsafeDependencesActive += InstDep.NumUnsafeDependencesStartOrEnd;
assert(NumUnsafeDependencesActive >= 0 &&
"Negative number of dependences active");
// Add partitions for values used outside. These partitions can be out of
// order from the original program order. This is OK because if the
// partition uses a load we will merge this partition with the original
// partition of the load that we set up in the previous loop (see
// mergeToAvoidDuplicatedLoads).
auto DefsUsedOutside = findDefsUsedOutsideOfLoop(L);
for (auto *Inst : DefsUsedOutside)
LLVM_DEBUG(dbgs() << "Seeded partitions:\n" << Partitions);
if (Partitions.getSize() < 2)
return fail("CantIsolateUnsafeDeps",
"cannot isolate unsafe dependencies");
// Run the merge heuristics: Merge non-cyclic adjacent partitions since we
// should be able to vectorize these together.
LLVM_DEBUG(dbgs() << "\nMerged partitions:\n" << Partitions);
if (Partitions.getSize() < 2)
return fail("CantIsolateUnsafeDeps",
"cannot isolate unsafe dependencies");
// Now, populate the partitions with non-memory operations.
LLVM_DEBUG(dbgs() << "\nPopulated partitions:\n" << Partitions);
// In order to preserve original lexical order for loads, keep them in the
// partition that we set up in the MemoryInstructionDependences loop.
if (Partitions.mergeToAvoidDuplicatedLoads()) {
LLVM_DEBUG(dbgs() << "\nPartitions merged to ensure unique loads:\n"
<< Partitions);
if (Partitions.getSize() < 2)
return fail("CantIsolateUnsafeDeps",
"cannot isolate unsafe dependencies");
// Don't distribute the loop if we need too many SCEV run-time checks, or
// any if it's illegal.
const SCEVPredicate &Pred = LAI->getPSE().getPredicate();
if (LAI->hasConvergentOp() && !Pred.isAlwaysTrue()) {
return fail("RuntimeCheckWithConvergent",
"may not insert runtime check with convergent operation");
if (Pred.getComplexity() > (IsForced.value_or(false)
? PragmaDistributeSCEVCheckThreshold
: DistributeSCEVCheckThreshold))
return fail("TooManySCEVRuntimeChecks",
"too many SCEV run-time checks needed.\n");
if (!IsForced.value_or(false) && hasDisableAllTransformsHint(L))
return fail("HeuristicDisabled", "distribution heuristic disabled");
LLVM_DEBUG(dbgs() << "\nDistributing loop: " << *L << "\n");
// We're done forming the partitions set up the reverse mapping from
// instructions to partitions.
// If we need run-time checks, version the loop now.
auto PtrToPartition = Partitions.computePartitionSetForPointers(*LAI);
const auto *RtPtrChecking = LAI->getRuntimePointerChecking();
const auto &AllChecks = RtPtrChecking->getChecks();
auto Checks = includeOnlyCrossPartitionChecks(AllChecks, PtrToPartition,
if (LAI->hasConvergentOp() && !Checks.empty()) {
return fail("RuntimeCheckWithConvergent",
"may not insert runtime check with convergent operation");
// To keep things simple have an empty preheader before we version or clone
// the loop. (Also split if this has no predecessor, i.e. entry, because we
// rely on PH having a predecessor.)
if (!PH->getSinglePredecessor() || &*PH->begin() != PH->getTerminator())
SplitBlock(PH, PH->getTerminator(), DT, LI);
if (!Pred.isAlwaysTrue() || !Checks.empty()) {
assert(!LAI->hasConvergentOp() && "inserting illegal loop versioning");
MDNode *OrigLoopID = L->getLoopID();
LLVM_DEBUG(dbgs() << "\nPointers:\n");
LLVM_DEBUG(LAI->getRuntimePointerChecking()->printChecks(dbgs(), Checks));
LoopVersioning LVer(*LAI, Checks, L, LI, DT, SE);
// The unversioned loop will not be changed, so we inherit all attributes
// from the original loop, but remove the loop distribution metadata to
// avoid to distribute it again.
MDNode *UnversionedLoopID = *makeFollowupLoopID(
{LLVMLoopDistributeFollowupAll, LLVMLoopDistributeFollowupFallback},
"llvm.loop.distribute.", true);
// Create identical copies of the original loop for each partition and hook
// them up sequentially.
// Now, we remove the instruction from each loop that don't belong to that
// partition.
LLVM_DEBUG(dbgs() << "\nAfter removing unused Instrs:\n");
if (LDistVerify) {
// Report the success.
ORE->emit([&]() {
return OptimizationRemark(LDIST_NAME, "Distribute", L->getStartLoc(),
<< "distributed loop";
return true;
/// Provide diagnostics then \return with false.
bool fail(StringRef RemarkName, StringRef Message) {
LLVMContext &Ctx = F->getContext();
bool Forced = isForced().value_or(false);
LLVM_DEBUG(dbgs() << "Skipping; " << Message << "\n");
// With Rpass-missed report that distribution failed.
ORE->emit([&]() {
return OptimizationRemarkMissed(LDIST_NAME, "NotDistributed",
L->getStartLoc(), L->getHeader())
<< "loop not distributed: use -Rpass-analysis=loop-distribute for "
"more "
// With Rpass-analysis report why. This is on by default if distribution
// was requested explicitly.
Forced ? OptimizationRemarkAnalysis::AlwaysPrint : LDIST_NAME,
RemarkName, L->getStartLoc(), L->getHeader())
<< "loop not distributed: " << Message);
// Also issue a warning if distribution was requested explicitly but it
// failed.
if (Forced)
*F, L->getStartLoc(), "loop not distributed: failed "
"explicitly specified loop distribution"));
return false;
/// Return if distribution forced to be enabled/disabled for the loop.
/// If the optional has a value, it indicates whether distribution was forced
/// to be enabled (true) or disabled (false). If the optional has no value
/// distribution was not forced either way.
const std::optional<bool> &isForced() const { return IsForced; }
/// Filter out checks between pointers from the same partition.
/// \p PtrToPartition contains the partition number for pointers. Partition
/// number -1 means that the pointer is used in multiple partitions. In this
/// case we can't safely omit the check.
SmallVector<RuntimePointerCheck, 4> includeOnlyCrossPartitionChecks(
const SmallVectorImpl<RuntimePointerCheck> &AllChecks,
const SmallVectorImpl<int> &PtrToPartition,
const RuntimePointerChecking *RtPtrChecking) {
SmallVector<RuntimePointerCheck, 4> Checks;
copy_if(AllChecks, std::back_inserter(Checks),
[&](const RuntimePointerCheck &Check) {
for (unsigned PtrIdx1 : Check.first->Members)
for (unsigned PtrIdx2 : Check.second->Members)
// Only include this check if there is a pair of pointers
// that require checking and the pointers fall into
// separate partitions.
// (Note that we already know at this point that the two
// pointer groups need checking but it doesn't follow
// that each pair of pointers within the two groups need
// checking as well.
// In other words we don't want to include a check just
// because there is a pair of pointers between the two
// pointer groups that require checks and a different
// pair whose pointers fall into different partitions.)
if (RtPtrChecking->needsChecking(PtrIdx1, PtrIdx2) &&
PtrToPartition, PtrIdx1, PtrIdx2))
return true;
return false;
return Checks;
/// Check whether the loop metadata is forcing distribution to be
/// enabled/disabled.
void setForced() {
std::optional<const MDOperand *> Value =
findStringMetadataForLoop(L, "llvm.loop.distribute.enable");
if (!Value)
const MDOperand *Op = *Value;
assert(Op && mdconst::hasa<ConstantInt>(*Op) && "invalid metadata");
IsForced = mdconst::extract<ConstantInt>(*Op)->getZExtValue();
Loop *L;
Function *F;
// Analyses used.
LoopInfo *LI;
const LoopAccessInfo *LAI = nullptr;
DominatorTree *DT;
ScalarEvolution *SE;
LoopAccessInfoManager &LAIs;
OptimizationRemarkEmitter *ORE;
/// Indicates whether distribution is forced to be enabled/disabled for
/// the loop.
/// If the optional has a value, it indicates whether distribution was forced
/// to be enabled (true) or disabled (false). If the optional has no value
/// distribution was not forced either way.
std::optional<bool> IsForced;
} // end anonymous namespace
/// Shared implementation between new and old PMs.
static bool runImpl(Function &F, LoopInfo *LI, DominatorTree *DT,
ScalarEvolution *SE, OptimizationRemarkEmitter *ORE,
LoopAccessInfoManager &LAIs) {
// Build up a worklist of inner-loops to vectorize. This is necessary as the
// act of distributing a loop creates new loops and can invalidate iterators
// across the loops.
SmallVector<Loop *, 8> Worklist;
for (Loop *TopLevelLoop : *LI)
for (Loop *L : depth_first(TopLevelLoop))
// We only handle inner-most loops.
if (L->isInnermost())
// Now walk the identified inner loops.
bool Changed = false;
for (Loop *L : Worklist) {
LoopDistributeForLoop LDL(L, &F, LI, DT, SE, LAIs, ORE);
// If distribution was forced for the specific loop to be
// enabled/disabled, follow that. Otherwise use the global flag.
if (LDL.isForced().value_or(EnableLoopDistribute))
Changed |= LDL.processLoop();
// Process each loop nest in the function.
return Changed;
namespace {
/// The pass class.
class LoopDistributeLegacy : public FunctionPass {
static char ID;
LoopDistributeLegacy() : FunctionPass(ID) {
// The default is set by the caller.
bool runOnFunction(Function &F) override {
if (skipFunction(F))
return false;
auto *LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
auto *DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
auto *SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE();
auto *ORE = &getAnalysis<OptimizationRemarkEmitterWrapperPass>().getORE();
auto &LAIs = getAnalysis<LoopAccessLegacyAnalysis>().getLAIs();
return runImpl(F, LI, DT, SE, ORE, LAIs);
void getAnalysisUsage(AnalysisUsage &AU) const override {
} // end anonymous namespace
PreservedAnalyses LoopDistributePass::run(Function &F,
FunctionAnalysisManager &AM) {
auto &LI = AM.getResult<LoopAnalysis>(F);
auto &DT = AM.getResult<DominatorTreeAnalysis>(F);
auto &SE = AM.getResult<ScalarEvolutionAnalysis>(F);
auto &ORE = AM.getResult<OptimizationRemarkEmitterAnalysis>(F);
LoopAccessInfoManager &LAIs = AM.getResult<LoopAccessAnalysis>(F);
bool Changed = runImpl(F, &LI, &DT, &SE, &ORE, LAIs);
if (!Changed)
return PreservedAnalyses::all();
PreservedAnalyses PA;
return PA;
char LoopDistributeLegacy::ID;
static const char ldist_name[] = "Loop Distribution";
INITIALIZE_PASS_BEGIN(LoopDistributeLegacy, LDIST_NAME, ldist_name, false,
INITIALIZE_PASS_END(LoopDistributeLegacy, LDIST_NAME, ldist_name, false, false)
FunctionPass *llvm::createLoopDistributePass() { return new LoopDistributeLegacy(); }