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//===- LoopPeel.cpp -------------------------------------------------------===//
// 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
// Loop Peeling Utilities.
#include "llvm/Transforms/Utils/LoopPeel.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/Optional.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/LoopIterator.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/Analysis/ScalarEvolutionExpressions.h"
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/InstrTypes.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/MDBuilder.h"
#include "llvm/IR/Metadata.h"
#include "llvm/IR/PatternMatch.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/Cloning.h"
#include "llvm/Transforms/Utils/LoopSimplify.h"
#include "llvm/Transforms/Utils/LoopUtils.h"
#include "llvm/Transforms/Utils/UnrollLoop.h"
#include "llvm/Transforms/Utils/ValueMapper.h"
#include <algorithm>
#include <cassert>
#include <cstdint>
#include <limits>
using namespace llvm;
using namespace llvm::PatternMatch;
#define DEBUG_TYPE "loop-peel"
STATISTIC(NumPeeled, "Number of loops peeled");
static cl::opt<unsigned> UnrollPeelCount(
"unroll-peel-count", cl::Hidden,
cl::desc("Set the unroll peeling count, for testing purposes"));
static cl::opt<bool>
UnrollAllowPeeling("unroll-allow-peeling", cl::init(true), cl::Hidden,
cl::desc("Allows loops to be peeled when the dynamic "
"trip count is known to be low."));
static cl::opt<bool>
cl::init(false), cl::Hidden,
cl::desc("Allows loop nests to be peeled."));
static cl::opt<unsigned> UnrollPeelMaxCount(
"unroll-peel-max-count", cl::init(7), cl::Hidden,
cl::desc("Max average trip count which will cause loop peeling."));
static cl::opt<unsigned> UnrollForcePeelCount(
"unroll-force-peel-count", cl::init(0), cl::Hidden,
cl::desc("Force a peel count regardless of profiling information."));
static cl::opt<bool> UnrollPeelMultiDeoptExit(
"unroll-peel-multi-deopt-exit", cl::init(true), cl::Hidden,
cl::desc("Allow peeling of loops with multiple deopt exits."));
static const char *PeeledCountMetaData = "llvm.loop.peeled.count";
// Designates that a Phi is estimated to become invariant after an "infinite"
// number of loop iterations (i.e. only may become an invariant if the loop is
// fully unrolled).
static const unsigned InfiniteIterationsToInvariance =
// Check whether we are capable of peeling this loop.
bool llvm::canPeel(Loop *L) {
// Make sure the loop is in simplified form
if (!L->isLoopSimplifyForm())
return false;
if (UnrollPeelMultiDeoptExit) {
SmallVector<BasicBlock *, 4> Exits;
if (!Exits.empty()) {
// Latch's terminator is a conditional branch, Latch is exiting and
// all non Latch exits ends up with deoptimize.
const BasicBlock *Latch = L->getLoopLatch();
const BranchInst *T = dyn_cast<BranchInst>(Latch->getTerminator());
return T && T->isConditional() && L->isLoopExiting(Latch) &&
all_of(Exits, [](const BasicBlock *BB) {
return BB->getTerminatingDeoptimizeCall();
// Only peel loops that contain a single exit
if (!L->getExitingBlock() || !L->getUniqueExitBlock())
return false;
// Don't try to peel loops where the latch is not the exiting block.
// This can be an indication of two different things:
// 1) The loop is not rotated.
// 2) The loop contains irreducible control flow that involves the latch.
const BasicBlock *Latch = L->getLoopLatch();
if (Latch != L->getExitingBlock())
return false;
// Peeling is only supported if the latch is a branch.
if (!isa<BranchInst>(Latch->getTerminator()))
return false;
return true;
// This function calculates the number of iterations after which the given Phi
// becomes an invariant. The pre-calculated values are memorized in the map. The
// function (shortcut is I) is calculated according to the following definition:
// Given %x = phi <Inputs from above the loop>, ..., [%y, %back.edge].
// If %y is a loop invariant, then I(%x) = 1.
// If %y is a Phi from the loop header, I(%x) = I(%y) + 1.
// Otherwise, I(%x) is infinite.
// TODO: Actually if %y is an expression that depends only on Phi %z and some
// loop invariants, we can estimate I(%x) = I(%z) + 1. The example
// looks like:
// %x = phi(0, %a), <-- becomes invariant starting from 3rd iteration.
// %y = phi(0, 5),
// %a = %y + 1.
static unsigned calculateIterationsToInvariance(
PHINode *Phi, Loop *L, BasicBlock *BackEdge,
SmallDenseMap<PHINode *, unsigned> &IterationsToInvariance) {
assert(Phi->getParent() == L->getHeader() &&
"Non-loop Phi should not be checked for turning into invariant.");
assert(BackEdge == L->getLoopLatch() && "Wrong latch?");
// If we already know the answer, take it from the map.
auto I = IterationsToInvariance.find(Phi);
if (I != IterationsToInvariance.end())
return I->second;
// Otherwise we need to analyze the input from the back edge.
Value *Input = Phi->getIncomingValueForBlock(BackEdge);
// Place infinity to map to avoid infinite recursion for cycled Phis. Such
// cycles can never stop on an invariant.
IterationsToInvariance[Phi] = InfiniteIterationsToInvariance;
unsigned ToInvariance = InfiniteIterationsToInvariance;
if (L->isLoopInvariant(Input))
ToInvariance = 1u;
else if (PHINode *IncPhi = dyn_cast<PHINode>(Input)) {
// Only consider Phis in header block.
if (IncPhi->getParent() != L->getHeader())
return InfiniteIterationsToInvariance;
// If the input becomes an invariant after X iterations, then our Phi
// becomes an invariant after X + 1 iterations.
unsigned InputToInvariance = calculateIterationsToInvariance(
IncPhi, L, BackEdge, IterationsToInvariance);
if (InputToInvariance != InfiniteIterationsToInvariance)
ToInvariance = InputToInvariance + 1u;
// If we found that this Phi lies in an invariant chain, update the map.
if (ToInvariance != InfiniteIterationsToInvariance)
IterationsToInvariance[Phi] = ToInvariance;
return ToInvariance;
// Return the number of iterations to peel off that make conditions in the
// body true/false. For example, if we peel 2 iterations off the loop below,
// the condition i < 2 can be evaluated at compile time.
// for (i = 0; i < n; i++)
// if (i < 2)
// ..
// else
// ..
// }
static unsigned countToEliminateCompares(Loop &L, unsigned MaxPeelCount,
ScalarEvolution &SE) {
assert(L.isLoopSimplifyForm() && "Loop needs to be in loop simplify form");
unsigned DesiredPeelCount = 0;
for (auto *BB : L.blocks()) {
auto *BI = dyn_cast<BranchInst>(BB->getTerminator());
if (!BI || BI->isUnconditional())
// Ignore loop exit condition.
if (L.getLoopLatch() == BB)
Value *Condition = BI->getCondition();
Value *LeftVal, *RightVal;
CmpInst::Predicate Pred;
if (!match(Condition, m_ICmp(Pred, m_Value(LeftVal), m_Value(RightVal))))
const SCEV *LeftSCEV = SE.getSCEV(LeftVal);
const SCEV *RightSCEV = SE.getSCEV(RightVal);
// Do not consider predicates that are known to be true or false
// independently of the loop iteration.
if (SE.evaluatePredicate(Pred, LeftSCEV, RightSCEV))
// Check if we have a condition with one AddRec and one non AddRec
// expression. Normalize LeftSCEV to be the AddRec.
if (!isa<SCEVAddRecExpr>(LeftSCEV)) {
if (isa<SCEVAddRecExpr>(RightSCEV)) {
std::swap(LeftSCEV, RightSCEV);
Pred = ICmpInst::getSwappedPredicate(Pred);
} else
const SCEVAddRecExpr *LeftAR = cast<SCEVAddRecExpr>(LeftSCEV);
// Avoid huge SCEV computations in the loop below, make sure we only
// consider AddRecs of the loop we are trying to peel.
if (!LeftAR->isAffine() || LeftAR->getLoop() != &L)
if (!(ICmpInst::isEquality(Pred) && LeftAR->hasNoSelfWrap()) &&
!SE.getMonotonicPredicateType(LeftAR, Pred))
// Check if extending the current DesiredPeelCount lets us evaluate Pred
// or !Pred in the loop body statically.
unsigned NewPeelCount = DesiredPeelCount;
const SCEV *IterVal = LeftAR->evaluateAtIteration(
SE.getConstant(LeftSCEV->getType(), NewPeelCount), SE);
// If the original condition is not known, get the negated predicate
// (which holds on the else branch) and check if it is known. This allows
// us to peel of iterations that make the original condition false.
if (!SE.isKnownPredicate(Pred, IterVal, RightSCEV))
Pred = ICmpInst::getInversePredicate(Pred);
const SCEV *Step = LeftAR->getStepRecurrence(SE);
const SCEV *NextIterVal = SE.getAddExpr(IterVal, Step);
auto PeelOneMoreIteration = [&IterVal, &NextIterVal, &SE, Step,
&NewPeelCount]() {
IterVal = NextIterVal;
NextIterVal = SE.getAddExpr(IterVal, Step);
auto CanPeelOneMoreIteration = [&NewPeelCount, &MaxPeelCount]() {
return NewPeelCount < MaxPeelCount;
while (CanPeelOneMoreIteration() &&
SE.isKnownPredicate(Pred, IterVal, RightSCEV))
// With *that* peel count, does the predicate !Pred become known in the
// first iteration of the loop body after peeling?
if (!SE.isKnownPredicate(ICmpInst::getInversePredicate(Pred), IterVal,
continue; // If not, give up.
// However, for equality comparisons, that isn't always sufficient to
// eliminate the comparsion in loop body, we may need to peel one more
// iteration. See if that makes !Pred become unknown again.
if (ICmpInst::isEquality(Pred) &&
!SE.isKnownPredicate(ICmpInst::getInversePredicate(Pred), NextIterVal,
RightSCEV) &&
!SE.isKnownPredicate(Pred, IterVal, RightSCEV) &&
SE.isKnownPredicate(Pred, NextIterVal, RightSCEV)) {
if (!CanPeelOneMoreIteration())
continue; // Need to peel one more iteration, but can't. Give up.
PeelOneMoreIteration(); // Great!
DesiredPeelCount = std::max(DesiredPeelCount, NewPeelCount);
return DesiredPeelCount;
// Return the number of iterations we want to peel off.
void llvm::computePeelCount(Loop *L, unsigned LoopSize,
TargetTransformInfo::PeelingPreferences &PP,
unsigned &TripCount, ScalarEvolution &SE,
unsigned Threshold) {
assert(LoopSize > 0 && "Zero loop size is not allowed!");
// Save the PP.PeelCount value set by the target in
// TTI.getPeelingPreferences or by the flag -unroll-peel-count.
unsigned TargetPeelCount = PP.PeelCount;
PP.PeelCount = 0;
if (!canPeel(L))
// Only try to peel innermost loops by default.
// The constraint can be relaxed by the target in TTI.getUnrollingPreferences
// or by the flag -unroll-allow-loop-nests-peeling.
if (!PP.AllowLoopNestsPeeling && !L->isInnermost())
// If the user provided a peel count, use that.
bool UserPeelCount = UnrollForcePeelCount.getNumOccurrences() > 0;
if (UserPeelCount) {
LLVM_DEBUG(dbgs() << "Force-peeling first " << UnrollForcePeelCount
<< " iterations.\n");
PP.PeelCount = UnrollForcePeelCount;
PP.PeelProfiledIterations = true;
// Skip peeling if it's disabled.
if (!PP.AllowPeeling)
unsigned AlreadyPeeled = 0;
if (auto Peeled = getOptionalIntLoopAttribute(L, PeeledCountMetaData))
AlreadyPeeled = *Peeled;
// Stop if we already peeled off the maximum number of iterations.
if (AlreadyPeeled >= UnrollPeelMaxCount)
// Here we try to get rid of Phis which become invariants after 1, 2, ..., N
// iterations of the loop. For this we compute the number for iterations after
// which every Phi is guaranteed to become an invariant, and try to peel the
// maximum number of iterations among these values, thus turning all those
// Phis into invariants.
// First, check that we can peel at least one iteration.
if (2 * LoopSize <= Threshold && UnrollPeelMaxCount > 0) {
// Store the pre-calculated values here.
SmallDenseMap<PHINode *, unsigned> IterationsToInvariance;
// Now go through all Phis to calculate their the number of iterations they
// need to become invariants.
// Start the max computation with the UP.PeelCount value set by the target
// in TTI.getUnrollingPreferences or by the flag -unroll-peel-count.
unsigned DesiredPeelCount = TargetPeelCount;
BasicBlock *BackEdge = L->getLoopLatch();
assert(BackEdge && "Loop is not in simplified form?");
for (auto BI = L->getHeader()->begin(); isa<PHINode>(&*BI); ++BI) {
PHINode *Phi = cast<PHINode>(&*BI);
unsigned ToInvariance = calculateIterationsToInvariance(
Phi, L, BackEdge, IterationsToInvariance);
if (ToInvariance != InfiniteIterationsToInvariance)
DesiredPeelCount = std::max(DesiredPeelCount, ToInvariance);
// Pay respect to limitations implied by loop size and the max peel count.
unsigned MaxPeelCount = UnrollPeelMaxCount;
MaxPeelCount = std::min(MaxPeelCount, Threshold / LoopSize - 1);
DesiredPeelCount = std::max(DesiredPeelCount,
countToEliminateCompares(*L, MaxPeelCount, SE));
if (DesiredPeelCount > 0) {
DesiredPeelCount = std::min(DesiredPeelCount, MaxPeelCount);
// Consider max peel count limitation.
assert(DesiredPeelCount > 0 && "Wrong loop size estimation?");
if (DesiredPeelCount + AlreadyPeeled <= UnrollPeelMaxCount) {
LLVM_DEBUG(dbgs() << "Peel " << DesiredPeelCount
<< " iteration(s) to turn"
<< " some Phis into invariants.\n");
PP.PeelCount = DesiredPeelCount;
PP.PeelProfiledIterations = false;
// Bail if we know the statically calculated trip count.
// In this case we rather prefer partial unrolling.
if (TripCount)
// Do not apply profile base peeling if it is disabled.
if (!PP.PeelProfiledIterations)
// If we don't know the trip count, but have reason to believe the average
// trip count is low, peeling should be beneficial, since we will usually
// hit the peeled section.
// We only do this in the presence of profile information, since otherwise
// our estimates of the trip count are not reliable enough.
if (L->getHeader()->getParent()->hasProfileData()) {
Optional<unsigned> PeelCount = getLoopEstimatedTripCount(L);
if (!PeelCount)
LLVM_DEBUG(dbgs() << "Profile-based estimated trip count is " << *PeelCount
<< "\n");
if (*PeelCount) {
if ((*PeelCount + AlreadyPeeled <= UnrollPeelMaxCount) &&
(LoopSize * (*PeelCount + 1) <= Threshold)) {
LLVM_DEBUG(dbgs() << "Peeling first " << *PeelCount
<< " iterations.\n");
PP.PeelCount = *PeelCount;
LLVM_DEBUG(dbgs() << "Requested peel count: " << *PeelCount << "\n");
LLVM_DEBUG(dbgs() << "Already peel count: " << AlreadyPeeled << "\n");
LLVM_DEBUG(dbgs() << "Max peel count: " << UnrollPeelMaxCount << "\n");
LLVM_DEBUG(dbgs() << "Peel cost: " << LoopSize * (*PeelCount + 1)
<< "\n");
LLVM_DEBUG(dbgs() << "Max peel cost: " << Threshold << "\n");
/// Update the branch weights of the latch of a peeled-off loop
/// iteration.
/// This sets the branch weights for the latch of the recently peeled off loop
/// iteration correctly.
/// Let F is a weight of the edge from latch to header.
/// Let E is a weight of the edge from latch to exit.
/// F/(F+E) is a probability to go to loop and E/(F+E) is a probability to
/// go to exit.
/// Then, Estimated TripCount = F / E.
/// For I-th (counting from 0) peeled off iteration we set the the weights for
/// the peeled latch as (TC - I, 1). It gives us reasonable distribution,
/// The probability to go to exit 1/(TC-I) increases. At the same time
/// the estimated trip count of remaining loop reduces by I.
/// To avoid dealing with division rounding we can just multiple both part
/// of weights to E and use weight as (F - I * E, E).
/// \param Header The copy of the header block that belongs to next iteration.
/// \param LatchBR The copy of the latch branch that belongs to this iteration.
/// \param[in,out] FallThroughWeight The weight of the edge from latch to
/// header before peeling (in) and after peeled off one iteration (out).
static void updateBranchWeights(BasicBlock *Header, BranchInst *LatchBR,
uint64_t ExitWeight,
uint64_t &FallThroughWeight) {
// FallThroughWeight is 0 means that there is no branch weights on original
// latch block or estimated trip count is zero.
if (!FallThroughWeight)
unsigned HeaderIdx = (LatchBR->getSuccessor(0) == Header ? 0 : 1);
MDBuilder MDB(LatchBR->getContext());
MDNode *WeightNode =
HeaderIdx ? MDB.createBranchWeights(ExitWeight, FallThroughWeight)
: MDB.createBranchWeights(FallThroughWeight, ExitWeight);
LatchBR->setMetadata(LLVMContext::MD_prof, WeightNode);
FallThroughWeight =
FallThroughWeight > ExitWeight ? FallThroughWeight - ExitWeight : 1;
/// Initialize the weights.
/// \param Header The header block.
/// \param LatchBR The latch branch.
/// \param[out] ExitWeight The weight of the edge from Latch to Exit.
/// \param[out] FallThroughWeight The weight of the edge from Latch to Header.
static void initBranchWeights(BasicBlock *Header, BranchInst *LatchBR,
uint64_t &ExitWeight,
uint64_t &FallThroughWeight) {
uint64_t TrueWeight, FalseWeight;
if (!LatchBR->extractProfMetadata(TrueWeight, FalseWeight))
unsigned HeaderIdx = LatchBR->getSuccessor(0) == Header ? 0 : 1;
ExitWeight = HeaderIdx ? TrueWeight : FalseWeight;
FallThroughWeight = HeaderIdx ? FalseWeight : TrueWeight;
/// Update the weights of original Latch block after peeling off all iterations.
/// \param Header The header block.
/// \param LatchBR The latch branch.
/// \param ExitWeight The weight of the edge from Latch to Exit.
/// \param FallThroughWeight The weight of the edge from Latch to Header.
static void fixupBranchWeights(BasicBlock *Header, BranchInst *LatchBR,
uint64_t ExitWeight,
uint64_t FallThroughWeight) {
// FallThroughWeight is 0 means that there is no branch weights on original
// latch block or estimated trip count is zero.
if (!FallThroughWeight)
// Sets the branch weights on the loop exit.
MDBuilder MDB(LatchBR->getContext());
unsigned HeaderIdx = LatchBR->getSuccessor(0) == Header ? 0 : 1;
MDNode *WeightNode =
HeaderIdx ? MDB.createBranchWeights(ExitWeight, FallThroughWeight)
: MDB.createBranchWeights(FallThroughWeight, ExitWeight);
LatchBR->setMetadata(LLVMContext::MD_prof, WeightNode);
/// Clones the body of the loop L, putting it between \p InsertTop and \p
/// InsertBot.
/// \param IterNumber The serial number of the iteration currently being
/// peeled off.
/// \param ExitEdges The exit edges of the original loop.
/// \param[out] NewBlocks A list of the blocks in the newly created clone
/// \param[out] VMap The value map between the loop and the new clone.
/// \param LoopBlocks A helper for DFS-traversal of the loop.
/// \param LVMap A value-map that maps instructions from the original loop to
/// instructions in the last peeled-off iteration.
static void cloneLoopBlocks(
Loop *L, unsigned IterNumber, BasicBlock *InsertTop, BasicBlock *InsertBot,
SmallVectorImpl<std::pair<BasicBlock *, BasicBlock *>> &ExitEdges,
SmallVectorImpl<BasicBlock *> &NewBlocks, LoopBlocksDFS &LoopBlocks,
ValueToValueMapTy &VMap, ValueToValueMapTy &LVMap, DominatorTree *DT,
LoopInfo *LI, ArrayRef<MDNode *> LoopLocalNoAliasDeclScopes) {
BasicBlock *Header = L->getHeader();
BasicBlock *Latch = L->getLoopLatch();
BasicBlock *PreHeader = L->getLoopPreheader();
Function *F = Header->getParent();
LoopBlocksDFS::RPOIterator BlockBegin = LoopBlocks.beginRPO();
LoopBlocksDFS::RPOIterator BlockEnd = LoopBlocks.endRPO();
Loop *ParentLoop = L->getParentLoop();
// For each block in the original loop, create a new copy,
// and update the value map with the newly created values.
for (LoopBlocksDFS::RPOIterator BB = BlockBegin; BB != BlockEnd; ++BB) {
BasicBlock *NewBB = CloneBasicBlock(*BB, VMap, ".peel", F);
// If an original block is an immediate child of the loop L, its copy
// is a child of a ParentLoop after peeling. If a block is a child of
// a nested loop, it is handled in the cloneLoop() call below.
if (ParentLoop && LI->getLoopFor(*BB) == L)
ParentLoop->addBasicBlockToLoop(NewBB, *LI);
VMap[*BB] = NewBB;
// If dominator tree is available, insert nodes to represent cloned blocks.
if (DT) {
if (Header == *BB)
DT->addNewBlock(NewBB, InsertTop);
else {
DomTreeNode *IDom = DT->getNode(*BB)->getIDom();
// VMap must contain entry for IDom, as the iteration order is RPO.
DT->addNewBlock(NewBB, cast<BasicBlock>(VMap[IDom->getBlock()]));
// 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("Peel") + Twine(IterNumber)).str();
cloneAndAdaptNoAliasScopes(LoopLocalNoAliasDeclScopes, NewBlocks,
Header->getContext(), Ext);
// Recursively create the new Loop objects for nested loops, if any,
// to preserve LoopInfo.
for (Loop *ChildLoop : *L) {
cloneLoop(ChildLoop, ParentLoop, VMap, LI, nullptr);
// Hook-up the control flow for the newly inserted blocks.
// The new header is hooked up directly to the "top", which is either
// the original loop preheader (for the first iteration) or the previous
// iteration's exiting block (for every other iteration)
InsertTop->getTerminator()->setSuccessor(0, cast<BasicBlock>(VMap[Header]));
// Similarly, for the latch:
// The original exiting edge is still hooked up to the loop exit.
// The backedge now goes to the "bottom", which is either the loop's real
// header (for the last peeled iteration) or the copied header of the next
// iteration (for every other iteration)
BasicBlock *NewLatch = cast<BasicBlock>(VMap[Latch]);
BranchInst *LatchBR = cast<BranchInst>(NewLatch->getTerminator());
for (unsigned idx = 0, e = LatchBR->getNumSuccessors(); idx < e; ++idx)
if (LatchBR->getSuccessor(idx) == Header) {
LatchBR->setSuccessor(idx, InsertBot);
if (DT)
DT->changeImmediateDominator(InsertBot, NewLatch);
// The new copy of the loop body starts with a bunch of PHI nodes
// that pick an incoming value from either the preheader, or the previous
// loop iteration. Since this copy is no longer part of the loop, we
// resolve this statically:
// For the first iteration, we use the value from the preheader directly.
// For any other iteration, we replace the phi with the value generated by
// the immediately preceding clone of the loop body (which represents
// the previous iteration).
for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) {
PHINode *NewPHI = cast<PHINode>(VMap[&*I]);
if (IterNumber == 0) {
VMap[&*I] = NewPHI->getIncomingValueForBlock(PreHeader);
} else {
Value *LatchVal = NewPHI->getIncomingValueForBlock(Latch);
Instruction *LatchInst = dyn_cast<Instruction>(LatchVal);
if (LatchInst && L->contains(LatchInst))
VMap[&*I] = LVMap[LatchInst];
VMap[&*I] = LatchVal;
// Fix up the outgoing values - we need to add a value for the iteration
// we've just created. Note that this must happen *after* the incoming
// values are adjusted, since the value going out of the latch may also be
// a value coming into the header.
for (auto Edge : ExitEdges)
for (PHINode &PHI : Edge.second->phis()) {
Value *LatchVal = PHI.getIncomingValueForBlock(Edge.first);
Instruction *LatchInst = dyn_cast<Instruction>(LatchVal);
if (LatchInst && L->contains(LatchInst))
LatchVal = VMap[LatchVal];
PHI.addIncoming(LatchVal, cast<BasicBlock>(VMap[Edge.first]));
// LastValueMap is updated with the values for the current loop
// which are used the next time this function is called.
for (auto KV : VMap)
LVMap[KV.first] = KV.second;
TargetTransformInfo::PeelingPreferences llvm::gatherPeelingPreferences(
Loop *L, ScalarEvolution &SE, const TargetTransformInfo &TTI,
Optional<bool> UserAllowPeeling,
Optional<bool> UserAllowProfileBasedPeeling, bool UnrollingSpecficValues) {
TargetTransformInfo::PeelingPreferences PP;
// Set the default values.
PP.PeelCount = 0;
PP.AllowPeeling = true;
PP.AllowLoopNestsPeeling = false;
PP.PeelProfiledIterations = true;
// Get the target specifc values.
TTI.getPeelingPreferences(L, SE, PP);
// User specified values using cl::opt.
if (UnrollingSpecficValues) {
if (UnrollPeelCount.getNumOccurrences() > 0)
PP.PeelCount = UnrollPeelCount;
if (UnrollAllowPeeling.getNumOccurrences() > 0)
PP.AllowPeeling = UnrollAllowPeeling;
if (UnrollAllowLoopNestsPeeling.getNumOccurrences() > 0)
PP.AllowLoopNestsPeeling = UnrollAllowLoopNestsPeeling;
// User specifed values provided by argument.
if (UserAllowPeeling.hasValue())
PP.AllowPeeling = *UserAllowPeeling;
if (UserAllowProfileBasedPeeling.hasValue())
PP.PeelProfiledIterations = *UserAllowProfileBasedPeeling;
return PP;
/// Peel off the first \p PeelCount iterations of loop \p L.
/// Note that this does not peel them off as a single straight-line block.
/// Rather, each iteration is peeled off separately, and needs to check the
/// exit condition.
/// For loops that dynamically execute \p PeelCount iterations or less
/// this provides a benefit, since the peeled off iterations, which account
/// for the bulk of dynamic execution, can be further simplified by scalar
/// optimizations.
bool llvm::peelLoop(Loop *L, unsigned PeelCount, LoopInfo *LI,
ScalarEvolution *SE, DominatorTree *DT, AssumptionCache *AC,
bool PreserveLCSSA) {
assert(PeelCount > 0 && "Attempt to peel out zero iterations?");
assert(canPeel(L) && "Attempt to peel a loop which is not peelable?");
LoopBlocksDFS LoopBlocks(L);
BasicBlock *Header = L->getHeader();
BasicBlock *PreHeader = L->getLoopPreheader();
BasicBlock *Latch = L->getLoopLatch();
SmallVector<std::pair<BasicBlock *, BasicBlock *>, 4> ExitEdges;
DenseMap<BasicBlock *, BasicBlock *> ExitIDom;
if (DT) {
// We'd like to determine the idom of exit block after peeling one
// iteration.
// Let Exit is exit block.
// Let ExitingSet - is a set of predecessors of Exit block. They are exiting
// blocks.
// Let Latch' and ExitingSet' are copies after a peeling.
// We'd like to find an idom'(Exit) - idom of Exit after peeling.
// It is an evident that idom'(Exit) will be the nearest common dominator
// of ExitingSet and ExitingSet'.
// idom(Exit) is a nearest common dominator of ExitingSet.
// idom(Exit)' is a nearest common dominator of ExitingSet'.
// Taking into account that we have a single Latch, Latch' will dominate
// Header and idom(Exit).
// So the idom'(Exit) is nearest common dominator of idom(Exit)' and Latch'.
// All these basic blocks are in the same loop, so what we find is
// (nearest common dominator of idom(Exit) and Latch)'.
// In the loop below we remember nearest common dominator of idom(Exit) and
// Latch to update idom of Exit later.
assert(L->hasDedicatedExits() && "No dedicated exits?");
for (auto Edge : ExitEdges) {
if (ExitIDom.count(Edge.second))
BasicBlock *BB = DT->findNearestCommonDominator(
DT->getNode(Edge.second)->getIDom()->getBlock(), Latch);
assert(L->contains(BB) && "IDom is not in a loop");
ExitIDom[Edge.second] = BB;
Function *F = Header->getParent();
// Set up all the necessary basic blocks. It is convenient to split the
// preheader into 3 parts - two blocks to anchor the peeled copy of the loop
// body, and a new preheader for the "real" loop.
// Peeling the first iteration transforms.
// PreHeader:
// ...
// Header:
// LoopBody
// If (cond) goto Header
// Exit:
// into
// InsertTop:
// LoopBody
// If (!cond) goto Exit
// InsertBot:
// NewPreHeader:
// ...
// Header:
// LoopBody
// If (cond) goto Header
// Exit:
// Each following iteration will split the current bottom anchor in two,
// and put the new copy of the loop body between these two blocks. That is,
// after peeling another iteration from the example above, we'll split
// InsertBot, and get:
// InsertTop:
// LoopBody
// If (!cond) goto Exit
// InsertBot:
// LoopBody
// If (!cond) goto Exit
// NewPreHeader:
// ...
// Header:
// LoopBody
// If (cond) goto Header
// Exit:
BasicBlock *InsertTop = SplitEdge(PreHeader, Header, DT, LI);
BasicBlock *InsertBot =
SplitBlock(InsertTop, InsertTop->getTerminator(), DT, LI);
BasicBlock *NewPreHeader =
SplitBlock(InsertBot, InsertBot->getTerminator(), DT, LI);
InsertTop->setName(Header->getName() + ".peel.begin");
InsertBot->setName(Header->getName() + "");
NewPreHeader->setName(PreHeader->getName() + ".peel.newph");
ValueToValueMapTy LVMap;
// If we have branch weight information, we'll want to update it for the
// newly created branches.
BranchInst *LatchBR =
uint64_t ExitWeight = 0, FallThroughWeight = 0;
initBranchWeights(Header, LatchBR, ExitWeight, FallThroughWeight);
// 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 each peeled-off iteration, make a copy of the loop.
for (unsigned Iter = 0; Iter < PeelCount; ++Iter) {
SmallVector<BasicBlock *, 8> NewBlocks;
ValueToValueMapTy VMap;
cloneLoopBlocks(L, Iter, InsertTop, InsertBot, ExitEdges, NewBlocks,
LoopBlocks, VMap, LVMap, DT, LI,
// Remap to use values from the current iteration instead of the
// previous one.
remapInstructionsInBlocks(NewBlocks, VMap);
if (DT) {
// Latches of the cloned loops dominate over the loop exit, so idom of the
// latter is the first cloned loop body, as original PreHeader dominates
// the original loop body.
if (Iter == 0)
for (auto Exit : ExitIDom)
auto *LatchBRCopy = cast<BranchInst>(VMap[LatchBR]);
updateBranchWeights(InsertBot, LatchBRCopy, ExitWeight, FallThroughWeight);
// Remove Loop metadata from the latch branch instruction
// because it is not the Loop's latch branch anymore.
LatchBRCopy->setMetadata(LLVMContext::MD_loop, nullptr);
InsertTop = InsertBot;
InsertBot = SplitBlock(InsertBot, InsertBot->getTerminator(), DT, LI);
InsertBot->setName(Header->getName() + "");
NewBlocks[0]->getIterator(), F->end());
// Now adjust the phi nodes in the loop header to get their initial values
// from the last peeled-off iteration instead of the preheader.
for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) {
PHINode *PHI = cast<PHINode>(I);
Value *NewVal = PHI->getIncomingValueForBlock(Latch);
Instruction *LatchInst = dyn_cast<Instruction>(NewVal);
if (LatchInst && L->contains(LatchInst))
NewVal = LVMap[LatchInst];
PHI->setIncomingValueForBlock(NewPreHeader, NewVal);
fixupBranchWeights(Header, LatchBR, ExitWeight, FallThroughWeight);
// Update Metadata for count of peeled off iterations.
unsigned AlreadyPeeled = 0;
if (auto Peeled = getOptionalIntLoopAttribute(L, PeeledCountMetaData))
AlreadyPeeled = *Peeled;
addStringMetadataToLoop(L, PeeledCountMetaData, AlreadyPeeled + PeelCount);
if (Loop *ParentLoop = L->getParentLoop())
L = ParentLoop;
// We modified the loop, update SE.
// Finally DomtTree must be correct.
// FIXME: Incrementally update loop-simplify
simplifyLoop(L, DT, LI, SE, AC, nullptr, PreserveLCSSA);
return true;