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//===-- UnrollLoop.cpp - Loop unrolling utilities -------------------------===//
// 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 some loop unrolling utilities. It does not define any
// actual pass or policy, but provides a single function to perform loop
// unrolling.
// The process of unrolling can produce extraneous basic blocks linked with
// unconditional branches. This will be corrected in the future.
#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/ScopedHashTable.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/ADT/Twine.h"
#include "llvm/ADT/ilist_iterator.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/AssumptionCache.h"
#include "llvm/Analysis/DomTreeUpdater.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/LoopIterator.h"
#include "llvm/Analysis/MemorySSA.h"
#include "llvm/Analysis/OptimizationRemarkEmitter.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/CFG.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DebugInfoMetadata.h"
#include "llvm/IR/DebugLoc.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/IntrinsicInst.h"
#include "llvm/IR/Metadata.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/PatternMatch.h"
#include "llvm/IR/Use.h"
#include "llvm/IR/User.h"
#include "llvm/IR/ValueHandle.h"
#include "llvm/IR/ValueMap.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/GenericDomTree.h"
#include "llvm/Support/MathExtras.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/Cloning.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Transforms/Utils/LoopSimplify.h"
#include "llvm/Transforms/Utils/LoopUtils.h"
#include "llvm/Transforms/Utils/SimplifyIndVar.h"
#include "llvm/Transforms/Utils/UnrollLoop.h"
#include "llvm/Transforms/Utils/ValueMapper.h"
#include <algorithm>
#include <assert.h>
#include <numeric>
#include <type_traits>
#include <vector>
namespace llvm {
class DataLayout;
class Value;
} // namespace llvm
using namespace llvm;
#define DEBUG_TYPE "loop-unroll"
// TODO: Should these be here or in LoopUnroll?
STATISTIC(NumCompletelyUnrolled, "Number of loops completely unrolled");
STATISTIC(NumUnrolled, "Number of loops unrolled (completely or otherwise)");
STATISTIC(NumUnrolledNotLatch, "Number of loops unrolled without a conditional "
"latch (completely or otherwise)");
static cl::opt<bool>
UnrollRuntimeEpilog("unroll-runtime-epilog", cl::init(false), cl::Hidden,
cl::desc("Allow runtime unrolled loops to be unrolled "
"with epilog instead of prolog."));
static cl::opt<bool>
UnrollVerifyDomtree("unroll-verify-domtree", cl::Hidden,
cl::desc("Verify domtree after unrolling"),
static cl::opt<bool>
UnrollVerifyLoopInfo("unroll-verify-loopinfo", cl::Hidden,
cl::desc("Verify loopinfo after unrolling"),
/// Check if unrolling created a situation where we need to insert phi nodes to
/// preserve LCSSA form.
/// \param Blocks is a vector of basic blocks representing unrolled loop.
/// \param L is the outer loop.
/// It's possible that some of the blocks are in L, and some are not. In this
/// case, if there is a use is outside L, and definition is inside L, we need to
/// insert a phi-node, otherwise LCSSA will be broken.
/// The function is just a helper function for llvm::UnrollLoop that returns
/// true if this situation occurs, indicating that LCSSA needs to be fixed.
static bool needToInsertPhisForLCSSA(Loop *L,
const std::vector<BasicBlock *> &Blocks,
LoopInfo *LI) {
for (BasicBlock *BB : Blocks) {
if (LI->getLoopFor(BB) == L)
for (Instruction &I : *BB) {
for (Use &U : I.operands()) {
if (const auto *Def = dyn_cast<Instruction>(U)) {
Loop *DefLoop = LI->getLoopFor(Def->getParent());
if (!DefLoop)
if (DefLoop->contains(L))
return true;
return false;
/// Adds ClonedBB to LoopInfo, creates a new loop for ClonedBB if necessary
/// and adds a mapping from the original loop to the new loop to NewLoops.
/// Returns nullptr if no new loop was created and a pointer to the
/// original loop OriginalBB was part of otherwise.
const Loop* llvm::addClonedBlockToLoopInfo(BasicBlock *OriginalBB,
BasicBlock *ClonedBB, LoopInfo *LI,
NewLoopsMap &NewLoops) {
// Figure out which loop New is in.
const Loop *OldLoop = LI->getLoopFor(OriginalBB);
assert(OldLoop && "Should (at least) be in the loop being unrolled!");
Loop *&NewLoop = NewLoops[OldLoop];
if (!NewLoop) {
// Found a new sub-loop.
assert(OriginalBB == OldLoop->getHeader() &&
"Header should be first in RPO");
NewLoop = LI->AllocateLoop();
Loop *NewLoopParent = NewLoops.lookup(OldLoop->getParentLoop());
if (NewLoopParent)
NewLoop->addBasicBlockToLoop(ClonedBB, *LI);
return OldLoop;
} else {
NewLoop->addBasicBlockToLoop(ClonedBB, *LI);
return nullptr;
/// The function chooses which type of unroll (epilog or prolog) is more
/// profitabale.
/// Epilog unroll is more profitable when there is PHI that starts from
/// constant. In this case epilog will leave PHI start from constant,
/// but prolog will convert it to non-constant.
/// loop:
/// PN = PHI [I, Latch], [CI, PreHeader]
/// I = foo(PN)
/// ...
/// Epilog unroll case.
/// loop:
/// PN = PHI [I2, Latch], [CI, PreHeader]
/// I1 = foo(PN)
/// I2 = foo(I1)
/// ...
/// Prolog unroll case.
/// NewPN = PHI [PrologI, Prolog], [CI, PreHeader]
/// loop:
/// PN = PHI [I2, Latch], [NewPN, PreHeader]
/// I1 = foo(PN)
/// I2 = foo(I1)
/// ...
static bool isEpilogProfitable(Loop *L) {
BasicBlock *PreHeader = L->getLoopPreheader();
BasicBlock *Header = L->getHeader();
assert(PreHeader && Header);
for (const PHINode &PN : Header->phis()) {
if (isa<ConstantInt>(PN.getIncomingValueForBlock(PreHeader)))
return true;
return false;
struct LoadValue {
Instruction *DefI = nullptr;
unsigned Generation = 0;
LoadValue() = default;
LoadValue(Instruction *Inst, unsigned Generation)
: DefI(Inst), Generation(Generation) {}
class StackNode {
ScopedHashTable<const SCEV *, LoadValue>::ScopeTy LoadScope;
unsigned CurrentGeneration;
unsigned ChildGeneration;
DomTreeNode *Node;
DomTreeNode::const_iterator ChildIter;
DomTreeNode::const_iterator EndIter;
bool Processed = false;
StackNode(ScopedHashTable<const SCEV *, LoadValue> &AvailableLoads,
unsigned cg, DomTreeNode *N, DomTreeNode::const_iterator Child,
DomTreeNode::const_iterator End)
: LoadScope(AvailableLoads), CurrentGeneration(cg), ChildGeneration(cg),
Node(N), ChildIter(Child), EndIter(End) {}
// Accessors.
unsigned currentGeneration() const { return CurrentGeneration; }
unsigned childGeneration() const { return ChildGeneration; }
void childGeneration(unsigned generation) { ChildGeneration = generation; }
DomTreeNode *node() { return Node; }
DomTreeNode::const_iterator childIter() const { return ChildIter; }
DomTreeNode *nextChild() {
DomTreeNode *Child = *ChildIter;
return Child;
DomTreeNode::const_iterator end() const { return EndIter; }
bool isProcessed() const { return Processed; }
void process() { Processed = true; }
Value *getMatchingValue(LoadValue LV, LoadInst *LI, unsigned CurrentGeneration,
BatchAAResults &BAA,
function_ref<MemorySSA *()> GetMSSA) {
if (!LV.DefI)
return nullptr;
if (LV.DefI->getType() != LI->getType())
return nullptr;
if (LV.Generation != CurrentGeneration) {
MemorySSA *MSSA = GetMSSA();
if (!MSSA)
return nullptr;
auto *EarlierMA = MSSA->getMemoryAccess(LV.DefI);
MemoryAccess *LaterDef =
MSSA->getWalker()->getClobberingMemoryAccess(LI, BAA);
if (!MSSA->dominates(LaterDef, EarlierMA))
return nullptr;
return LV.DefI;
void loadCSE(Loop *L, DominatorTree &DT, ScalarEvolution &SE, LoopInfo &LI,
BatchAAResults &BAA, function_ref<MemorySSA *()> GetMSSA) {
ScopedHashTable<const SCEV *, LoadValue> AvailableLoads;
SmallVector<std::unique_ptr<StackNode>> NodesToProcess;
DomTreeNode *HeaderD = DT.getNode(L->getHeader());
NodesToProcess.emplace_back(new StackNode(AvailableLoads, 0, HeaderD,
HeaderD->begin(), HeaderD->end()));
unsigned CurrentGeneration = 0;
while (!NodesToProcess.empty()) {
StackNode *NodeToProcess = &*NodesToProcess.back();
CurrentGeneration = NodeToProcess->currentGeneration();
if (!NodeToProcess->isProcessed()) {
// Process the node.
// If this block has a single predecessor, then the predecessor is the
// parent
// of the domtree node and all of the live out memory values are still
// current in this block. If this block has multiple predecessors, then
// they could have invalidated the live-out memory values of our parent
// value. For now, just be conservative and invalidate memory if this
// block has multiple predecessors.
if (!NodeToProcess->node()->getBlock()->getSinglePredecessor())
for (auto &I : make_early_inc_range(*NodeToProcess->node()->getBlock())) {
auto *Load = dyn_cast<LoadInst>(&I);
if (!Load || !Load->isSimple()) {
if (I.mayWriteToMemory())
const SCEV *PtrSCEV = SE.getSCEV(Load->getPointerOperand());
LoadValue LV = AvailableLoads.lookup(PtrSCEV);
if (Value *M =
getMatchingValue(LV, Load, CurrentGeneration, BAA, GetMSSA)) {
if (LI.replacementPreservesLCSSAForm(Load, M)) {
} else {
AvailableLoads.insert(PtrSCEV, LoadValue(Load, CurrentGeneration));
} else if (NodeToProcess->childIter() != NodeToProcess->end()) {
// Push the next child onto the stack.
DomTreeNode *Child = NodeToProcess->nextChild();
if (!L->contains(Child->getBlock()))
new StackNode(AvailableLoads, NodeToProcess->childGeneration(), Child,
Child->begin(), Child->end()));
} else {
// It has been processed, and there are no more children to process,
// so delete it and pop it off the stack.
/// Perform some cleanup and simplifications on loops after unrolling. It is
/// useful to simplify the IV's in the new loop, as well as do a quick
/// simplify/dce pass of the instructions.
void llvm::simplifyLoopAfterUnroll(Loop *L, bool SimplifyIVs, LoopInfo *LI,
ScalarEvolution *SE, DominatorTree *DT,
AssumptionCache *AC,
const TargetTransformInfo *TTI,
AAResults *AA) {
using namespace llvm::PatternMatch;
// Simplify any new induction variables in the partially unrolled loop.
if (SE && SimplifyIVs) {
SmallVector<WeakTrackingVH, 16> DeadInsts;
simplifyLoopIVs(L, SE, DT, LI, TTI, DeadInsts);
// Aggressively clean up dead instructions that simplifyLoopIVs already
// identified. Any remaining should be cleaned up below.
while (!DeadInsts.empty()) {
Value *V = DeadInsts.pop_back_val();
if (Instruction *Inst = dyn_cast_or_null<Instruction>(V))
if (AA) {
std::unique_ptr<MemorySSA> MSSA = nullptr;
BatchAAResults BAA(*AA);
loadCSE(L, *DT, *SE, *LI, BAA, [L, AA, DT, &MSSA]() -> MemorySSA * {
if (!MSSA)
MSSA.reset(new MemorySSA(*L, AA, DT));
return &*MSSA;
// At this point, the code is well formed. Perform constprop, instsimplify,
// and dce.
const DataLayout &DL = L->getHeader()->getModule()->getDataLayout();
SmallVector<WeakTrackingVH, 16> DeadInsts;
for (BasicBlock *BB : L->getBlocks()) {
// Remove repeated debug instructions after loop unrolling.
if (BB->getParent()->getSubprogram())
for (Instruction &Inst : llvm::make_early_inc_range(*BB)) {
if (Value *V = simplifyInstruction(&Inst, {DL, nullptr, DT, AC}))
if (LI->replacementPreservesLCSSAForm(&Inst, V))
if (isInstructionTriviallyDead(&Inst))
// Fold ((add X, C1), C2) to (add X, C1+C2). This is very common in
// unrolled loops, and handling this early allows following code to
// identify the IV as a "simple recurrence" without first folding away
// a long chain of adds.
Value *X;
const APInt *C1, *C2;
if (match(&Inst, m_Add(m_Add(m_Value(X), m_APInt(C1)), m_APInt(C2)))) {
auto *InnerI = dyn_cast<Instruction>(Inst.getOperand(0));
auto *InnerOBO = cast<OverflowingBinaryOperator>(Inst.getOperand(0));
bool SignedOverflow;
APInt NewC = C1->sadd_ov(*C2, SignedOverflow);
Inst.setOperand(0, X);
Inst.setOperand(1, ConstantInt::get(Inst.getType(), NewC));
Inst.setHasNoUnsignedWrap(Inst.hasNoUnsignedWrap() &&
Inst.setHasNoSignedWrap(Inst.hasNoSignedWrap() &&
InnerOBO->hasNoSignedWrap() &&
if (InnerI && isInstructionTriviallyDead(InnerI))
// We can't do recursive deletion until we're done iterating, as we might
// have a phi which (potentially indirectly) uses instructions later in
// the block we're iterating through.
/// Unroll the given loop by Count. The loop must be in LCSSA form. Unrolling
/// can only fail when the loop's latch block is not terminated by a conditional
/// branch instruction. However, if the trip count (and multiple) are not known,
/// loop unrolling will mostly produce more code that is no faster.
/// If Runtime is true then UnrollLoop will try to insert a prologue or
/// epilogue that ensures the latch has a trip multiple of Count. UnrollLoop
/// will not runtime-unroll the loop if computing the run-time trip count will
/// be expensive and AllowExpensiveTripCount is false.
/// The LoopInfo Analysis that is passed will be kept consistent.
/// This utility preserves LoopInfo. It will also preserve ScalarEvolution and
/// DominatorTree if they are non-null.
/// If RemainderLoop is non-null, it will receive the remainder loop (if
/// required and not fully unrolled).
llvm::UnrollLoop(Loop *L, UnrollLoopOptions ULO, LoopInfo *LI,
ScalarEvolution *SE, DominatorTree *DT, AssumptionCache *AC,
const TargetTransformInfo *TTI, OptimizationRemarkEmitter *ORE,
bool PreserveLCSSA, Loop **RemainderLoop, AAResults *AA) {
assert(DT && "DomTree is required");
if (!L->getLoopPreheader()) {
LLVM_DEBUG(dbgs() << " Can't unroll; loop preheader-insertion failed.\n");
return LoopUnrollResult::Unmodified;
if (!L->getLoopLatch()) {
LLVM_DEBUG(dbgs() << " Can't unroll; loop exit-block-insertion failed.\n");
return LoopUnrollResult::Unmodified;
// Loops with indirectbr cannot be cloned.
if (!L->isSafeToClone()) {
LLVM_DEBUG(dbgs() << " Can't unroll; Loop body cannot be cloned.\n");
return LoopUnrollResult::Unmodified;
if (L->getHeader()->hasAddressTaken()) {
// The loop-rotate pass can be helpful to avoid this in many cases.
dbgs() << " Won't unroll loop: address of header block is taken.\n");
return LoopUnrollResult::Unmodified;
assert(ULO.Count > 0);
// All these values should be taken only after peeling because they might have
// changed.
BasicBlock *Preheader = L->getLoopPreheader();
BasicBlock *Header = L->getHeader();
BasicBlock *LatchBlock = L->getLoopLatch();
SmallVector<BasicBlock *, 4> ExitBlocks;
std::vector<BasicBlock *> OriginalLoopBlocks = L->getBlocks();
const unsigned MaxTripCount = SE->getSmallConstantMaxTripCount(L);
const bool MaxOrZero = SE->isBackedgeTakenCountMaxOrZero(L);
unsigned EstimatedLoopInvocationWeight = 0;
std::optional<unsigned> OriginalTripCount =
llvm::getLoopEstimatedTripCount(L, &EstimatedLoopInvocationWeight);
// Effectively "DCE" unrolled iterations that are beyond the max tripcount
// and will never be executed.
if (MaxTripCount && ULO.Count > MaxTripCount)
ULO.Count = MaxTripCount;
struct ExitInfo {
unsigned TripCount;
unsigned TripMultiple;
unsigned BreakoutTrip;
bool ExitOnTrue;
BasicBlock *FirstExitingBlock = nullptr;
SmallVector<BasicBlock *> ExitingBlocks;
DenseMap<BasicBlock *, ExitInfo> ExitInfos;
SmallVector<BasicBlock *, 4> ExitingBlocks;
for (auto *ExitingBlock : ExitingBlocks) {
// The folding code is not prepared to deal with non-branch instructions
// right now.
auto *BI = dyn_cast<BranchInst>(ExitingBlock->getTerminator());
if (!BI)
ExitInfo &Info = ExitInfos.try_emplace(ExitingBlock).first->second;
Info.TripCount = SE->getSmallConstantTripCount(L, ExitingBlock);
Info.TripMultiple = SE->getSmallConstantTripMultiple(L, ExitingBlock);
if (Info.TripCount != 0) {
Info.BreakoutTrip = Info.TripCount % ULO.Count;
Info.TripMultiple = 0;
} else {
Info.BreakoutTrip = Info.TripMultiple =
(unsigned)std::gcd(ULO.Count, Info.TripMultiple);
Info.ExitOnTrue = !L->contains(BI->getSuccessor(0));
LLVM_DEBUG(dbgs() << " Exiting block %" << ExitingBlock->getName()
<< ": TripCount=" << Info.TripCount
<< ", TripMultiple=" << Info.TripMultiple
<< ", BreakoutTrip=" << Info.BreakoutTrip << "\n");
// Are we eliminating the loop control altogether? Note that we can know
// we're eliminating the backedge without knowing exactly which iteration
// of the unrolled body exits.
const bool CompletelyUnroll = ULO.Count == MaxTripCount;
const bool PreserveOnlyFirst = CompletelyUnroll && MaxOrZero;
// There's no point in performing runtime unrolling if this unroll count
// results in a full unroll.
if (CompletelyUnroll)
ULO.Runtime = false;
// Go through all exits of L and see if there are any phi-nodes there. We just
// conservatively assume that they're inserted to preserve LCSSA form, which
// means that complete unrolling might break this form. We need to either fix
// it in-place after the transformation, or entirely rebuild LCSSA. TODO: For
// now we just recompute LCSSA for the outer loop, but it should be possible
// to fix it in-place.
bool NeedToFixLCSSA =
PreserveLCSSA && CompletelyUnroll &&
[](const BasicBlock *BB) { return isa<PHINode>(BB->begin()); });
// The current loop unroll pass can unroll loops that have
// (1) single latch; and
// (2a) latch is unconditional; or
// (2b) latch is conditional and is an exiting block
// FIXME: The implementation can be extended to work with more complicated
// cases, e.g. loops with multiple latches.
BranchInst *LatchBI = dyn_cast<BranchInst>(LatchBlock->getTerminator());
// A conditional branch which exits the loop, which can be optimized to an
// unconditional branch in the unrolled loop in some cases.
bool LatchIsExiting = L->isLoopExiting(LatchBlock);
if (!LatchBI || (LatchBI->isConditional() && !LatchIsExiting)) {
dbgs() << "Can't unroll; a conditional latch must exit the loop");
return LoopUnrollResult::Unmodified;
// Loops containing convergent instructions cannot use runtime unrolling,
// as the prologue/epilogue may add additional control-dependencies to
// convergent operations.
bool HasConvergent = false;
for (auto &BB : L->blocks())
for (auto &I : *BB)
if (auto *CB = dyn_cast<CallBase>(&I))
HasConvergent |= CB->isConvergent();
assert((!HasConvergent || !ULO.Runtime) &&
"Can't runtime unroll if loop contains a convergent operation.");
bool EpilogProfitability =
UnrollRuntimeEpilog.getNumOccurrences() ? UnrollRuntimeEpilog
: isEpilogProfitable(L);
if (ULO.Runtime &&
!UnrollRuntimeLoopRemainder(L, ULO.Count, ULO.AllowExpensiveTripCount,
EpilogProfitability, ULO.UnrollRemainder,
PreserveLCSSA, RemainderLoop)) {
if (ULO.Force)
ULO.Runtime = false;
else {
LLVM_DEBUG(dbgs() << "Won't unroll; remainder loop could not be "
"generated when assuming runtime trip count\n");
return LoopUnrollResult::Unmodified;
using namespace ore;
// Report the unrolling decision.
if (CompletelyUnroll) {
LLVM_DEBUG(dbgs() << "COMPLETELY UNROLLING loop %" << Header->getName()
<< " with trip count " << ULO.Count << "!\n");
if (ORE)
ORE->emit([&]() {
return OptimizationRemark(DEBUG_TYPE, "FullyUnrolled", L->getStartLoc(),
<< "completely unrolled loop with "
<< NV("UnrollCount", ULO.Count) << " iterations";
} else {
LLVM_DEBUG(dbgs() << "UNROLLING loop %" << Header->getName() << " by "
<< ULO.Count);
if (ULO.Runtime)
LLVM_DEBUG(dbgs() << " with run-time trip count");
LLVM_DEBUG(dbgs() << "!\n");
if (ORE)
ORE->emit([&]() {
OptimizationRemark Diag(DEBUG_TYPE, "PartialUnrolled", L->getStartLoc(),
Diag << "unrolled loop by a factor of " << NV("UnrollCount", ULO.Count);
if (ULO.Runtime)
Diag << " with run-time trip count";
return Diag;
// We are going to make changes to this loop. SCEV may be keeping cached info
// about it, in particular about backedge taken count. The changes we make
// are guaranteed to invalidate this information for our loop. It is tempting
// to only invalidate the loop being unrolled, but it is incorrect as long as
// all exiting branches from all inner loops have impact on the outer loops,
// and if something changes inside them then any of outer loops may also
// change. When we forget outermost loop, we also forget all contained loops
// and this is what we need here.
if (SE) {
if (ULO.ForgetAllSCEV)
else {
if (!LatchIsExiting)
// For the first iteration of the loop, we should use the precloned values for
// PHI nodes. Insert associations now.
ValueToValueMapTy LastValueMap;
std::vector<PHINode*> OrigPHINode;
for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) {
std::vector<BasicBlock *> Headers;
std::vector<BasicBlock *> Latches;
// The current on-the-fly SSA update requires blocks to be processed in
// reverse postorder so that LastValueMap contains the correct value at each
// exit.
LoopBlocksDFS DFS(L);
// Stash the DFS iterators before adding blocks to the loop.
LoopBlocksDFS::RPOIterator BlockBegin = DFS.beginRPO();
LoopBlocksDFS::RPOIterator BlockEnd = DFS.endRPO();
std::vector<BasicBlock*> UnrolledLoopBlocks = L->getBlocks();
// Loop Unrolling might create new loops. While we do preserve LoopInfo, we
// might break loop-simplified form for these loops (as they, e.g., would
// share the same exit blocks). We'll keep track of loops for which we can
// break this so that later we can re-simplify them.
SmallSetVector<Loop *, 4> LoopsToSimplify;
for (Loop *SubLoop : *L)
// When a FSDiscriminator is enabled, we don't need to add the multiply
// factors to the discriminators.
if (Header->getParent()->shouldEmitDebugInfoForProfiling() &&
for (BasicBlock *BB : L->getBlocks())
for (Instruction &I : *BB)
if (!I.isDebugOrPseudoInst())
if (const DILocation *DIL = I.getDebugLoc()) {
auto NewDIL = DIL->cloneByMultiplyingDuplicationFactor(ULO.Count);
if (NewDIL)
<< "Failed to create new discriminator: "
<< DIL->getFilename() << " Line: " << DIL->getLine());
// Identify what noalias metadata is inside the loop: if it is inside the
// loop, the associated metadata must be cloned for each iteration.
SmallVector<MDNode *, 6> LoopLocalNoAliasDeclScopes;
identifyNoAliasScopesToClone(L->getBlocks(), LoopLocalNoAliasDeclScopes);
// We place the unrolled iterations immediately after the original loop
// latch. This is a reasonable default placement if we don't have block
// frequencies, and if we do, well the layout will be adjusted later.
auto BlockInsertPt = std::next(LatchBlock->getIterator());
for (unsigned It = 1; It != ULO.Count; ++It) {
SmallVector<BasicBlock *, 8> NewBlocks;
SmallDenseMap<const Loop *, Loop *, 4> NewLoops;
NewLoops[L] = L;
for (LoopBlocksDFS::RPOIterator BB = BlockBegin; BB != BlockEnd; ++BB) {
ValueToValueMapTy VMap;
BasicBlock *New = CloneBasicBlock(*BB, VMap, "." + Twine(It));
Header->getParent()->insert(BlockInsertPt, New);
assert((*BB != Header || LI->getLoopFor(*BB) == L) &&
"Header should not be in a sub-loop");
// Tell LI about New.
const Loop *OldLoop = addClonedBlockToLoopInfo(*BB, New, LI, NewLoops);
if (OldLoop)
if (*BB == Header)
// Loop over all of the PHI nodes in the block, changing them to use
// the incoming values from the previous block.
for (PHINode *OrigPHI : OrigPHINode) {
PHINode *NewPHI = cast<PHINode>(VMap[OrigPHI]);
Value *InVal = NewPHI->getIncomingValueForBlock(LatchBlock);
if (Instruction *InValI = dyn_cast<Instruction>(InVal))
if (It > 1 && L->contains(InValI))
InVal = LastValueMap[InValI];
VMap[OrigPHI] = InVal;
// Update our running map of newest clones
LastValueMap[*BB] = New;
for (ValueToValueMapTy::iterator VI = VMap.begin(), VE = VMap.end();
VI != VE; ++VI)
LastValueMap[VI->first] = VI->second;
// Add phi entries for newly created values to all exit blocks.
for (BasicBlock *Succ : successors(*BB)) {
if (L->contains(Succ))
for (PHINode &PHI : Succ->phis()) {
Value *Incoming = PHI.getIncomingValueForBlock(*BB);
ValueToValueMapTy::iterator It = LastValueMap.find(Incoming);
if (It != LastValueMap.end())
Incoming = It->second;
PHI.addIncoming(Incoming, New);
// Keep track of new headers and latches as we create them, so that
// we can insert the proper branches later.
if (*BB == Header)
if (*BB == LatchBlock)
// Keep track of the exiting block and its successor block contained in
// the loop for the current iteration.
auto ExitInfoIt = ExitInfos.find(*BB);
if (ExitInfoIt != ExitInfos.end())
// Update DomTree: since we just copy the loop body, and each copy has a
// dedicated entry block (copy of the header block), this header's copy
// dominates all copied blocks. That means, dominance relations in the
// copied body are the same as in the original body.
if (*BB == Header)
DT->addNewBlock(New, Latches[It - 1]);
else {
auto BBDomNode = DT->getNode(*BB);
auto BBIDom = BBDomNode->getIDom();
BasicBlock *OriginalBBIDom = BBIDom->getBlock();
New, cast<BasicBlock>(LastValueMap[cast<Value>(OriginalBBIDom)]));
// Remap all instructions in the most recent iteration
remapInstructionsInBlocks(NewBlocks, LastValueMap);
for (BasicBlock *NewBlock : NewBlocks)
for (Instruction &I : *NewBlock)
if (auto *II = dyn_cast<AssumeInst>(&I))
// Identify what other metadata depends on the cloned version. After
// cloning, replace the metadata with the corrected version for both
// memory instructions and noalias intrinsics.
std::string ext = (Twine("It") + Twine(It)).str();
cloneAndAdaptNoAliasScopes(LoopLocalNoAliasDeclScopes, NewBlocks,
Header->getContext(), ext);
// Loop over the PHI nodes in the original block, setting incoming values.
for (PHINode *PN : OrigPHINode) {
if (CompletelyUnroll) {
} else if (ULO.Count > 1) {
Value *InVal = PN->removeIncomingValue(LatchBlock, false);
// If this value was defined in the loop, take the value defined by the
// last iteration of the loop.
if (Instruction *InValI = dyn_cast<Instruction>(InVal)) {
if (L->contains(InValI))
InVal = LastValueMap[InVal];
assert(Latches.back() == LastValueMap[LatchBlock] && "bad last latch");
PN->addIncoming(InVal, Latches.back());
// Connect latches of the unrolled iterations to the headers of the next
// iteration. Currently they point to the header of the same iteration.
for (unsigned i = 0, e = Latches.size(); i != e; ++i) {
unsigned j = (i + 1) % e;
Latches[i]->getTerminator()->replaceSuccessorWith(Headers[i], Headers[j]);
// Update dominators of blocks we might reach through exits.
// Immediate dominator of such block might change, because we add more
// routes which can lead to the exit: we can now reach it from the copied
// iterations too.
if (ULO.Count > 1) {
for (auto *BB : OriginalLoopBlocks) {
auto *BBDomNode = DT->getNode(BB);
SmallVector<BasicBlock *, 16> ChildrenToUpdate;
for (auto *ChildDomNode : BBDomNode->children()) {
auto *ChildBB = ChildDomNode->getBlock();
if (!L->contains(ChildBB))
// The new idom of the block will be the nearest common dominator
// of all copies of the previous idom. This is equivalent to the
// nearest common dominator of the previous idom and the first latch,
// which dominates all copies of the previous idom.
BasicBlock *NewIDom = DT->findNearestCommonDominator(BB, LatchBlock);
for (auto *ChildBB : ChildrenToUpdate)
DT->changeImmediateDominator(ChildBB, NewIDom);
assert(!UnrollVerifyDomtree ||
SmallVector<DominatorTree::UpdateType> DTUpdates;
auto SetDest = [&](BasicBlock *Src, bool WillExit, bool ExitOnTrue) {
auto *Term = cast<BranchInst>(Src->getTerminator());
const unsigned Idx = ExitOnTrue ^ WillExit;
BasicBlock *Dest = Term->getSuccessor(Idx);
BasicBlock *DeadSucc = Term->getSuccessor(1-Idx);
// Remove predecessors from all non-Dest successors.
DeadSucc->removePredecessor(Src, /* KeepOneInputPHIs */ true);
// Replace the conditional branch with an unconditional one.
BranchInst::Create(Dest, Term->getIterator());
DTUpdates.emplace_back(DominatorTree::Delete, Src, DeadSucc);
auto WillExit = [&](const ExitInfo &Info, unsigned i, unsigned j,
bool IsLatch) -> std::optional<bool> {
if (CompletelyUnroll) {
if (PreserveOnlyFirst) {
if (i == 0)
return std::nullopt;
return j == 0;
// Complete (but possibly inexact) unrolling
if (j == 0)
return true;
if (Info.TripCount && j != Info.TripCount)
return false;
return std::nullopt;
if (ULO.Runtime) {
// If runtime unrolling inserts a prologue, information about non-latch
// exits may be stale.
if (IsLatch && j != 0)
return false;
return std::nullopt;
if (j != Info.BreakoutTrip &&
(Info.TripMultiple == 0 || j % Info.TripMultiple != 0)) {
// If we know the trip count or a multiple of it, we can safely use an
// unconditional branch for some iterations.
return false;
return std::nullopt;
// Fold branches for iterations where we know that they will exit or not
// exit.
for (auto &Pair : ExitInfos) {
ExitInfo &Info = Pair.second;
for (unsigned i = 0, e = Info.ExitingBlocks.size(); i != e; ++i) {
// The branch destination.
unsigned j = (i + 1) % e;
bool IsLatch = Pair.first == LatchBlock;
std::optional<bool> KnownWillExit = WillExit(Info, i, j, IsLatch);
if (!KnownWillExit) {
if (!Info.FirstExitingBlock)
Info.FirstExitingBlock = Info.ExitingBlocks[i];
// We don't fold known-exiting branches for non-latch exits here,
// because this ensures that both all loop blocks and all exit blocks
// remain reachable in the CFG.
// TODO: We could fold these branches, but it would require much more
// sophisticated updates to LoopInfo.
if (*KnownWillExit && !IsLatch) {
if (!Info.FirstExitingBlock)
Info.FirstExitingBlock = Info.ExitingBlocks[i];
SetDest(Info.ExitingBlocks[i], *KnownWillExit, Info.ExitOnTrue);
DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Lazy);
DomTreeUpdater *DTUToUse = &DTU;
if (ExitingBlocks.size() == 1 && ExitInfos.size() == 1) {
// Manually update the DT if there's a single exiting node. In that case
// there's a single exit node and it is sufficient to update the nodes
// immediately dominated by the original exiting block. They will become
// dominated by the first exiting block that leaves the loop after
// unrolling. Note that the CFG inside the loop does not change, so there's
// no need to update the DT inside the unrolled loop.
DTUToUse = nullptr;
auto &[OriginalExit, Info] = *ExitInfos.begin();
if (!Info.FirstExitingBlock)
Info.FirstExitingBlock = Info.ExitingBlocks.back();
for (auto *C : to_vector(DT->getNode(OriginalExit)->children())) {
if (L->contains(C->getBlock()))
} else {
// When completely unrolling, the last latch becomes unreachable.
if (!LatchIsExiting && CompletelyUnroll) {
// There is no need to update the DT here, because there must be a unique
// latch. Hence if the latch is not exiting it must directly branch back to
// the original loop header and does not dominate any nodes.
assert(LatchBlock->getSingleSuccessor() && "Loop with multiple latches?");
changeToUnreachable(Latches.back()->getTerminator(), PreserveLCSSA);
// Merge adjacent basic blocks, if possible.
for (BasicBlock *Latch : Latches) {
BranchInst *Term = dyn_cast<BranchInst>(Latch->getTerminator());
assert((Term ||
(CompletelyUnroll && !LatchIsExiting && Latch == Latches.back())) &&
"Need a branch as terminator, except when fully unrolling with "
"unconditional latch");
if (Term && Term->isUnconditional()) {
BasicBlock *Dest = Term->getSuccessor(0);
BasicBlock *Fold = Dest->getUniquePredecessor();
if (MergeBlockIntoPredecessor(Dest, /*DTU=*/DTUToUse, LI,
/*MSSAU=*/nullptr, /*MemDep=*/nullptr,
DTUToUse ? nullptr : DT)) {
// Dest has been folded into Fold. Update our worklists accordingly.
std::replace(Latches.begin(), Latches.end(), Dest, Fold);
llvm::erase(UnrolledLoopBlocks, Dest);
if (DTUToUse) {
// Apply updates to the DomTree.
DT = &DTU.getDomTree();
assert(!UnrollVerifyDomtree ||
// At this point, the code is well formed. We now simplify the unrolled loop,
// doing constant propagation and dead code elimination as we go.
simplifyLoopAfterUnroll(L, !CompletelyUnroll && ULO.Count > 1, LI, SE, DT, AC,
NumCompletelyUnrolled += CompletelyUnroll;
Loop *OuterL = L->getParentLoop();
// Update LoopInfo if the loop is completely removed.
if (CompletelyUnroll) {
// We shouldn't try to use `L` anymore.
L = nullptr;
} else if (OriginalTripCount) {
// Update the trip count. Note that the remainder has already logic
// computing it in `UnrollRuntimeLoopRemainder`.
setLoopEstimatedTripCount(L, *OriginalTripCount / ULO.Count,
// LoopInfo should not be valid, confirm that.
if (UnrollVerifyLoopInfo)
// After complete unrolling most of the blocks should be contained in OuterL.
// However, some of them might happen to be out of OuterL (e.g. if they
// precede a loop exit). In this case we might need to insert PHI nodes in
// order to preserve LCSSA form.
// We don't need to check this if we already know that we need to fix LCSSA
// form.
// TODO: For now we just recompute LCSSA for the outer loop in this case, but
// it should be possible to fix it in-place.
if (PreserveLCSSA && OuterL && CompletelyUnroll && !NeedToFixLCSSA)
NeedToFixLCSSA |= ::needToInsertPhisForLCSSA(OuterL, UnrolledLoopBlocks, LI);
// Make sure that loop-simplify form is preserved. We want to simplify
// at least one layer outside of the loop that was unrolled so that any
// changes to the parent loop exposed by the unrolling are considered.
if (OuterL) {
// OuterL includes all loops for which we can break loop-simplify, so
// it's sufficient to simplify only it (it'll recursively simplify inner
// loops too).
if (NeedToFixLCSSA) {
// LCSSA must be performed on the outermost affected loop. The unrolled
// loop's last loop latch is guaranteed to be in the outermost loop
// after LoopInfo's been updated by LoopInfo::erase.
Loop *LatchLoop = LI->getLoopFor(Latches.back());
Loop *FixLCSSALoop = OuterL;
if (!FixLCSSALoop->contains(LatchLoop))
while (FixLCSSALoop->getParentLoop() != LatchLoop)
FixLCSSALoop = FixLCSSALoop->getParentLoop();
formLCSSARecursively(*FixLCSSALoop, *DT, LI, SE);
} else if (PreserveLCSSA) {
assert(OuterL->isLCSSAForm(*DT) &&
"Loops should be in LCSSA form after loop-unroll.");
// TODO: That potentially might be compile-time expensive. We should try
// to fix the loop-simplified form incrementally.
simplifyLoop(OuterL, DT, LI, SE, AC, nullptr, PreserveLCSSA);
} else {
// Simplify loops for which we might've broken loop-simplify form.
for (Loop *SubLoop : LoopsToSimplify)
simplifyLoop(SubLoop, DT, LI, SE, AC, nullptr, PreserveLCSSA);
return CompletelyUnroll ? LoopUnrollResult::FullyUnrolled
: LoopUnrollResult::PartiallyUnrolled;
/// Given an llvm.loop loop id metadata node, returns the loop hint metadata
/// node with the given name (for example, "llvm.loop.unroll.count"). If no
/// such metadata node exists, then nullptr is returned.
MDNode *llvm::GetUnrollMetadata(MDNode *LoopID, StringRef Name) {
// First operand should refer to the loop id itself.
assert(LoopID->getNumOperands() > 0 && "requires at least one operand");
assert(LoopID->getOperand(0) == LoopID && "invalid loop id");
for (unsigned i = 1, e = LoopID->getNumOperands(); i < e; ++i) {
MDNode *MD = dyn_cast<MDNode>(LoopID->getOperand(i));
if (!MD)
MDString *S = dyn_cast<MDString>(MD->getOperand(0));
if (!S)
if (Name == S->getString())
return MD;
return nullptr;