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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/Optional.h"
#include "llvm/ADT/STLExtras.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/ADT/iterator_range.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/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/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/LoopPeel.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 <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"),
/// 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;
/// 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) {
// 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))
// At this point, the code is well formed. We now do a quick sweep over the
// inserted code, doing constant propagation and dead code elimination as we
// go.
const DataLayout &DL = L->getHeader()->getModule()->getDataLayout();
for (BasicBlock *BB : L->getBlocks()) {
for (BasicBlock::iterator I = BB->begin(), E = BB->end(); I != E;) {
Instruction *Inst = &*I++;
if (Value *V = SimplifyInstruction(Inst, {DL, nullptr, DT, AC}))
if (LI->replacementPreservesLCSSAForm(Inst, V))
if (isInstructionTriviallyDead(Inst))
// TODO: after peeling or unrolling, previously loop variant conditions are
// likely to fold to constants, eagerly propagating those here will require
// fewer cleanup passes to be run. Alternatively, a LoopEarlyCSE might be
// appropriate.
/// 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.
/// TripCount is the upper bound of the iteration on which control exits
/// LatchBlock. Control may exit the loop prior to TripCount iterations either
/// via an early branch in other loop block or via LatchBlock terminator. This
/// is relaxed from the general definition of trip count which is the number of
/// times the loop header executes. Note that UnrollLoop assumes that the loop
/// counter test is in LatchBlock in order to remove unnecesssary instances of
/// the test. If control can exit the loop from the LatchBlock's terminator
/// prior to TripCount iterations, flag PreserveCondBr needs to be set.
/// PreserveCondBr indicates whether the conditional branch of the LatchBlock
/// needs to be preserved. It is needed when we use trip count upper bound to
/// fully unroll the loop. If PreserveOnlyFirst is also set then only the first
/// conditional branch needs to be preserved.
/// Similarly, TripMultiple divides the number of times that the LatchBlock may
/// execute without exiting the loop.
/// If AllowRuntime is true then UnrollLoop will consider unrolling loops that
/// have a runtime (i.e. not compile time constant) trip count. Unrolling these
/// loops require a unroll "prologue" that runs "RuntimeTripCount % Count"
/// iterations before branching into the unrolled loop. UnrollLoop will not
/// runtime-unroll the loop if computing RuntimeTripCount will be expensive and
/// AllowExpensiveTripCount is false.
/// If we want to perform PGO-based loop peeling, PeelCount is set to the
/// number of iterations we want to peel off.
/// 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).
LoopUnrollResult llvm::UnrollLoop(Loop *L, UnrollLoopOptions ULO, LoopInfo *LI,
ScalarEvolution *SE, DominatorTree *DT,
AssumptionCache *AC,
const TargetTransformInfo *TTI,
OptimizationRemarkEmitter *ORE,
bool PreserveLCSSA, Loop **RemainderLoop) {
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;
if (ULO.TripCount != 0)
LLVM_DEBUG(dbgs() << " Trip Count = " << ULO.TripCount << "\n");
if (ULO.TripMultiple != 1)
LLVM_DEBUG(dbgs() << " Trip Multiple = " << ULO.TripMultiple << "\n");
// Effectively "DCE" unrolled iterations that are beyond the tripcount
// and will never be executed.
if (ULO.TripCount != 0 && ULO.Count > ULO.TripCount)
ULO.Count = ULO.TripCount;
// Don't enter the unroll code if there is nothing to do.
if (ULO.TripCount == 0 && ULO.Count < 2 && ULO.PeelCount == 0) {
LLVM_DEBUG(dbgs() << "Won't unroll; almost nothing to do\n");
return LoopUnrollResult::Unmodified;
assert(ULO.Count > 0);
assert(ULO.TripMultiple > 0);
assert(ULO.TripCount == 0 || ULO.TripCount % ULO.TripMultiple == 0);
// Are we eliminating the loop control altogether?
bool CompletelyUnroll = ULO.Count == ULO.TripCount;
// We assume a run-time trip count if the compiler cannot
// figure out the loop trip count and the unroll-runtime
// flag is specified.
bool RuntimeTripCount =
(ULO.TripCount == 0 && ULO.Count > 0 && ULO.AllowRuntime);
assert((!RuntimeTripCount || !ULO.PeelCount) &&
"Did not expect runtime trip-count unrolling "
"and peeling for the same loop");
bool Peeled = false;
if (ULO.PeelCount) {
Peeled = peelLoop(L, ULO.PeelCount, LI, SE, DT, AC, PreserveLCSSA);
// Successful peeling may result in a change in the loop preheader/trip
// counts. If we later unroll the loop, we want these to be updated.
if (Peeled) {
// According to our guards and profitability checks the only
// meaningful exit should be latch block. Other exits go to deopt,
// so we do not worry about them.
BasicBlock *ExitingBlock = L->getLoopLatch();
assert(ExitingBlock && "Loop without exiting block?");
assert(L->isLoopExiting(ExitingBlock) && "Latch is not exiting?");
ULO.TripCount = SE->getSmallConstantTripCount(L, ExitingBlock);
ULO.TripMultiple = SE->getSmallConstantTripMultiple(L, ExitingBlock);
// 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();
// 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.
BranchInst *ExitingBI = nullptr;
bool LatchIsExiting = L->isLoopExiting(LatchBlock);
if (LatchIsExiting)
ExitingBI = LatchBI;
else if (BasicBlock *ExitingBlock = L->getExitingBlock())
ExitingBI = dyn_cast<BranchInst>(ExitingBlock->getTerminator());
if (!LatchBI || (LatchBI->isConditional() && !LatchIsExiting)) {
// If the peeling guard is changed this assert may be relaxed or even
// deleted.
assert(!Peeled && "Peeling guard changed!");
dbgs() << "Can't unroll; a conditional latch must exit the loop");
return LoopUnrollResult::Unmodified;
if (ExitingBI)
dbgs() << " Exiting Block = " << ExitingBI->getParent()->getName()
<< "\n";
dbgs() << " No single exiting block\n";
// Loops containing convergent instructions must have a count that divides
// their TripMultiple.
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.TripMultiple % ULO.Count == 0) &&
"Unroll count must divide trip multiple if loop contains a "
"convergent operation.");
bool EpilogProfitability =
UnrollRuntimeEpilog.getNumOccurrences() ? UnrollRuntimeEpilog
: isEpilogProfitable(L);
if (RuntimeTripCount && ULO.TripMultiple % ULO.Count != 0 &&
!UnrollRuntimeLoopRemainder(L, ULO.Count, ULO.AllowExpensiveTripCount,
EpilogProfitability, ULO.UnrollRemainder,
PreserveLCSSA, RemainderLoop)) {
if (ULO.Force)
RuntimeTripCount = false;
else {
LLVM_DEBUG(dbgs() << "Won't unroll; remainder loop could not be "
"generated when assuming runtime trip count\n");
return LoopUnrollResult::Unmodified;
// If we know the trip count, we know the multiple...
unsigned BreakoutTrip = 0;
if (ULO.TripCount != 0) {
BreakoutTrip = ULO.TripCount % ULO.Count;
ULO.TripMultiple = 0;
} else {
// Figure out what multiple to use.
BreakoutTrip = ULO.TripMultiple =
(unsigned)GreatestCommonDivisor64(ULO.Count, ULO.TripMultiple);
using namespace ore;
// Report the unrolling decision.
if (CompletelyUnroll) {
LLVM_DEBUG(dbgs() << "COMPLETELY UNROLLING loop %" << Header->getName()
<< " with trip count " << ULO.TripCount << "!\n");
if (ORE)
ORE->emit([&]() {
return OptimizationRemark(DEBUG_TYPE, "FullyUnrolled", L->getStartLoc(),
<< "completely unrolled loop with "
<< NV("UnrollCount", ULO.TripCount) << " iterations";
} else if (ULO.PeelCount) {
LLVM_DEBUG(dbgs() << "PEELING loop %" << Header->getName()
<< " with iteration count " << ULO.PeelCount << "!\n");
if (ORE)
ORE->emit([&]() {
return OptimizationRemark(DEBUG_TYPE, "Peeled", L->getStartLoc(),
<< " peeled loop by " << NV("PeelCount", ULO.PeelCount)
<< " iterations";
} else {
auto DiagBuilder = [&]() {
OptimizationRemark Diag(DEBUG_TYPE, "PartialUnrolled", L->getStartLoc(),
return Diag << "unrolled loop by a factor of "
<< NV("UnrollCount", ULO.Count);
LLVM_DEBUG(dbgs() << "UNROLLING loop %" << Header->getName() << " by "
<< ULO.Count);
if (ULO.TripMultiple == 0 || BreakoutTrip != ULO.TripMultiple) {
LLVM_DEBUG(dbgs() << " with a breakout at trip " << BreakoutTrip);
if (ORE)
ORE->emit([&]() {
return DiagBuilder() << " with a breakout at trip "
<< NV("BreakoutTrip", BreakoutTrip);
} else if (ULO.TripMultiple != 1) {
LLVM_DEBUG(dbgs() << " with " << ULO.TripMultiple << " trips per branch");
if (ORE)
ORE->emit([&]() {
return DiagBuilder()
<< " with " << NV("TripMultiple", ULO.TripMultiple)
<< " trips per branch";
} else if (RuntimeTripCount) {
LLVM_DEBUG(dbgs() << " with run-time trip count");
if (ORE)
[&]() { return DiagBuilder() << " with run-time trip count"; });
LLVM_DEBUG(dbgs() << "!\n");
// 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)
if (!LatchIsExiting)
Optional<bool> ContinueOnTrue = None;
BasicBlock *LoopExit = nullptr;
if (ExitingBI) {
ContinueOnTrue = L->contains(ExitingBI->getSuccessor(0));
LoopExit = ExitingBI->getSuccessor(*ContinueOnTrue);
// 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 *> ExitingBlocks;
std::vector<BasicBlock *> ExitingSucc;
std::vector<BasicBlock *> Latches;
if (ExitingBI) {
// 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)
if (Header->getParent()->isDebugInfoForProfiling())
for (BasicBlock *BB : L->getBlocks())
for (Instruction &I : *BB)
if (!isa<DbgInfoIntrinsic>(&I))
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);
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));
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.
if (ExitingBI) {
if (*BB == ExitingBlocks[0])
if (*BB == ExitingSucc[0])
// 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 (DT) {
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<IntrinsicInst>(&I))
if (II->getIntrinsicID() == Intrinsic::assume)
// 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());
auto setDest = [](BasicBlock *Src, BasicBlock *Dest, BasicBlock *BlockInLoop,
bool NeedConditional, Optional<bool> ContinueOnTrue,
bool IsDestLoopExit) {
auto *Term = cast<BranchInst>(Src->getTerminator());
if (NeedConditional) {
// Update the conditional branch's successor for the following
// iteration.
assert(ContinueOnTrue.hasValue() &&
"Expecting valid ContinueOnTrue when NeedConditional is true");
Term->setSuccessor(!(*ContinueOnTrue), Dest);
} else {
// Remove phi operands at this loop exit
if (!IsDestLoopExit) {
BasicBlock *BB = Src;
for (BasicBlock *Succ : successors(BB)) {
// Preserve the incoming value from BB if we are jumping to the block
// in the current loop.
if (Succ == BlockInLoop)
for (PHINode &Phi : Succ->phis())
Phi.removeIncomingValue(BB, false);
// Replace the conditional branch with an unconditional one.
BranchInst::Create(Dest, Term);
// Connect latches of the unrolled iterations to the headers of the next
// iteration. If the latch is also the exiting block, the conditional branch
// may have to be preserved.
for (unsigned i = 0, e = Latches.size(); i != e; ++i) {
// The branch destination.
unsigned j = (i + 1) % e;
BasicBlock *Dest = Headers[j];
bool NeedConditional = LatchIsExiting;
if (LatchIsExiting) {
if (RuntimeTripCount && j != 0)
NeedConditional = false;
// For a complete unroll, make the last iteration end with a branch
// to the exit block.
if (CompletelyUnroll) {
if (j == 0)
Dest = LoopExit;
// If using trip count upper bound to completely unroll, we need to
// keep the conditional branch except the last one because the loop
// may exit after any iteration.
assert(NeedConditional &&
"NeedCondition cannot be modified by both complete "
"unrolling and runtime unrolling");
NeedConditional =
(ULO.PreserveCondBr && j && !(ULO.PreserveOnlyFirst && i != 0));
} else if (j != BreakoutTrip &&
(ULO.TripMultiple == 0 || j % ULO.TripMultiple != 0)) {
// If we know the trip count or a multiple of it, we can safely use an
// unconditional branch for some iterations.
NeedConditional = false;
setDest(Latches[i], Dest, Headers[i], NeedConditional, ContinueOnTrue,
Dest == LoopExit);
if (!LatchIsExiting) {
// If the latch is not exiting, we may be able to simplify the conditional
// branches in the unrolled exiting blocks.
for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) {
// The branch destination.
unsigned j = (i + 1) % e;
bool NeedConditional = true;
if (RuntimeTripCount && j != 0)
NeedConditional = false;
if (CompletelyUnroll)
// We cannot drop the conditional branch for the last condition, as we
// may have to execute the loop body depending on the condition.
NeedConditional = j == 0 || ULO.PreserveCondBr;
else if (j != BreakoutTrip &&
(ULO.TripMultiple == 0 || j % ULO.TripMultiple != 0))
// If we know the trip count or a multiple of it, we can safely use an
// unconditional branch for some iterations.
NeedConditional = false;
// Conditional branches from non-latch exiting block have successors
// either in the same loop iteration or outside the loop. The branches are
// already correct.
if (NeedConditional)
setDest(ExitingBlocks[i], ExitingSucc[i], ExitingSucc[i], NeedConditional,
None, false);
// When completely unrolling, the last latch becomes unreachable.
if (CompletelyUnroll) {
BranchInst *Term = cast<BranchInst>(Latches.back()->getTerminator());
new UnreachableInst(Term->getContext(), Term);
// 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 (DT && 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))
BasicBlock *NewIDom;
if (ExitingBI && BB == ExitingBlocks[0]) {
// The latch is special because we emit unconditional branches in
// some cases where the original loop contained a conditional branch.
// Since the latch is always at the bottom of the loop, if the latch
// dominated an exit before unrolling, the new dominator of that exit
// must also be a latch. Specifically, the dominator is the first
// latch which ends in a conditional branch, or the last latch if
// there is no such latch.
// For loops exiting from non latch exiting block, we limit the
// branch simplification to single exiting block loops.
NewIDom = ExitingBlocks.back();
for (unsigned i = 0, e = ExitingBlocks.size(); i != e; ++i) {
Instruction *Term = ExitingBlocks[i]->getTerminator();
if (isa<BranchInst>(Term) && cast<BranchInst>(Term)->isConditional()) {
NewIDom =
DT->findNearestCommonDominator(ExitingBlocks[i], Latches[i]);
} else {
// 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.
NewIDom = DT->findNearestCommonDominator(BB, LatchBlock);
for (auto *ChildBB : ChildrenToUpdate)
DT->changeImmediateDominator(ChildBB, NewIDom);
assert(!DT || !UnrollVerifyDomtree ||
DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Lazy);
// 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, LI)) {
// Dest has been folded into Fold. Update our worklists accordingly.
std::replace(Latches.begin(), Latches.end(), Dest, Fold);
llvm::erase_value(UnrolledLoopBlocks, Dest);
// Apply updates to the DomTree.
DT = &DTU.getDomTree();
// 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 || Peeled), LI,
NumCompletelyUnrolled += CompletelyUnroll;
Loop *OuterL = L->getParentLoop();
// Update LoopInfo if the loop is completely removed.
if (CompletelyUnroll)
// 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);
// If we have a pass and a DominatorTree we should re-simplify impacted loops
// to ensure subsequent analyses can rely on this form. 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 (DT) {
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.equals(S->getString()))
return MD;
return nullptr;