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//===- LoopInfo.cpp - Natural Loop Calculator -----------------------------===//
// 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 defines the LoopInfo class that is used to identify natural loops
// and determine the loop depth of various nodes of the CFG. Note that the
// loops identified may actually be several natural loops that share the same
// header node... not just a single natural loop.
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/ADT/DepthFirstIterator.h"
#include "llvm/ADT/ScopeExit.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/Analysis/IVDescriptors.h"
#include "llvm/Analysis/LoopInfoImpl.h"
#include "llvm/Analysis/LoopIterator.h"
#include "llvm/Analysis/LoopNestAnalysis.h"
#include "llvm/Analysis/MemorySSA.h"
#include "llvm/Analysis/MemorySSAUpdater.h"
#include "llvm/Analysis/ScalarEvolutionExpressions.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/Config/llvm-config.h"
#include "llvm/IR/CFG.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DebugLoc.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/IRPrintingPasses.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/Metadata.h"
#include "llvm/IR/PassManager.h"
#include "llvm/IR/PrintPasses.h"
#include "llvm/InitializePasses.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include <algorithm>
using namespace llvm;
// Explicitly instantiate methods in LoopInfoImpl.h for IR-level Loops.
template class llvm::LoopBase<BasicBlock, Loop>;
template class llvm::LoopInfoBase<BasicBlock, Loop>;
// Always verify loopinfo if expensive checking is enabled.
bool llvm::VerifyLoopInfo = true;
bool llvm::VerifyLoopInfo = false;
static cl::opt<bool, true>
VerifyLoopInfoX("verify-loop-info", cl::location(VerifyLoopInfo),
cl::Hidden, cl::desc("Verify loop info (time consuming)"));
// Loop implementation
bool Loop::isLoopInvariant(const Value *V) const {
if (const Instruction *I = dyn_cast<Instruction>(V))
return !contains(I);
return true; // All non-instructions are loop invariant
bool Loop::hasLoopInvariantOperands(const Instruction *I) const {
return all_of(I->operands(), [this](Value *V) { return isLoopInvariant(V); });
bool Loop::makeLoopInvariant(Value *V, bool &Changed, Instruction *InsertPt,
MemorySSAUpdater *MSSAU) const {
if (Instruction *I = dyn_cast<Instruction>(V))
return makeLoopInvariant(I, Changed, InsertPt, MSSAU);
return true; // All non-instructions are loop-invariant.
bool Loop::makeLoopInvariant(Instruction *I, bool &Changed,
Instruction *InsertPt,
MemorySSAUpdater *MSSAU) const {
// Test if the value is already loop-invariant.
if (isLoopInvariant(I))
return true;
if (!isSafeToSpeculativelyExecute(I))
return false;
if (I->mayReadFromMemory())
return false;
// EH block instructions are immobile.
if (I->isEHPad())
return false;
// Determine the insertion point, unless one was given.
if (!InsertPt) {
BasicBlock *Preheader = getLoopPreheader();
// Without a preheader, hoisting is not feasible.
if (!Preheader)
return false;
InsertPt = Preheader->getTerminator();
// Don't hoist instructions with loop-variant operands.
for (Value *Operand : I->operands())
if (!makeLoopInvariant(Operand, Changed, InsertPt, MSSAU))
return false;
// Hoist.
if (MSSAU)
if (auto *MUD = MSSAU->getMemorySSA()->getMemoryAccess(I))
MSSAU->moveToPlace(MUD, InsertPt->getParent(),
// There is possibility of hoisting this instruction above some arbitrary
// condition. Any metadata defined on it can be control dependent on this
// condition. Conservatively strip it here so that we don't give any wrong
// information to the optimizer.
Changed = true;
return true;
bool Loop::getIncomingAndBackEdge(BasicBlock *&Incoming,
BasicBlock *&Backedge) const {
BasicBlock *H = getHeader();
Incoming = nullptr;
Backedge = nullptr;
pred_iterator PI = pred_begin(H);
assert(PI != pred_end(H) && "Loop must have at least one backedge!");
Backedge = *PI++;
if (PI == pred_end(H))
return false; // dead loop
Incoming = *PI++;
if (PI != pred_end(H))
return false; // multiple backedges?
if (contains(Incoming)) {
if (contains(Backedge))
return false;
std::swap(Incoming, Backedge);
} else if (!contains(Backedge))
return false;
assert(Incoming && Backedge && "expected non-null incoming and backedges");
return true;
PHINode *Loop::getCanonicalInductionVariable() const {
BasicBlock *H = getHeader();
BasicBlock *Incoming = nullptr, *Backedge = nullptr;
if (!getIncomingAndBackEdge(Incoming, Backedge))
return nullptr;
// Loop over all of the PHI nodes, looking for a canonical indvar.
for (BasicBlock::iterator I = H->begin(); isa<PHINode>(I); ++I) {
PHINode *PN = cast<PHINode>(I);
if (ConstantInt *CI =
if (CI->isZero())
if (Instruction *Inc =
if (Inc->getOpcode() == Instruction::Add && Inc->getOperand(0) == PN)
if (ConstantInt *CI = dyn_cast<ConstantInt>(Inc->getOperand(1)))
if (CI->isOne())
return PN;
return nullptr;
/// Get the latch condition instruction.
ICmpInst *Loop::getLatchCmpInst() const {
if (BasicBlock *Latch = getLoopLatch())
if (BranchInst *BI = dyn_cast_or_null<BranchInst>(Latch->getTerminator()))
if (BI->isConditional())
return dyn_cast<ICmpInst>(BI->getCondition());
return nullptr;
/// Return the final value of the loop induction variable if found.
static Value *findFinalIVValue(const Loop &L, const PHINode &IndVar,
const Instruction &StepInst) {
ICmpInst *LatchCmpInst = L.getLatchCmpInst();
if (!LatchCmpInst)
return nullptr;
Value *Op0 = LatchCmpInst->getOperand(0);
Value *Op1 = LatchCmpInst->getOperand(1);
if (Op0 == &IndVar || Op0 == &StepInst)
return Op1;
if (Op1 == &IndVar || Op1 == &StepInst)
return Op0;
return nullptr;
Optional<Loop::LoopBounds> Loop::LoopBounds::getBounds(const Loop &L,
PHINode &IndVar,
ScalarEvolution &SE) {
InductionDescriptor IndDesc;
if (!InductionDescriptor::isInductionPHI(&IndVar, &L, &SE, IndDesc))
return None;
Value *InitialIVValue = IndDesc.getStartValue();
Instruction *StepInst = IndDesc.getInductionBinOp();
if (!InitialIVValue || !StepInst)
return None;
const SCEV *Step = IndDesc.getStep();
Value *StepInstOp1 = StepInst->getOperand(1);
Value *StepInstOp0 = StepInst->getOperand(0);
Value *StepValue = nullptr;
if (SE.getSCEV(StepInstOp1) == Step)
StepValue = StepInstOp1;
else if (SE.getSCEV(StepInstOp0) == Step)
StepValue = StepInstOp0;
Value *FinalIVValue = findFinalIVValue(L, IndVar, *StepInst);
if (!FinalIVValue)
return None;
return LoopBounds(L, *InitialIVValue, *StepInst, StepValue, *FinalIVValue,
using Direction = Loop::LoopBounds::Direction;
ICmpInst::Predicate Loop::LoopBounds::getCanonicalPredicate() const {
BasicBlock *Latch = L.getLoopLatch();
assert(Latch && "Expecting valid latch");
BranchInst *BI = dyn_cast_or_null<BranchInst>(Latch->getTerminator());
assert(BI && BI->isConditional() && "Expecting conditional latch branch");
ICmpInst *LatchCmpInst = dyn_cast<ICmpInst>(BI->getCondition());
assert(LatchCmpInst &&
"Expecting the latch compare instruction to be a CmpInst");
// Need to inverse the predicate when first successor is not the loop
// header
ICmpInst::Predicate Pred = (BI->getSuccessor(0) == L.getHeader())
? LatchCmpInst->getPredicate()
: LatchCmpInst->getInversePredicate();
if (LatchCmpInst->getOperand(0) == &getFinalIVValue())
Pred = ICmpInst::getSwappedPredicate(Pred);
// Need to flip strictness of the predicate when the latch compare instruction
// is not using StepInst
if (LatchCmpInst->getOperand(0) == &getStepInst() ||
LatchCmpInst->getOperand(1) == &getStepInst())
return Pred;
// Cannot flip strictness of NE and EQ
if (Pred != ICmpInst::ICMP_NE && Pred != ICmpInst::ICMP_EQ)
return ICmpInst::getFlippedStrictnessPredicate(Pred);
Direction D = getDirection();
if (D == Direction::Increasing)
return ICmpInst::ICMP_SLT;
if (D == Direction::Decreasing)
return ICmpInst::ICMP_SGT;
// If cannot determine the direction, then unable to find the canonical
// predicate
Direction Loop::LoopBounds::getDirection() const {
if (const SCEVAddRecExpr *StepAddRecExpr =
if (const SCEV *StepRecur = StepAddRecExpr->getStepRecurrence(SE)) {
if (SE.isKnownPositive(StepRecur))
return Direction::Increasing;
if (SE.isKnownNegative(StepRecur))
return Direction::Decreasing;
return Direction::Unknown;
Optional<Loop::LoopBounds> Loop::getBounds(ScalarEvolution &SE) const {
if (PHINode *IndVar = getInductionVariable(SE))
return LoopBounds::getBounds(*this, *IndVar, SE);
return None;
PHINode *Loop::getInductionVariable(ScalarEvolution &SE) const {
if (!isLoopSimplifyForm())
return nullptr;
BasicBlock *Header = getHeader();
assert(Header && "Expected a valid loop header");
ICmpInst *CmpInst = getLatchCmpInst();
if (!CmpInst)
return nullptr;
Value *LatchCmpOp0 = CmpInst->getOperand(0);
Value *LatchCmpOp1 = CmpInst->getOperand(1);
for (PHINode &IndVar : Header->phis()) {
InductionDescriptor IndDesc;
if (!InductionDescriptor::isInductionPHI(&IndVar, this, &SE, IndDesc))
BasicBlock *Latch = getLoopLatch();
Value *StepInst = IndVar.getIncomingValueForBlock(Latch);
// case 1:
// IndVar = phi[{InitialValue, preheader}, {StepInst, latch}]
// StepInst = IndVar + step
// cmp = StepInst < FinalValue
if (StepInst == LatchCmpOp0 || StepInst == LatchCmpOp1)
return &IndVar;
// case 2:
// IndVar = phi[{InitialValue, preheader}, {StepInst, latch}]
// StepInst = IndVar + step
// cmp = IndVar < FinalValue
if (&IndVar == LatchCmpOp0 || &IndVar == LatchCmpOp1)
return &IndVar;
return nullptr;
bool Loop::getInductionDescriptor(ScalarEvolution &SE,
InductionDescriptor &IndDesc) const {
if (PHINode *IndVar = getInductionVariable(SE))
return InductionDescriptor::isInductionPHI(IndVar, this, &SE, IndDesc);
return false;
bool Loop::isAuxiliaryInductionVariable(PHINode &AuxIndVar,
ScalarEvolution &SE) const {
// Located in the loop header
BasicBlock *Header = getHeader();
if (AuxIndVar.getParent() != Header)
return false;
// No uses outside of the loop
for (User *U : AuxIndVar.users())
if (const Instruction *I = dyn_cast<Instruction>(U))
if (!contains(I))
return false;
InductionDescriptor IndDesc;
if (!InductionDescriptor::isInductionPHI(&AuxIndVar, this, &SE, IndDesc))
return false;
// The step instruction opcode should be add or sub.
if (IndDesc.getInductionOpcode() != Instruction::Add &&
IndDesc.getInductionOpcode() != Instruction::Sub)
return false;
// Incremented by a loop invariant step for each loop iteration
return SE.isLoopInvariant(IndDesc.getStep(), this);
BranchInst *Loop::getLoopGuardBranch() const {
if (!isLoopSimplifyForm())
return nullptr;
BasicBlock *Preheader = getLoopPreheader();
assert(Preheader && getLoopLatch() &&
"Expecting a loop with valid preheader and latch");
// Loop should be in rotate form.
if (!isRotatedForm())
return nullptr;
// Disallow loops with more than one unique exit block, as we do not verify
// that GuardOtherSucc post dominates all exit blocks.
BasicBlock *ExitFromLatch = getUniqueExitBlock();
if (!ExitFromLatch)
return nullptr;
BasicBlock *GuardBB = Preheader->getUniquePredecessor();
if (!GuardBB)
return nullptr;
assert(GuardBB->getTerminator() && "Expecting valid guard terminator");
BranchInst *GuardBI = dyn_cast<BranchInst>(GuardBB->getTerminator());
if (!GuardBI || GuardBI->isUnconditional())
return nullptr;
BasicBlock *GuardOtherSucc = (GuardBI->getSuccessor(0) == Preheader)
? GuardBI->getSuccessor(1)
: GuardBI->getSuccessor(0);
// Check if ExitFromLatch (or any BasicBlock which is an empty unique
// successor of ExitFromLatch) is equal to GuardOtherSucc. If
// skipEmptyBlockUntil returns GuardOtherSucc, then the guard branch for the
// loop is GuardBI (return GuardBI), otherwise return nullptr.
if (&LoopNest::skipEmptyBlockUntil(ExitFromLatch, GuardOtherSucc,
/*CheckUniquePred=*/true) ==
return GuardBI;
return nullptr;
bool Loop::isCanonical(ScalarEvolution &SE) const {
InductionDescriptor IndDesc;
if (!getInductionDescriptor(SE, IndDesc))
return false;
ConstantInt *Init = dyn_cast_or_null<ConstantInt>(IndDesc.getStartValue());
if (!Init || !Init->isZero())
return false;
if (IndDesc.getInductionOpcode() != Instruction::Add)
return false;
ConstantInt *Step = IndDesc.getConstIntStepValue();
if (!Step || !Step->isOne())
return false;
return true;
// Check that 'BB' doesn't have any uses outside of the 'L'
static bool isBlockInLCSSAForm(const Loop &L, const BasicBlock &BB,
const DominatorTree &DT) {
for (const Instruction &I : BB) {
// Tokens can't be used in PHI nodes and live-out tokens prevent loop
// optimizations, so for the purposes of considered LCSSA form, we
// can ignore them.
if (I.getType()->isTokenTy())
for (const Use &U : I.uses()) {
const Instruction *UI = cast<Instruction>(U.getUser());
const BasicBlock *UserBB = UI->getParent();
// For practical purposes, we consider that the use in a PHI
// occurs in the respective predecessor block. For more info,
// see the `phi` doc in LangRef and the LCSSA doc.
if (const PHINode *P = dyn_cast<PHINode>(UI))
UserBB = P->getIncomingBlock(U);
// Check the current block, as a fast-path, before checking whether
// the use is anywhere in the loop. Most values are used in the same
// block they are defined in. Also, blocks not reachable from the
// entry are special; uses in them don't need to go through PHIs.
if (UserBB != &BB && !L.contains(UserBB) &&
return false;
return true;
bool Loop::isLCSSAForm(const DominatorTree &DT) const {
// For each block we check that it doesn't have any uses outside of this loop.
return all_of(this->blocks(), [&](const BasicBlock *BB) {
return isBlockInLCSSAForm(*this, *BB, DT);
bool Loop::isRecursivelyLCSSAForm(const DominatorTree &DT,
const LoopInfo &LI) const {
// For each block we check that it doesn't have any uses outside of its
// innermost loop. This process will transitively guarantee that the current
// loop and all of the nested loops are in LCSSA form.
return all_of(this->blocks(), [&](const BasicBlock *BB) {
return isBlockInLCSSAForm(*LI.getLoopFor(BB), *BB, DT);
bool Loop::isLoopSimplifyForm() const {
// Normal-form loops have a preheader, a single backedge, and all of their
// exits have all their predecessors inside the loop.
return getLoopPreheader() && getLoopLatch() && hasDedicatedExits();
// Routines that reform the loop CFG and split edges often fail on indirectbr.
bool Loop::isSafeToClone() const {
// Return false if any loop blocks contain indirectbrs, or there are any calls
// to noduplicate functions.
// FIXME: it should be ok to clone CallBrInst's if we correctly update the
// operand list to reflect the newly cloned labels.
for (BasicBlock *BB : this->blocks()) {
if (isa<IndirectBrInst>(BB->getTerminator()) ||
return false;
for (Instruction &I : *BB)
if (auto *CB = dyn_cast<CallBase>(&I))
if (CB->cannotDuplicate())
return false;
return true;
MDNode *Loop::getLoopID() const {
MDNode *LoopID = nullptr;
// Go through the latch blocks and check the terminator for the metadata.
SmallVector<BasicBlock *, 4> LatchesBlocks;
for (BasicBlock *BB : LatchesBlocks) {
Instruction *TI = BB->getTerminator();
MDNode *MD = TI->getMetadata(LLVMContext::MD_loop);
if (!MD)
return nullptr;
if (!LoopID)
LoopID = MD;
else if (MD != LoopID)
return nullptr;
if (!LoopID || LoopID->getNumOperands() == 0 ||
LoopID->getOperand(0) != LoopID)
return nullptr;
return LoopID;
void Loop::setLoopID(MDNode *LoopID) const {
assert((!LoopID || LoopID->getNumOperands() > 0) &&
"Loop ID needs at least one operand");
assert((!LoopID || LoopID->getOperand(0) == LoopID) &&
"Loop ID should refer to itself");
SmallVector<BasicBlock *, 4> LoopLatches;
for (BasicBlock *BB : LoopLatches)
BB->getTerminator()->setMetadata(LLVMContext::MD_loop, LoopID);
void Loop::setLoopAlreadyUnrolled() {
LLVMContext &Context = getHeader()->getContext();
MDNode *DisableUnrollMD =
MDNode::get(Context, MDString::get(Context, "llvm.loop.unroll.disable"));
MDNode *LoopID = getLoopID();
MDNode *NewLoopID = makePostTransformationMetadata(
Context, LoopID, {"llvm.loop.unroll."}, {DisableUnrollMD});
void Loop::setLoopMustProgress() {
LLVMContext &Context = getHeader()->getContext();
MDNode *MustProgress = findOptionMDForLoop(this, "llvm.loop.mustprogress");
if (MustProgress)
MDNode *MustProgressMD =
MDNode::get(Context, MDString::get(Context, "llvm.loop.mustprogress"));
MDNode *LoopID = getLoopID();
MDNode *NewLoopID =
makePostTransformationMetadata(Context, LoopID, {}, {MustProgressMD});
bool Loop::isAnnotatedParallel() const {
MDNode *DesiredLoopIdMetadata = getLoopID();
if (!DesiredLoopIdMetadata)
return false;
MDNode *ParallelAccesses =
findOptionMDForLoop(this, "llvm.loop.parallel_accesses");
SmallPtrSet<MDNode *, 4>
ParallelAccessGroups; // For scalable 'contains' check.
if (ParallelAccesses) {
for (const MDOperand &MD : drop_begin(ParallelAccesses->operands())) {
MDNode *AccGroup = cast<MDNode>(MD.get());
assert(isValidAsAccessGroup(AccGroup) &&
"List item must be an access group");
// The loop branch contains the parallel loop metadata. In order to ensure
// that any parallel-loop-unaware optimization pass hasn't added loop-carried
// dependencies (thus converted the loop back to a sequential loop), check
// that all the memory instructions in the loop belong to an access group that
// is parallel to this loop.
for (BasicBlock *BB : this->blocks()) {
for (Instruction &I : *BB) {
if (!I.mayReadOrWriteMemory())
if (MDNode *AccessGroup = I.getMetadata(LLVMContext::MD_access_group)) {
auto ContainsAccessGroup = [&ParallelAccessGroups](MDNode *AG) -> bool {
if (AG->getNumOperands() == 0) {
assert(isValidAsAccessGroup(AG) && "Item must be an access group");
return ParallelAccessGroups.count(AG);
for (const MDOperand &AccessListItem : AG->operands()) {
MDNode *AccGroup = cast<MDNode>(AccessListItem.get());
assert(isValidAsAccessGroup(AccGroup) &&
"List item must be an access group");
if (ParallelAccessGroups.count(AccGroup))
return true;
return false;
if (ContainsAccessGroup(AccessGroup))
// The memory instruction can refer to the loop identifier metadata
// directly or indirectly through another list metadata (in case of
// nested parallel loops). The loop identifier metadata refers to
// itself so we can check both cases with the same routine.
MDNode *LoopIdMD =
if (!LoopIdMD)
return false;
if (!llvm::is_contained(LoopIdMD->operands(), DesiredLoopIdMetadata))
return false;
return true;
DebugLoc Loop::getStartLoc() const { return getLocRange().getStart(); }
Loop::LocRange Loop::getLocRange() const {
// If we have a debug location in the loop ID, then use it.
if (MDNode *LoopID = getLoopID()) {
DebugLoc Start;
// We use the first DebugLoc in the header as the start location of the loop
// and if there is a second DebugLoc in the header we use it as end location
// of the loop.
for (unsigned i = 1, ie = LoopID->getNumOperands(); i < ie; ++i) {
if (DILocation *L = dyn_cast<DILocation>(LoopID->getOperand(i))) {
if (!Start)
Start = DebugLoc(L);
return LocRange(Start, DebugLoc(L));
if (Start)
return LocRange(Start);
// Try the pre-header first.
if (BasicBlock *PHeadBB = getLoopPreheader())
if (DebugLoc DL = PHeadBB->getTerminator()->getDebugLoc())
return LocRange(DL);
// If we have no pre-header or there are no instructions with debug
// info in it, try the header.
if (BasicBlock *HeadBB = getHeader())
return LocRange(HeadBB->getTerminator()->getDebugLoc());
return LocRange();
#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
LLVM_DUMP_METHOD void Loop::dump() const { print(dbgs()); }
LLVM_DUMP_METHOD void Loop::dumpVerbose() const {
print(dbgs(), /*Verbose=*/true);
// UnloopUpdater implementation
namespace {
/// Find the new parent loop for all blocks within the "unloop" whose last
/// backedges has just been removed.
class UnloopUpdater {
Loop &Unloop;
LoopInfo *LI;
LoopBlocksDFS DFS;
// Map unloop's immediate subloops to their nearest reachable parents. Nested
// loops within these subloops will not change parents. However, an immediate
// subloop's new parent will be the nearest loop reachable from either its own
// exits *or* any of its nested loop's exits.
DenseMap<Loop *, Loop *> SubloopParents;
// Flag the presence of an irreducible backedge whose destination is a block
// directly contained by the original unloop.
bool FoundIB;
UnloopUpdater(Loop *UL, LoopInfo *LInfo)
: Unloop(*UL), LI(LInfo), DFS(UL), FoundIB(false) {}
void updateBlockParents();
void removeBlocksFromAncestors();
void updateSubloopParents();
Loop *getNearestLoop(BasicBlock *BB, Loop *BBLoop);
} // end anonymous namespace
/// Update the parent loop for all blocks that are directly contained within the
/// original "unloop".
void UnloopUpdater::updateBlockParents() {
if (Unloop.getNumBlocks()) {
// Perform a post order CFG traversal of all blocks within this loop,
// propagating the nearest loop from successors to predecessors.
LoopBlocksTraversal Traversal(DFS, LI);
for (BasicBlock *POI : Traversal) {
Loop *L = LI->getLoopFor(POI);
Loop *NL = getNearestLoop(POI, L);
if (NL != L) {
// For reducible loops, NL is now an ancestor of Unloop.
assert((NL != &Unloop && (!NL || NL->contains(&Unloop))) &&
"uninitialized successor");
LI->changeLoopFor(POI, NL);
} else {
// Or the current block is part of a subloop, in which case its parent
// is unchanged.
assert((FoundIB || Unloop.contains(L)) && "uninitialized successor");
// Each irreducible loop within the unloop induces a round of iteration using
// the DFS result cached by Traversal.
bool Changed = FoundIB;
for (unsigned NIters = 0; Changed; ++NIters) {
assert(NIters < Unloop.getNumBlocks() && "runaway iterative algorithm");
// Iterate over the postorder list of blocks, propagating the nearest loop
// from successors to predecessors as before.
Changed = false;
for (LoopBlocksDFS::POIterator POI = DFS.beginPostorder(),
POE = DFS.endPostorder();
POI != POE; ++POI) {
Loop *L = LI->getLoopFor(*POI);
Loop *NL = getNearestLoop(*POI, L);
if (NL != L) {
assert(NL != &Unloop && (!NL || NL->contains(&Unloop)) &&
"uninitialized successor");
LI->changeLoopFor(*POI, NL);
Changed = true;
/// Remove unloop's blocks from all ancestors below their new parents.
void UnloopUpdater::removeBlocksFromAncestors() {
// Remove all unloop's blocks (including those in nested subloops) from
// ancestors below the new parent loop.
for (BasicBlock *BB : Unloop.blocks()) {
Loop *OuterParent = LI->getLoopFor(BB);
if (Unloop.contains(OuterParent)) {
while (OuterParent->getParentLoop() != &Unloop)
OuterParent = OuterParent->getParentLoop();
OuterParent = SubloopParents[OuterParent];
// Remove blocks from former Ancestors except Unloop itself which will be
// deleted.
for (Loop *OldParent = Unloop.getParentLoop(); OldParent != OuterParent;
OldParent = OldParent->getParentLoop()) {
assert(OldParent && "new loop is not an ancestor of the original");
/// Update the parent loop for all subloops directly nested within unloop.
void UnloopUpdater::updateSubloopParents() {
while (!Unloop.isInnermost()) {
Loop *Subloop = *std::prev(Unloop.end());
assert(SubloopParents.count(Subloop) && "DFS failed to visit subloop");
if (Loop *Parent = SubloopParents[Subloop])
/// Return the nearest parent loop among this block's successors. If a successor
/// is a subloop header, consider its parent to be the nearest parent of the
/// subloop's exits.
/// For subloop blocks, simply update SubloopParents and return NULL.
Loop *UnloopUpdater::getNearestLoop(BasicBlock *BB, Loop *BBLoop) {
// Initially for blocks directly contained by Unloop, NearLoop == Unloop and
// is considered uninitialized.
Loop *NearLoop = BBLoop;
Loop *Subloop = nullptr;
if (NearLoop != &Unloop && Unloop.contains(NearLoop)) {
Subloop = NearLoop;
// Find the subloop ancestor that is directly contained within Unloop.
while (Subloop->getParentLoop() != &Unloop) {
Subloop = Subloop->getParentLoop();
assert(Subloop && "subloop is not an ancestor of the original loop");
// Get the current nearest parent of the Subloop exits, initially Unloop.
NearLoop = SubloopParents.insert({Subloop, &Unloop}).first->second;
succ_iterator I = succ_begin(BB), E = succ_end(BB);
if (I == E) {
assert(!Subloop && "subloop blocks must have a successor");
NearLoop = nullptr; // unloop blocks may now exit the function.
for (; I != E; ++I) {
if (*I == BB)
continue; // self loops are uninteresting
Loop *L = LI->getLoopFor(*I);
if (L == &Unloop) {
// This successor has not been processed. This path must lead to an
// irreducible backedge.
assert((FoundIB || !DFS.hasPostorder(*I)) && "should have seen IB");
FoundIB = true;
if (L != &Unloop && Unloop.contains(L)) {
// Successor is in a subloop.
if (Subloop)
continue; // Branching within subloops. Ignore it.
// BB branches from the original into a subloop header.
assert(L->getParentLoop() == &Unloop && "cannot skip into nested loops");
// Get the current nearest parent of the Subloop's exits.
L = SubloopParents[L];
// L could be Unloop if the only exit was an irreducible backedge.
if (L == &Unloop) {
// Handle critical edges from Unloop into a sibling loop.
if (L && !L->contains(&Unloop)) {
L = L->getParentLoop();
// Remember the nearest parent loop among successors or subloop exits.
if (NearLoop == &Unloop || !NearLoop || NearLoop->contains(L))
NearLoop = L;
if (Subloop) {
SubloopParents[Subloop] = NearLoop;
return BBLoop;
return NearLoop;
LoopInfo::LoopInfo(const DomTreeBase<BasicBlock> &DomTree) { analyze(DomTree); }
bool LoopInfo::invalidate(Function &F, const PreservedAnalyses &PA,
FunctionAnalysisManager::Invalidator &) {
// Check whether the analysis, all analyses on functions, or the function's
// CFG have been preserved.
auto PAC = PA.getChecker<LoopAnalysis>();
return !(PAC.preserved() || PAC.preservedSet<AllAnalysesOn<Function>>() ||
void LoopInfo::erase(Loop *Unloop) {
assert(!Unloop->isInvalid() && "Loop has already been erased!");
auto InvalidateOnExit = make_scope_exit([&]() { destroy(Unloop); });
// First handle the special case of no parent loop to simplify the algorithm.
if (Unloop->isOutermost()) {
// Since BBLoop had no parent, Unloop blocks are no longer in a loop.
for (BasicBlock *BB : Unloop->blocks()) {
// Don't reparent blocks in subloops.
if (getLoopFor(BB) != Unloop)
// Blocks no longer have a parent but are still referenced by Unloop until
// the Unloop object is deleted.
changeLoopFor(BB, nullptr);
// Remove the loop from the top-level LoopInfo object.
for (iterator I = begin();; ++I) {
assert(I != end() && "Couldn't find loop");
if (*I == Unloop) {
// Move all of the subloops to the top-level.
while (!Unloop->isInnermost())
// Update the parent loop for all blocks within the loop. Blocks within
// subloops will not change parents.
UnloopUpdater Updater(Unloop, this);
// Remove blocks from former ancestor loops.
// Add direct subloops as children in their new parent loop.
// Remove unloop from its parent loop.
Loop *ParentLoop = Unloop->getParentLoop();
for (Loop::iterator I = ParentLoop->begin();; ++I) {
assert(I != ParentLoop->end() && "Couldn't find loop");
if (*I == Unloop) {
LoopInfo::wouldBeOutOfLoopUseRequiringLCSSA(const Value *V,
const BasicBlock *ExitBB) const {
if (V->getType()->isTokenTy())
// We can't form PHIs of token type, so the definition of LCSSA excludes
// values of that type.
return false;
const Instruction *I = dyn_cast<Instruction>(V);
if (!I)
return false;
const Loop *L = getLoopFor(I->getParent());
if (!L)
return false;
if (L->contains(ExitBB))
// Could be an exit bb of a subloop and contained in defining loop
return false;
// We found a (new) out-of-loop use location, for a value defined in-loop.
// (Note that because of LCSSA, we don't have to account for values defined
// in sibling loops. Such values will have LCSSA phis of their own in the
// common parent loop.)
return true;
AnalysisKey LoopAnalysis::Key;
LoopInfo LoopAnalysis::run(Function &F, FunctionAnalysisManager &AM) {
// FIXME: Currently we create a LoopInfo from scratch for every function.
// This may prove to be too wasteful due to deallocating and re-allocating
// memory each time for the underlying map and vector datastructures. At some
// point it may prove worthwhile to use a freelist and recycle LoopInfo
// objects. I don't want to add that kind of complexity until the scope of
// the problem is better understood.
LoopInfo LI;
return LI;
PreservedAnalyses LoopPrinterPass::run(Function &F,
FunctionAnalysisManager &AM) {
return PreservedAnalyses::all();
void llvm::printLoop(Loop &L, raw_ostream &OS, const std::string &Banner) {
if (forcePrintModuleIR()) {
// handling -print-module-scope
OS << Banner << " (loop: ";
L.getHeader()->printAsOperand(OS, false);
OS << ")\n";
// printing whole module
OS << *L.getHeader()->getModule();
OS << Banner;
auto *PreHeader = L.getLoopPreheader();
if (PreHeader) {
OS << "\n; Preheader:";
OS << "\n; Loop:";
for (auto *Block : L.blocks())
if (Block)
OS << "Printing <null> block";
SmallVector<BasicBlock *, 8> ExitBlocks;
if (!ExitBlocks.empty()) {
OS << "\n; Exit blocks";
for (auto *Block : ExitBlocks)
if (Block)
OS << "Printing <null> block";
MDNode *llvm::findOptionMDForLoopID(MDNode *LoopID, StringRef Name) {
// No loop metadata node, no loop properties.
if (!LoopID)
return nullptr;
// First operand should refer to the metadata node itself, for legacy reasons.
assert(LoopID->getNumOperands() > 0 && "requires at least one operand");
assert(LoopID->getOperand(0) == LoopID && "invalid loop id");
// Iterate over the metdata node operands and look for MDString metadata.
for (unsigned i = 1, e = LoopID->getNumOperands(); i < e; ++i) {
MDNode *MD = dyn_cast<MDNode>(LoopID->getOperand(i));
if (!MD || MD->getNumOperands() < 1)
MDString *S = dyn_cast<MDString>(MD->getOperand(0));
if (!S)
// Return the operand node if MDString holds expected metadata.
if (Name.equals(S->getString()))
return MD;
// Loop property not found.
return nullptr;
MDNode *llvm::findOptionMDForLoop(const Loop *TheLoop, StringRef Name) {
return findOptionMDForLoopID(TheLoop->getLoopID(), Name);
/// Find string metadata for loop
/// If it has a value (e.g. {"llvm.distribute", 1} return the value as an
/// operand or null otherwise. If the string metadata is not found return
/// Optional's not-a-value.
Optional<const MDOperand *> llvm::findStringMetadataForLoop(const Loop *TheLoop,
StringRef Name) {
MDNode *MD = findOptionMDForLoop(TheLoop, Name);
if (!MD)
return None;
switch (MD->getNumOperands()) {
case 1:
return nullptr;
case 2:
return &MD->getOperand(1);
llvm_unreachable("loop metadata has 0 or 1 operand");
Optional<bool> llvm::getOptionalBoolLoopAttribute(const Loop *TheLoop,
StringRef Name) {
MDNode *MD = findOptionMDForLoop(TheLoop, Name);
if (!MD)
return None;
switch (MD->getNumOperands()) {
case 1:
// When the value is absent it is interpreted as 'attribute set'.
return true;
case 2:
if (ConstantInt *IntMD =
return IntMD->getZExtValue();
return true;
llvm_unreachable("unexpected number of options");
bool llvm::getBooleanLoopAttribute(const Loop *TheLoop, StringRef Name) {
return getOptionalBoolLoopAttribute(TheLoop, Name).getValueOr(false);
llvm::Optional<int> llvm::getOptionalIntLoopAttribute(const Loop *TheLoop,
StringRef Name) {
const MDOperand *AttrMD =
findStringMetadataForLoop(TheLoop, Name).getValueOr(nullptr);
if (!AttrMD)
return None;
ConstantInt *IntMD = mdconst::extract_or_null<ConstantInt>(AttrMD->get());
if (!IntMD)
return None;
return IntMD->getSExtValue();
int llvm::getIntLoopAttribute(const Loop *TheLoop, StringRef Name,
int Default) {
return getOptionalIntLoopAttribute(TheLoop, Name).getValueOr(Default);
static const char *LLVMLoopMustProgress = "llvm.loop.mustprogress";
bool llvm::hasMustProgress(const Loop *L) {
return getBooleanLoopAttribute(L, LLVMLoopMustProgress);
bool llvm::isMustProgress(const Loop *L) {
return L->getHeader()->getParent()->mustProgress() || hasMustProgress(L);
bool llvm::isValidAsAccessGroup(MDNode *Node) {
return Node->getNumOperands() == 0 && Node->isDistinct();
MDNode *llvm::makePostTransformationMetadata(LLVMContext &Context,
MDNode *OrigLoopID,
ArrayRef<StringRef> RemovePrefixes,
ArrayRef<MDNode *> AddAttrs) {
// First remove any existing loop metadata related to this transformation.
SmallVector<Metadata *, 4> MDs;
// Reserve first location for self reference to the LoopID metadata node.
// Remove metadata for the transformation that has been applied or that became
// outdated.
if (OrigLoopID) {
for (unsigned i = 1, ie = OrigLoopID->getNumOperands(); i < ie; ++i) {
bool IsVectorMetadata = false;
Metadata *Op = OrigLoopID->getOperand(i);
if (MDNode *MD = dyn_cast<MDNode>(Op)) {
const MDString *S = dyn_cast<MDString>(MD->getOperand(0));
if (S)
IsVectorMetadata =
llvm::any_of(RemovePrefixes, [S](StringRef Prefix) -> bool {
return S->getString().startswith(Prefix);
if (!IsVectorMetadata)
// Add metadata to avoid reapplying a transformation, such as
// llvm.loop.unroll.disable and llvm.loop.isvectorized.
MDs.append(AddAttrs.begin(), AddAttrs.end());
MDNode *NewLoopID = MDNode::getDistinct(Context, MDs);
// Replace the temporary node with a self-reference.
NewLoopID->replaceOperandWith(0, NewLoopID);
return NewLoopID;
// LoopInfo implementation
LoopInfoWrapperPass::LoopInfoWrapperPass() : FunctionPass(ID) {
char LoopInfoWrapperPass::ID = 0;
INITIALIZE_PASS_BEGIN(LoopInfoWrapperPass, "loops", "Natural Loop Information",
true, true)
INITIALIZE_PASS_END(LoopInfoWrapperPass, "loops", "Natural Loop Information",
true, true)
bool LoopInfoWrapperPass::runOnFunction(Function &) {
return false;
void LoopInfoWrapperPass::verifyAnalysis() const {
// LoopInfoWrapperPass is a FunctionPass, but verifying every loop in the
// function each time verifyAnalysis is called is very expensive. The
// -verify-loop-info option can enable this. In order to perform some
// checking by default, LoopPass has been taught to call verifyLoop manually
// during loop pass sequences.
if (VerifyLoopInfo) {
auto &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
void LoopInfoWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const {
void LoopInfoWrapperPass::print(raw_ostream &OS, const Module *) const {
PreservedAnalyses LoopVerifierPass::run(Function &F,
FunctionAnalysisManager &AM) {
LoopInfo &LI = AM.getResult<LoopAnalysis>(F);
auto &DT = AM.getResult<DominatorTreeAnalysis>(F);
return PreservedAnalyses::all();
// LoopBlocksDFS implementation
/// Traverse the loop blocks and store the DFS result.
/// Useful for clients that just want the final DFS result and don't need to
/// visit blocks during the initial traversal.
void LoopBlocksDFS::perform(LoopInfo *LI) {
LoopBlocksTraversal Traversal(*this, LI);
for (LoopBlocksTraversal::POTIterator POI = Traversal.begin(),
POE = Traversal.end();
POI != POE; ++POI)