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//===-- LoopUtils.cpp - Loop Utility functions -------------------------===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
// This file defines common loop utility functions.
#include "llvm/Transforms/Utils/LoopUtils.h"
#include "llvm/ADT/DenseSet.h"
#include "llvm/ADT/Optional.h"
#include "llvm/ADT/PriorityWorklist.h"
#include "llvm/ADT/ScopeExit.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/BasicAliasAnalysis.h"
#include "llvm/Analysis/DomTreeUpdater.h"
#include "llvm/Analysis/GlobalsModRef.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/LoopAccessAnalysis.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/Analysis/MemorySSA.h"
#include "llvm/Analysis/MemorySSAUpdater.h"
#include "llvm/Analysis/MustExecute.h"
#include "llvm/Analysis/ScalarEvolution.h"
#include "llvm/Analysis/ScalarEvolutionAliasAnalysis.h"
#include "llvm/Analysis/ScalarEvolutionExpressions.h"
#include "llvm/Analysis/TargetTransformInfo.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/DIBuilder.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/IR/MDBuilder.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/Operator.h"
#include "llvm/IR/PatternMatch.h"
#include "llvm/IR/ValueHandle.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/KnownBits.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/Local.h"
#include "llvm/Transforms/Utils/ScalarEvolutionExpander.h"
using namespace llvm;
using namespace llvm::PatternMatch;
#define DEBUG_TYPE "loop-utils"
static const char *LLVMLoopDisableNonforced = "llvm.loop.disable_nonforced";
static const char *LLVMLoopDisableLICM = "llvm.licm.disable";
static const char *LLVMLoopMustProgress = "llvm.loop.mustprogress";
bool llvm::formDedicatedExitBlocks(Loop *L, DominatorTree *DT, LoopInfo *LI,
MemorySSAUpdater *MSSAU,
bool PreserveLCSSA) {
bool Changed = false;
// We re-use a vector for the in-loop predecesosrs.
SmallVector<BasicBlock *, 4> InLoopPredecessors;
auto RewriteExit = [&](BasicBlock *BB) {
assert(InLoopPredecessors.empty() &&
"Must start with an empty predecessors list!");
auto Cleanup = make_scope_exit([&] { InLoopPredecessors.clear(); });
// See if there are any non-loop predecessors of this exit block and
// keep track of the in-loop predecessors.
bool IsDedicatedExit = true;
for (auto *PredBB : predecessors(BB))
if (L->contains(PredBB)) {
if (isa<IndirectBrInst>(PredBB->getTerminator()))
// We cannot rewrite exiting edges from an indirectbr.
return false;
if (isa<CallBrInst>(PredBB->getTerminator()))
// We cannot rewrite exiting edges from a callbr.
return false;
} else {
IsDedicatedExit = false;
assert(!InLoopPredecessors.empty() && "Must have *some* loop predecessor!");
// Nothing to do if this is already a dedicated exit.
if (IsDedicatedExit)
return false;
auto *NewExitBB = SplitBlockPredecessors(
BB, InLoopPredecessors, ".loopexit", DT, LI, MSSAU, PreserveLCSSA);
if (!NewExitBB)
dbgs() << "WARNING: Can't create a dedicated exit block for loop: "
<< *L << "\n");
LLVM_DEBUG(dbgs() << "LoopSimplify: Creating dedicated exit block "
<< NewExitBB->getName() << "\n");
return true;
// Walk the exit blocks directly rather than building up a data structure for
// them, but only visit each one once.
SmallPtrSet<BasicBlock *, 4> Visited;
for (auto *BB : L->blocks())
for (auto *SuccBB : successors(BB)) {
// We're looking for exit blocks so skip in-loop successors.
if (L->contains(SuccBB))
// Visit each exit block exactly once.
if (!Visited.insert(SuccBB).second)
Changed |= RewriteExit(SuccBB);
return Changed;
/// Returns the instructions that use values defined in the loop.
SmallVector<Instruction *, 8> llvm::findDefsUsedOutsideOfLoop(Loop *L) {
SmallVector<Instruction *, 8> UsedOutside;
for (auto *Block : L->getBlocks())
// FIXME: I believe that this could use copy_if if the Inst reference could
// be adapted into a pointer.
for (auto &Inst : *Block) {
auto Users = Inst.users();
if (any_of(Users, [&](User *U) {
auto *Use = cast<Instruction>(U);
return !L->contains(Use->getParent());
return UsedOutside;
void llvm::getLoopAnalysisUsage(AnalysisUsage &AU) {
// By definition, all loop passes need the LoopInfo analysis and the
// Dominator tree it depends on. Because they all participate in the loop
// pass manager, they must also preserve these.
// We must also preserve LoopSimplify and LCSSA. We locally access their IDs
// here because users shouldn't directly get them from this header.
extern char &LoopSimplifyID;
extern char &LCSSAID;
// This is used in the LPPassManager to perform LCSSA verification on passes
// which preserve lcssa form
// Loop passes are designed to run inside of a loop pass manager which means
// that any function analyses they require must be required by the first loop
// pass in the manager (so that it is computed before the loop pass manager
// runs) and preserved by all loop pasess in the manager. To make this
// reasonably robust, the set needed for most loop passes is maintained here.
// If your loop pass requires an analysis not listed here, you will need to
// carefully audit the loop pass manager nesting structure that results.
// FIXME: When all loop passes preserve MemorySSA, it can be required and
// preserved here instead of the individual handling in each pass.
/// Manually defined generic "LoopPass" dependency initialization. This is used
/// to initialize the exact set of passes from above in \c
/// getLoopAnalysisUsage. It can be used within a loop pass's initialization
/// with:
/// As-if "LoopPass" were a pass.
void llvm::initializeLoopPassPass(PassRegistry &Registry) {
/// Create MDNode for input string.
static MDNode *createStringMetadata(Loop *TheLoop, StringRef Name, unsigned V) {
LLVMContext &Context = TheLoop->getHeader()->getContext();
Metadata *MDs[] = {
MDString::get(Context, Name),
ConstantAsMetadata::get(ConstantInt::get(Type::getInt32Ty(Context), V))};
return MDNode::get(Context, MDs);
/// Set input string into loop metadata by keeping other values intact.
/// If the string is already in loop metadata update value if it is
/// different.
void llvm::addStringMetadataToLoop(Loop *TheLoop, const char *StringMD,
unsigned V) {
SmallVector<Metadata *, 4> MDs(1);
// If the loop already has metadata, retain it.
MDNode *LoopID = TheLoop->getLoopID();
if (LoopID) {
for (unsigned i = 1, ie = LoopID->getNumOperands(); i < ie; ++i) {
MDNode *Node = cast<MDNode>(LoopID->getOperand(i));
// If it is of form key = value, try to parse it.
if (Node->getNumOperands() == 2) {
MDString *S = dyn_cast<MDString>(Node->getOperand(0));
if (S && S->getString().equals(StringMD)) {
ConstantInt *IntMD =
if (IntMD && IntMD->getSExtValue() == V)
// It is already in place. Do nothing.
// We need to update the value, so just skip it here and it will
// be added after copying other existed nodes.
// Add new metadata.
MDs.push_back(createStringMetadata(TheLoop, StringMD, V));
// Replace current metadata node with new one.
LLVMContext &Context = TheLoop->getHeader()->getContext();
MDNode *NewLoopID = MDNode::get(Context, MDs);
// Set operand 0 to refer to the loop id itself.
NewLoopID->replaceOperandWith(0, NewLoopID);
/// Find string metadata for loop
/// If it has a value (e.g. {"llvm.distribute", 1} return the value as an
/// operand or null otherwise. If the string metadata is not found return
/// Optional's not-a-value.
Optional<const MDOperand *> llvm::findStringMetadataForLoop(const Loop *TheLoop,
StringRef Name) {
MDNode *MD = findOptionMDForLoop(TheLoop, Name);
if (!MD)
return None;
switch (MD->getNumOperands()) {
case 1:
return nullptr;
case 2:
return &MD->getOperand(1);
llvm_unreachable("loop metadata has 0 or 1 operand");
static Optional<bool> getOptionalBoolLoopAttribute(const Loop *TheLoop,
StringRef Name) {
MDNode *MD = findOptionMDForLoop(TheLoop, Name);
if (!MD)
return None;
switch (MD->getNumOperands()) {
case 1:
// When the value is absent it is interpreted as 'attribute set'.
return true;
case 2:
if (ConstantInt *IntMD =
return IntMD->getZExtValue();
return true;
llvm_unreachable("unexpected number of options");
bool llvm::getBooleanLoopAttribute(const Loop *TheLoop, StringRef Name) {
return getOptionalBoolLoopAttribute(TheLoop, Name).getValueOr(false);
llvm::getOptionalElementCountLoopAttribute(Loop *TheLoop) {
Optional<int> Width =
getOptionalIntLoopAttribute(TheLoop, "llvm.loop.vectorize.width");
if (Width.hasValue()) {
Optional<int> IsScalable = getOptionalIntLoopAttribute(
TheLoop, "llvm.loop.vectorize.scalable.enable");
return ElementCount::get(*Width, IsScalable.getValueOr(false));
return None;
llvm::Optional<int> llvm::getOptionalIntLoopAttribute(Loop *TheLoop,
StringRef Name) {
const MDOperand *AttrMD =
findStringMetadataForLoop(TheLoop, Name).getValueOr(nullptr);
if (!AttrMD)
return None;
ConstantInt *IntMD = mdconst::extract_or_null<ConstantInt>(AttrMD->get());
if (!IntMD)
return None;
return IntMD->getSExtValue();
Optional<MDNode *> llvm::makeFollowupLoopID(
MDNode *OrigLoopID, ArrayRef<StringRef> FollowupOptions,
const char *InheritOptionsExceptPrefix, bool AlwaysNew) {
if (!OrigLoopID) {
if (AlwaysNew)
return nullptr;
return None;
assert(OrigLoopID->getOperand(0) == OrigLoopID);
bool InheritAllAttrs = !InheritOptionsExceptPrefix;
bool InheritSomeAttrs =
InheritOptionsExceptPrefix && InheritOptionsExceptPrefix[0] != '\0';
SmallVector<Metadata *, 8> MDs;
bool Changed = false;
if (InheritAllAttrs || InheritSomeAttrs) {
for (const MDOperand &Existing : drop_begin(OrigLoopID->operands())) {
MDNode *Op = cast<MDNode>(Existing.get());
auto InheritThisAttribute = [InheritSomeAttrs,
InheritOptionsExceptPrefix](MDNode *Op) {
if (!InheritSomeAttrs)
return false;
// Skip malformatted attribute metadata nodes.
if (Op->getNumOperands() == 0)
return true;
Metadata *NameMD = Op->getOperand(0).get();
if (!isa<MDString>(NameMD))
return true;
StringRef AttrName = cast<MDString>(NameMD)->getString();
// Do not inherit excluded attributes.
return !AttrName.startswith(InheritOptionsExceptPrefix);
if (InheritThisAttribute(Op))
Changed = true;
} else {
// Modified if we dropped at least one attribute.
Changed = OrigLoopID->getNumOperands() > 1;
bool HasAnyFollowup = false;
for (StringRef OptionName : FollowupOptions) {
MDNode *FollowupNode = findOptionMDForLoopID(OrigLoopID, OptionName);
if (!FollowupNode)
HasAnyFollowup = true;
for (const MDOperand &Option : drop_begin(FollowupNode->operands())) {
Changed = true;
// Attributes of the followup loop not specified explicity, so signal to the
// transformation pass to add suitable attributes.
if (!AlwaysNew && !HasAnyFollowup)
return None;
// If no attributes were added or remove, the previous loop Id can be reused.
if (!AlwaysNew && !Changed)
return OrigLoopID;
// No attributes is equivalent to having no !llvm.loop metadata at all.
if (MDs.size() == 1)
return nullptr;
// Build the new loop ID.
MDTuple *FollowupLoopID = MDNode::get(OrigLoopID->getContext(), MDs);
FollowupLoopID->replaceOperandWith(0, FollowupLoopID);
return FollowupLoopID;
bool llvm::hasDisableAllTransformsHint(const Loop *L) {
return getBooleanLoopAttribute(L, LLVMLoopDisableNonforced);
bool llvm::hasDisableLICMTransformsHint(const Loop *L) {
return getBooleanLoopAttribute(L, LLVMLoopDisableLICM);
bool llvm::hasMustProgress(const Loop *L) {
return getBooleanLoopAttribute(L, LLVMLoopMustProgress);
TransformationMode llvm::hasUnrollTransformation(Loop *L) {
if (getBooleanLoopAttribute(L, "llvm.loop.unroll.disable"))
return TM_SuppressedByUser;
Optional<int> Count =
getOptionalIntLoopAttribute(L, "llvm.loop.unroll.count");
if (Count.hasValue())
return Count.getValue() == 1 ? TM_SuppressedByUser : TM_ForcedByUser;
if (getBooleanLoopAttribute(L, "llvm.loop.unroll.enable"))
return TM_ForcedByUser;
if (getBooleanLoopAttribute(L, "llvm.loop.unroll.full"))
return TM_ForcedByUser;
if (hasDisableAllTransformsHint(L))
return TM_Disable;
return TM_Unspecified;
TransformationMode llvm::hasUnrollAndJamTransformation(Loop *L) {
if (getBooleanLoopAttribute(L, "llvm.loop.unroll_and_jam.disable"))
return TM_SuppressedByUser;
Optional<int> Count =
getOptionalIntLoopAttribute(L, "llvm.loop.unroll_and_jam.count");
if (Count.hasValue())
return Count.getValue() == 1 ? TM_SuppressedByUser : TM_ForcedByUser;
if (getBooleanLoopAttribute(L, "llvm.loop.unroll_and_jam.enable"))
return TM_ForcedByUser;
if (hasDisableAllTransformsHint(L))
return TM_Disable;
return TM_Unspecified;
TransformationMode llvm::hasVectorizeTransformation(Loop *L) {
Optional<bool> Enable =
getOptionalBoolLoopAttribute(L, "llvm.loop.vectorize.enable");
if (Enable == false)
return TM_SuppressedByUser;
Optional<ElementCount> VectorizeWidth =
Optional<int> InterleaveCount =
getOptionalIntLoopAttribute(L, "llvm.loop.interleave.count");
// 'Forcing' vector width and interleave count to one effectively disables
// this tranformation.
if (Enable == true && VectorizeWidth && VectorizeWidth->isScalar() &&
InterleaveCount == 1)
return TM_SuppressedByUser;
if (getBooleanLoopAttribute(L, "llvm.loop.isvectorized"))
return TM_Disable;
if (Enable == true)
return TM_ForcedByUser;
if ((VectorizeWidth && VectorizeWidth->isScalar()) && InterleaveCount == 1)
return TM_Disable;
if ((VectorizeWidth && VectorizeWidth->isVector()) || InterleaveCount > 1)
return TM_Enable;
if (hasDisableAllTransformsHint(L))
return TM_Disable;
return TM_Unspecified;
TransformationMode llvm::hasDistributeTransformation(Loop *L) {
if (getBooleanLoopAttribute(L, "llvm.loop.distribute.enable"))
return TM_ForcedByUser;
if (hasDisableAllTransformsHint(L))
return TM_Disable;
return TM_Unspecified;
TransformationMode llvm::hasLICMVersioningTransformation(Loop *L) {
if (getBooleanLoopAttribute(L, "llvm.loop.licm_versioning.disable"))
return TM_SuppressedByUser;
if (hasDisableAllTransformsHint(L))
return TM_Disable;
return TM_Unspecified;
/// Does a BFS from a given node to all of its children inside a given loop.
/// The returned vector of nodes includes the starting point.
SmallVector<DomTreeNode *, 16>
llvm::collectChildrenInLoop(DomTreeNode *N, const Loop *CurLoop) {
SmallVector<DomTreeNode *, 16> Worklist;
auto AddRegionToWorklist = [&](DomTreeNode *DTN) {
// Only include subregions in the top level loop.
BasicBlock *BB = DTN->getBlock();
if (CurLoop->contains(BB))
for (size_t I = 0; I < Worklist.size(); I++) {
for (DomTreeNode *Child : Worklist[I]->children())
return Worklist;
void llvm::deleteDeadLoop(Loop *L, DominatorTree *DT, ScalarEvolution *SE,
LoopInfo *LI, MemorySSA *MSSA) {
assert((!DT || L->isLCSSAForm(*DT)) && "Expected LCSSA!");
auto *Preheader = L->getLoopPreheader();
assert(Preheader && "Preheader should exist!");
std::unique_ptr<MemorySSAUpdater> MSSAU;
if (MSSA)
MSSAU = std::make_unique<MemorySSAUpdater>(MSSA);
// Now that we know the removal is safe, remove the loop by changing the
// branch from the preheader to go to the single exit block.
// Because we're deleting a large chunk of code at once, the sequence in which
// we remove things is very important to avoid invalidation issues.
// Tell ScalarEvolution that the loop is deleted. Do this before
// deleting the loop so that ScalarEvolution can look at the loop
// to determine what it needs to clean up.
if (SE)
auto *OldBr = dyn_cast<BranchInst>(Preheader->getTerminator());
assert(OldBr && "Preheader must end with a branch");
assert(OldBr->isUnconditional() && "Preheader must have a single successor");
// Connect the preheader to the exit block. Keep the old edge to the header
// around to perform the dominator tree update in two separate steps
// -- #1 insertion of the edge preheader -> exit and #2 deletion of the edge
// preheader -> header.
// 0. Preheader 1. Preheader 2. Preheader
// | | | |
// V | V |
// Header <--\ | Header <--\ | Header <--\
// | | | | | | | | | | |
// | V | | | V | | | V |
// | Body --/ | | Body --/ | | Body --/
// V V V V V
// Exit Exit Exit
// By doing this is two separate steps we can perform the dominator tree
// update without using the batch update API.
// Even when the loop is never executed, we cannot remove the edge from the
// source block to the exit block. Consider the case where the unexecuted loop
// branches back to an outer loop. If we deleted the loop and removed the edge
// coming to this inner loop, this will break the outer loop structure (by
// deleting the backedge of the outer loop). If the outer loop is indeed a
// non-loop, it will be deleted in a future iteration of loop deletion pass.
IRBuilder<> Builder(OldBr);
auto *ExitBlock = L->getUniqueExitBlock();
DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager);
if (ExitBlock) {
assert(ExitBlock && "Should have a unique exit block!");
assert(L->hasDedicatedExits() && "Loop should have dedicated exits!");
Builder.CreateCondBr(Builder.getFalse(), L->getHeader(), ExitBlock);
// Remove the old branch. The conditional branch becomes a new terminator.
// Rewrite phis in the exit block to get their inputs from the Preheader
// instead of the exiting block.
for (PHINode &P : ExitBlock->phis()) {
// Set the zero'th element of Phi to be from the preheader and remove all
// other incoming values. Given the loop has dedicated exits, all other
// incoming values must be from the exiting blocks.
int PredIndex = 0;
P.setIncomingBlock(PredIndex, Preheader);
// Removes all incoming values from all other exiting blocks (including
// duplicate values from an exiting block).
// Nuke all entries except the zero'th entry which is the preheader entry.
// NOTE! We need to remove Incoming Values in the reverse order as done
// below, to keep the indices valid for deletion (removeIncomingValues
// updates getNumIncomingValues and shifts all values down into the
// operand being deleted).
for (unsigned i = 0, e = P.getNumIncomingValues() - 1; i != e; ++i)
P.removeIncomingValue(e - i, false);
assert((P.getNumIncomingValues() == 1 &&
P.getIncomingBlock(PredIndex) == Preheader) &&
"Should have exactly one value and that's from the preheader!");
if (DT) {
DTU.applyUpdates({{DominatorTree::Insert, Preheader, ExitBlock}});
if (MSSA) {
MSSAU->applyUpdates({{DominatorTree::Insert, Preheader, ExitBlock}},
if (VerifyMemorySSA)
// Disconnect the loop body by branching directly to its exit.
// Remove the old branch.
} else {
assert(L->hasNoExitBlocks() &&
"Loop should have either zero or one exit blocks.");
if (DT) {
DTU.applyUpdates({{DominatorTree::Delete, Preheader, L->getHeader()}});
if (MSSA) {
MSSAU->applyUpdates({{DominatorTree::Delete, Preheader, L->getHeader()}},
SmallSetVector<BasicBlock *, 8> DeadBlockSet(L->block_begin(),
if (VerifyMemorySSA)
// Use a map to unique and a vector to guarantee deterministic ordering.
llvm::SmallDenseSet<std::pair<DIVariable *, DIExpression *>, 4> DeadDebugSet;
llvm::SmallVector<DbgVariableIntrinsic *, 4> DeadDebugInst;
if (ExitBlock) {
// Given LCSSA form is satisfied, we should not have users of instructions
// within the dead loop outside of the loop. However, LCSSA doesn't take
// unreachable uses into account. We handle them here.
// We could do it after drop all references (in this case all users in the
// loop will be already eliminated and we have less work to do but according
// to API doc of User::dropAllReferences only valid operation after dropping
// references, is deletion. So let's substitute all usages of
// instruction from the loop with undef value of corresponding type first.
for (auto *Block : L->blocks())
for (Instruction &I : *Block) {
auto *Undef = UndefValue::get(I.getType());
for (Value::use_iterator UI = I.use_begin(), E = I.use_end();
UI != E;) {
Use &U = *UI;
if (auto *Usr = dyn_cast<Instruction>(U.getUser()))
if (L->contains(Usr->getParent()))
// If we have a DT then we can check that uses outside a loop only in
// unreachable block.
if (DT)
assert(!DT->isReachableFromEntry(U) &&
"Unexpected user in reachable block");
auto *DVI = dyn_cast<DbgVariableIntrinsic>(&I);
if (!DVI)
auto Key =
DeadDebugSet.find({DVI->getVariable(), DVI->getExpression()});
if (Key != DeadDebugSet.end())
DeadDebugSet.insert({DVI->getVariable(), DVI->getExpression()});
// After the loop has been deleted all the values defined and modified
// inside the loop are going to be unavailable.
// Since debug values in the loop have been deleted, inserting an undef
// dbg.value truncates the range of any dbg.value before the loop where the
// loop used to be. This is particularly important for constant values.
DIBuilder DIB(*ExitBlock->getModule());
Instruction *InsertDbgValueBefore = ExitBlock->getFirstNonPHI();
assert(InsertDbgValueBefore &&
"There should be a non-PHI instruction in exit block, else these "
"instructions will have no parent.");
for (auto *DVI : DeadDebugInst)
DVI->getVariable(), DVI->getExpression(),
DVI->getDebugLoc(), InsertDbgValueBefore);
// Remove the block from the reference counting scheme, so that we can
// delete it freely later.
for (auto *Block : L->blocks())
if (MSSA && VerifyMemorySSA)
if (LI) {
// Erase the instructions and the blocks without having to worry
// about ordering because we already dropped the references.
// NOTE: This iteration is safe because erasing the block does not remove
// its entry from the loop's block list. We do that in the next section.
for (Loop::block_iterator LpI = L->block_begin(), LpE = L->block_end();
LpI != LpE; ++LpI)
// Finally, the blocks from loopinfo. This has to happen late because
// otherwise our loop iterators won't work.
SmallPtrSet<BasicBlock *, 8> blocks;
blocks.insert(L->block_begin(), L->block_end());
for (BasicBlock *BB : blocks)
// The last step is to update LoopInfo now that we've eliminated this loop.
// Note: LoopInfo::erase remove the given loop and relink its subloops with
// its parent. While removeLoop/removeChildLoop remove the given loop but
// not relink its subloops, which is what we want.
if (Loop *ParentLoop = L->getParentLoop()) {
Loop::iterator I = find(*ParentLoop, L);
assert(I != ParentLoop->end() && "Couldn't find loop");
} else {
Loop::iterator I = find(*LI, L);
assert(I != LI->end() && "Couldn't find loop");
static Loop *getOutermostLoop(Loop *L) {
while (Loop *Parent = L->getParentLoop())
L = Parent;
return L;
void llvm::breakLoopBackedge(Loop *L, DominatorTree &DT, ScalarEvolution &SE,
LoopInfo &LI, MemorySSA *MSSA) {
auto *Latch = L->getLoopLatch();
assert(Latch && "multiple latches not yet supported");
auto *Header = L->getHeader();
Loop *OutermostLoop = getOutermostLoop(L);
// Note: By splitting the backedge, and then explicitly making it unreachable
// we gracefully handle corner cases such as non-bottom tested loops and the
// like. We also have the benefit of being able to reuse existing well tested
// code. It might be worth special casing the common bottom tested case at
// some point to avoid code churn.
std::unique_ptr<MemorySSAUpdater> MSSAU;
if (MSSA)
MSSAU = std::make_unique<MemorySSAUpdater>(MSSA);
auto *BackedgeBB = SplitEdge(Latch, Header, &DT, &LI, MSSAU.get());
DomTreeUpdater DTU(&DT, DomTreeUpdater::UpdateStrategy::Eager);
(void)changeToUnreachable(BackedgeBB->getTerminator(), /*UseTrap*/false,
/*PreserveLCSSA*/true, &DTU, MSSAU.get());
// Erase (and destroy) this loop instance. Handles relinking sub-loops
// and blocks within the loop as needed.
// If the loop we broke had a parent, then changeToUnreachable might have
// caused a block to be removed from the parent loop (see loop_nest_lcssa
// test case in zero-btc.ll for an example), thus changing the parent's
// exit blocks. If that happened, we need to rebuild LCSSA on the outermost
// loop which might have a had a block removed.
if (OutermostLoop != L)
formLCSSARecursively(*OutermostLoop, DT, &LI, &SE);
/// Checks if \p L has single exit through latch block except possibly
/// "deoptimizing" exits. Returns branch instruction terminating the loop
/// latch if above check is successful, nullptr otherwise.
static BranchInst *getExpectedExitLoopLatchBranch(Loop *L) {
BasicBlock *Latch = L->getLoopLatch();
if (!Latch)
return nullptr;
BranchInst *LatchBR = dyn_cast<BranchInst>(Latch->getTerminator());
if (!LatchBR || LatchBR->getNumSuccessors() != 2 || !L->isLoopExiting(Latch))
return nullptr;
assert((LatchBR->getSuccessor(0) == L->getHeader() ||
LatchBR->getSuccessor(1) == L->getHeader()) &&
"At least one edge out of the latch must go to the header");
SmallVector<BasicBlock *, 4> ExitBlocks;
if (any_of(ExitBlocks, [](const BasicBlock *EB) {
return !EB->getTerminatingDeoptimizeCall();
return nullptr;
return LatchBR;
llvm::getLoopEstimatedTripCount(Loop *L,
unsigned *EstimatedLoopInvocationWeight) {
// Support loops with an exiting latch and other existing exists only
// deoptimize.
BranchInst *LatchBranch = getExpectedExitLoopLatchBranch(L);
if (!LatchBranch)
return None;
// To estimate the number of times the loop body was executed, we want to
// know the number of times the backedge was taken, vs. the number of times
// we exited the loop.
uint64_t BackedgeTakenWeight, LatchExitWeight;
if (!LatchBranch->extractProfMetadata(BackedgeTakenWeight, LatchExitWeight))
return None;
if (LatchBranch->getSuccessor(0) != L->getHeader())
std::swap(BackedgeTakenWeight, LatchExitWeight);
if (!LatchExitWeight)
return None;
if (EstimatedLoopInvocationWeight)
*EstimatedLoopInvocationWeight = LatchExitWeight;
// Estimated backedge taken count is a ratio of the backedge taken weight by
// the weight of the edge exiting the loop, rounded to nearest.
uint64_t BackedgeTakenCount =
llvm::divideNearest(BackedgeTakenWeight, LatchExitWeight);
// Estimated trip count is one plus estimated backedge taken count.
return BackedgeTakenCount + 1;
bool llvm::setLoopEstimatedTripCount(Loop *L, unsigned EstimatedTripCount,
unsigned EstimatedloopInvocationWeight) {
// Support loops with an exiting latch and other existing exists only
// deoptimize.
BranchInst *LatchBranch = getExpectedExitLoopLatchBranch(L);
if (!LatchBranch)
return false;
// Calculate taken and exit weights.
unsigned LatchExitWeight = 0;
unsigned BackedgeTakenWeight = 0;
if (EstimatedTripCount > 0) {
LatchExitWeight = EstimatedloopInvocationWeight;
BackedgeTakenWeight = (EstimatedTripCount - 1) * LatchExitWeight;
// Make a swap if back edge is taken when condition is "false".
if (LatchBranch->getSuccessor(0) != L->getHeader())
std::swap(BackedgeTakenWeight, LatchExitWeight);
MDBuilder MDB(LatchBranch->getContext());
// Set/Update profile metadata.
MDB.createBranchWeights(BackedgeTakenWeight, LatchExitWeight));
return true;
bool llvm::hasIterationCountInvariantInParent(Loop *InnerLoop,
ScalarEvolution &SE) {
Loop *OuterL = InnerLoop->getParentLoop();
if (!OuterL)
return true;
// Get the backedge taken count for the inner loop
BasicBlock *InnerLoopLatch = InnerLoop->getLoopLatch();
const SCEV *InnerLoopBECountSC = SE.getExitCount(InnerLoop, InnerLoopLatch);
if (isa<SCEVCouldNotCompute>(InnerLoopBECountSC) ||
return false;
// Get whether count is invariant to the outer loop
ScalarEvolution::LoopDisposition LD =
SE.getLoopDisposition(InnerLoopBECountSC, OuterL);
if (LD != ScalarEvolution::LoopInvariant)
return false;
return true;
Value *llvm::createMinMaxOp(IRBuilderBase &Builder, RecurKind RK, Value *Left,
Value *Right) {
CmpInst::Predicate Pred;
switch (RK) {
llvm_unreachable("Unknown min/max recurrence kind");
case RecurKind::UMin:
Pred = CmpInst::ICMP_ULT;
case RecurKind::UMax:
Pred = CmpInst::ICMP_UGT;
case RecurKind::SMin:
Pred = CmpInst::ICMP_SLT;
case RecurKind::SMax:
Pred = CmpInst::ICMP_SGT;
case RecurKind::FMin:
Pred = CmpInst::FCMP_OLT;
case RecurKind::FMax:
Pred = CmpInst::FCMP_OGT;
Value *Cmp = Builder.CreateCmp(Pred, Left, Right, "rdx.minmax.cmp");
Value *Select = Builder.CreateSelect(Cmp, Left, Right, "");
return Select;
// Helper to generate an ordered reduction.
Value *llvm::getOrderedReduction(IRBuilderBase &Builder, Value *Acc, Value *Src,
unsigned Op, RecurKind RdxKind,
ArrayRef<Value *> RedOps) {
unsigned VF = cast<FixedVectorType>(Src->getType())->getNumElements();
// Extract and apply reduction ops in ascending order:
// e.g. ((((Acc + Scl[0]) + Scl[1]) + Scl[2]) + ) ... + Scl[VF-1]
Value *Result = Acc;
for (unsigned ExtractIdx = 0; ExtractIdx != VF; ++ExtractIdx) {
Value *Ext =
Builder.CreateExtractElement(Src, Builder.getInt32(ExtractIdx));
if (Op != Instruction::ICmp && Op != Instruction::FCmp) {
Result = Builder.CreateBinOp((Instruction::BinaryOps)Op, Result, Ext,
} else {
assert(RecurrenceDescriptor::isMinMaxRecurrenceKind(RdxKind) &&
"Invalid min/max");
Result = createMinMaxOp(Builder, RdxKind, Result, Ext);
if (!RedOps.empty())
propagateIRFlags(Result, RedOps);
return Result;
// Helper to generate a log2 shuffle reduction.
Value *llvm::getShuffleReduction(IRBuilderBase &Builder, Value *Src,
unsigned Op, RecurKind RdxKind,
ArrayRef<Value *> RedOps) {
unsigned VF = cast<FixedVectorType>(Src->getType())->getNumElements();
// VF is a power of 2 so we can emit the reduction using log2(VF) shuffles
// and vector ops, reducing the set of values being computed by half each
// round.
assert(isPowerOf2_32(VF) &&
"Reduction emission only supported for pow2 vectors!");
Value *TmpVec = Src;
SmallVector<int, 32> ShuffleMask(VF);
for (unsigned i = VF; i != 1; i >>= 1) {
// Move the upper half of the vector to the lower half.
for (unsigned j = 0; j != i / 2; ++j)
ShuffleMask[j] = i / 2 + j;
// Fill the rest of the mask with undef.
std::fill(&ShuffleMask[i / 2], ShuffleMask.end(), -1);
Value *Shuf = Builder.CreateShuffleVector(TmpVec, ShuffleMask, "rdx.shuf");
if (Op != Instruction::ICmp && Op != Instruction::FCmp) {
// The builder propagates its fast-math-flags setting.
TmpVec = Builder.CreateBinOp((Instruction::BinaryOps)Op, TmpVec, Shuf,
} else {
assert(RecurrenceDescriptor::isMinMaxRecurrenceKind(RdxKind) &&
"Invalid min/max");
TmpVec = createMinMaxOp(Builder, RdxKind, TmpVec, Shuf);
if (!RedOps.empty())
propagateIRFlags(TmpVec, RedOps);
// We may compute the reassociated scalar ops in a way that does not
// preserve nsw/nuw etc. Conservatively, drop those flags.
if (auto *ReductionInst = dyn_cast<Instruction>(TmpVec))
// The result is in the first element of the vector.
return Builder.CreateExtractElement(TmpVec, Builder.getInt32(0));
Value *llvm::createSimpleTargetReduction(IRBuilderBase &Builder,
const TargetTransformInfo *TTI,
Value *Src, RecurKind RdxKind,
ArrayRef<Value *> RedOps) {
TargetTransformInfo::ReductionFlags RdxFlags;
RdxFlags.IsMaxOp = RdxKind == RecurKind::SMax || RdxKind == RecurKind::UMax ||
RdxKind == RecurKind::FMax;
RdxFlags.IsSigned = RdxKind == RecurKind::SMax || RdxKind == RecurKind::SMin;
auto *SrcVecEltTy = cast<VectorType>(Src->getType())->getElementType();
switch (RdxKind) {
case RecurKind::Add:
return Builder.CreateAddReduce(Src);
case RecurKind::Mul:
return Builder.CreateMulReduce(Src);
case RecurKind::And:
return Builder.CreateAndReduce(Src);
case RecurKind::Or:
return Builder.CreateOrReduce(Src);
case RecurKind::Xor:
return Builder.CreateXorReduce(Src);
case RecurKind::FAdd:
return Builder.CreateFAddReduce(ConstantFP::getNegativeZero(SrcVecEltTy),
case RecurKind::FMul:
return Builder.CreateFMulReduce(ConstantFP::get(SrcVecEltTy, 1.0), Src);
case RecurKind::SMax:
return Builder.CreateIntMaxReduce(Src, true);
case RecurKind::SMin:
return Builder.CreateIntMinReduce(Src, true);
case RecurKind::UMax:
return Builder.CreateIntMaxReduce(Src, false);
case RecurKind::UMin:
return Builder.CreateIntMinReduce(Src, false);
case RecurKind::FMax:
return Builder.CreateFPMaxReduce(Src);
case RecurKind::FMin:
return Builder.CreateFPMinReduce(Src);
llvm_unreachable("Unhandled opcode");
Value *llvm::createTargetReduction(IRBuilderBase &B,
const TargetTransformInfo *TTI,
RecurrenceDescriptor &Desc, Value *Src) {
// TODO: Support in-order reductions based on the recurrence descriptor.
// All ops in the reduction inherit fast-math-flags from the recurrence
// descriptor.
IRBuilderBase::FastMathFlagGuard FMFGuard(B);
return createSimpleTargetReduction(B, TTI, Src, Desc.getRecurrenceKind());
Value *llvm::createOrderedReduction(IRBuilderBase &B,
RecurrenceDescriptor &Desc, Value *Src,
Value *Start) {
assert(Desc.getRecurrenceKind() == RecurKind::FAdd &&
"Unexpected reduction kind");
assert(Src->getType()->isVectorTy() && "Expected a vector type");
assert(!Start->getType()->isVectorTy() && "Expected a scalar type");
return B.CreateFAddReduce(Start, Src);
void llvm::propagateIRFlags(Value *I, ArrayRef<Value *> VL, Value *OpValue) {
auto *VecOp = dyn_cast<Instruction>(I);
if (!VecOp)
auto *Intersection = (OpValue == nullptr) ? dyn_cast<Instruction>(VL[0])
: dyn_cast<Instruction>(OpValue);
if (!Intersection)
const unsigned Opcode = Intersection->getOpcode();
for (auto *V : VL) {
auto *Instr = dyn_cast<Instruction>(V);
if (!Instr)
if (OpValue == nullptr || Opcode == Instr->getOpcode())
bool llvm::isKnownNegativeInLoop(const SCEV *S, const Loop *L,
ScalarEvolution &SE) {
const SCEV *Zero = SE.getZero(S->getType());
return SE.isAvailableAtLoopEntry(S, L) &&
SE.isLoopEntryGuardedByCond(L, ICmpInst::ICMP_SLT, S, Zero);
bool llvm::isKnownNonNegativeInLoop(const SCEV *S, const Loop *L,
ScalarEvolution &SE) {
const SCEV *Zero = SE.getZero(S->getType());
return SE.isAvailableAtLoopEntry(S, L) &&
SE.isLoopEntryGuardedByCond(L, ICmpInst::ICMP_SGE, S, Zero);
bool llvm::cannotBeMinInLoop(const SCEV *S, const Loop *L, ScalarEvolution &SE,
bool Signed) {
unsigned BitWidth = cast<IntegerType>(S->getType())->getBitWidth();
APInt Min = Signed ? APInt::getSignedMinValue(BitWidth) :
auto Predicate = Signed ? ICmpInst::ICMP_SGT : ICmpInst::ICMP_UGT;
return SE.isAvailableAtLoopEntry(S, L) &&
SE.isLoopEntryGuardedByCond(L, Predicate, S,
bool llvm::cannotBeMaxInLoop(const SCEV *S, const Loop *L, ScalarEvolution &SE,
bool Signed) {
unsigned BitWidth = cast<IntegerType>(S->getType())->getBitWidth();
APInt Max = Signed ? APInt::getSignedMaxValue(BitWidth) :
auto Predicate = Signed ? ICmpInst::ICMP_SLT : ICmpInst::ICMP_ULT;
return SE.isAvailableAtLoopEntry(S, L) &&
SE.isLoopEntryGuardedByCond(L, Predicate, S,
// rewriteLoopExitValues - Optimize IV users outside the loop.
// As a side effect, reduces the amount of IV processing within the loop.
// Return true if the SCEV expansion generated by the rewriter can replace the
// original value. SCEV guarantees that it produces the same value, but the way
// it is produced may be illegal IR. Ideally, this function will only be
// called for verification.
static bool isValidRewrite(ScalarEvolution *SE, Value *FromVal, Value *ToVal) {
// If an SCEV expression subsumed multiple pointers, its expansion could
// reassociate the GEP changing the base pointer. This is illegal because the
// final address produced by a GEP chain must be inbounds relative to its
// underlying object. Otherwise basic alias analysis, among other things,
// could fail in a dangerous way. Ultimately, SCEV will be improved to avoid
// producing an expression involving multiple pointers. Until then, we must
// bail out here.
// Retrieve the pointer operand of the GEP. Don't use getUnderlyingObject
// because it understands lcssa phis while SCEV does not.
Value *FromPtr = FromVal;
Value *ToPtr = ToVal;
if (auto *GEP = dyn_cast<GEPOperator>(FromVal))
FromPtr = GEP->getPointerOperand();
if (auto *GEP = dyn_cast<GEPOperator>(ToVal))
ToPtr = GEP->getPointerOperand();
if (FromPtr != FromVal || ToPtr != ToVal) {
// Quickly check the common case
if (FromPtr == ToPtr)
return true;
// SCEV may have rewritten an expression that produces the GEP's pointer
// operand. That's ok as long as the pointer operand has the same base
// pointer. Unlike getUnderlyingObject(), getPointerBase() will find the
// base of a recurrence. This handles the case in which SCEV expansion
// converts a pointer type recurrence into a nonrecurrent pointer base
// indexed by an integer recurrence.
// If the GEP base pointer is a vector of pointers, abort.
if (!FromPtr->getType()->isPointerTy() || !ToPtr->getType()->isPointerTy())
return false;
const SCEV *FromBase = SE->getPointerBase(SE->getSCEV(FromPtr));
const SCEV *ToBase = SE->getPointerBase(SE->getSCEV(ToPtr));
if (FromBase == ToBase)
return true;
LLVM_DEBUG(dbgs() << "rewriteLoopExitValues: GEP rewrite bail out "
<< *FromBase << " != " << *ToBase << "\n");
return false;
return true;
static bool hasHardUserWithinLoop(const Loop *L, const Instruction *I) {
SmallPtrSet<const Instruction *, 8> Visited;
SmallVector<const Instruction *, 8> WorkList;
while (!WorkList.empty()) {
const Instruction *Curr = WorkList.pop_back_val();
// This use is outside the loop, nothing to do.
if (!L->contains(Curr))
// Do we assume it is a "hard" use which will not be eliminated easily?
if (Curr->mayHaveSideEffects())
return true;
// Otherwise, add all its users to worklist.
for (auto U : Curr->users()) {
auto *UI = cast<Instruction>(U);
if (Visited.insert(UI).second)
return false;
// Collect information about PHI nodes which can be transformed in
// rewriteLoopExitValues.
struct RewritePhi {
PHINode *PN; // For which PHI node is this replacement?
unsigned Ith; // For which incoming value?
const SCEV *ExpansionSCEV; // The SCEV of the incoming value we are rewriting.
Instruction *ExpansionPoint; // Where we'd like to expand that SCEV?
bool HighCost; // Is this expansion a high-cost?
Value *Expansion = nullptr;
bool ValidRewrite = false;
RewritePhi(PHINode *P, unsigned I, const SCEV *Val, Instruction *ExpansionPt,
bool H)
: PN(P), Ith(I), ExpansionSCEV(Val), ExpansionPoint(ExpansionPt),
HighCost(H) {}
// Check whether it is possible to delete the loop after rewriting exit
// value. If it is possible, ignore ReplaceExitValue and do rewriting
// aggressively.
static bool canLoopBeDeleted(Loop *L, SmallVector<RewritePhi, 8> &RewritePhiSet) {
BasicBlock *Preheader = L->getLoopPreheader();
// If there is no preheader, the loop will not be deleted.
if (!Preheader)
return false;
// In LoopDeletion pass Loop can be deleted when ExitingBlocks.size() > 1.
// We obviate multiple ExitingBlocks case for simplicity.
// TODO: If we see testcase with multiple ExitingBlocks can be deleted
// after exit value rewriting, we can enhance the logic here.
SmallVector<BasicBlock *, 4> ExitingBlocks;
SmallVector<BasicBlock *, 8> ExitBlocks;
if (ExitBlocks.size() != 1 || ExitingBlocks.size() != 1)
return false;
BasicBlock *ExitBlock = ExitBlocks[0];
BasicBlock::iterator BI = ExitBlock->begin();
while (PHINode *P = dyn_cast<PHINode>(BI)) {
Value *Incoming = P->getIncomingValueForBlock(ExitingBlocks[0]);
// If the Incoming value of P is found in RewritePhiSet, we know it
// could be rewritten to use a loop invariant value in transformation
// phase later. Skip it in the loop invariant check below.
bool found = false;
for (const RewritePhi &Phi : RewritePhiSet) {
if (!Phi.ValidRewrite)
unsigned i = Phi.Ith;
if (Phi.PN == P && (Phi.PN)->getIncomingValue(i) == Incoming) {
found = true;
Instruction *I;
if (!found && (I = dyn_cast<Instruction>(Incoming)))
if (!L->hasLoopInvariantOperands(I))
return false;
for (auto *BB : L->blocks())
if (llvm::any_of(*BB, [](Instruction &I) {
return I.mayHaveSideEffects();
return false;
return true;
int llvm::rewriteLoopExitValues(Loop *L, LoopInfo *LI, TargetLibraryInfo *TLI,
ScalarEvolution *SE,
const TargetTransformInfo *TTI,
SCEVExpander &Rewriter, DominatorTree *DT,
ReplaceExitVal ReplaceExitValue,
SmallVector<WeakTrackingVH, 16> &DeadInsts) {
// Check a pre-condition.
assert(L->isRecursivelyLCSSAForm(*DT, *LI) &&
"Indvars did not preserve LCSSA!");
SmallVector<BasicBlock*, 8> ExitBlocks;
SmallVector<RewritePhi, 8> RewritePhiSet;
// Find all values that are computed inside the loop, but used outside of it.
// Because of LCSSA, these values will only occur in LCSSA PHI Nodes. Scan
// the exit blocks of the loop to find them.
for (BasicBlock *ExitBB : ExitBlocks) {
// If there are no PHI nodes in this exit block, then no values defined
// inside the loop are used on this path, skip it.
PHINode *PN = dyn_cast<PHINode>(ExitBB->begin());
if (!PN) continue;
unsigned NumPreds = PN->getNumIncomingValues();
// Iterate over all of the PHI nodes.
BasicBlock::iterator BBI = ExitBB->begin();
while ((PN = dyn_cast<PHINode>(BBI++))) {
if (PN->use_empty())
continue; // dead use, don't replace it
if (!SE->isSCEVable(PN->getType()))
// It's necessary to tell ScalarEvolution about this explicitly so that
// it can walk the def-use list and forget all SCEVs, as it may not be
// watching the PHI itself. Once the new exit value is in place, there
// may not be a def-use connection between the loop and every instruction
// which got a SCEVAddRecExpr for that loop.
// Iterate over all of the values in all the PHI nodes.
for (unsigned i = 0; i != NumPreds; ++i) {
// If the value being merged in is not integer or is not defined
// in the loop, skip it.
Value *InVal = PN->getIncomingValue(i);
if (!isa<Instruction>(InVal))
// If this pred is for a subloop, not L itself, skip it.
if (LI->getLoopFor(PN->getIncomingBlock(i)) != L)
continue; // The Block is in a subloop, skip it.
// Check that InVal is defined in the loop.
Instruction *Inst = cast<Instruction>(InVal);
if (!L->contains(Inst))
// Okay, this instruction has a user outside of the current loop
// and varies predictably *inside* the loop. Evaluate the value it
// contains when the loop exits, if possible. We prefer to start with
// expressions which are true for all exits (so as to maximize
// expression reuse by the SCEVExpander), but resort to per-exit
// evaluation if that fails.
const SCEV *ExitValue = SE->getSCEVAtScope(Inst, L->getParentLoop());
if (isa<SCEVCouldNotCompute>(ExitValue) ||
!SE->isLoopInvariant(ExitValue, L) ||
!isSafeToExpand(ExitValue, *SE)) {
// TODO: This should probably be sunk into SCEV in some way; maybe a
// getSCEVForExit(SCEV*, L, ExitingBB)? It can be generalized for
// most SCEV expressions and other recurrence types (e.g. shift
// recurrences). Is there existing code we can reuse?
const SCEV *ExitCount = SE->getExitCount(L, PN->getIncomingBlock(i));
if (isa<SCEVCouldNotCompute>(ExitCount))
if (auto *AddRec = dyn_cast<SCEVAddRecExpr>(SE->getSCEV(Inst)))
if (AddRec->getLoop() == L)
ExitValue = AddRec->evaluateAtIteration(ExitCount, *SE);
if (isa<SCEVCouldNotCompute>(ExitValue) ||
!SE->isLoopInvariant(ExitValue, L) ||
!isSafeToExpand(ExitValue, *SE))
// Computing the value outside of the loop brings no benefit if it is
// definitely used inside the loop in a way which can not be optimized
// away. Avoid doing so unless we know we have a value which computes
// the ExitValue already. TODO: This should be merged into SCEV
// expander to leverage its knowledge of existing expressions.
if (ReplaceExitValue != AlwaysRepl && !isa<SCEVConstant>(ExitValue) &&
!isa<SCEVUnknown>(ExitValue) && hasHardUserWithinLoop(L, Inst))
// Check if expansions of this SCEV would count as being high cost.
bool HighCost = Rewriter.isHighCostExpansion(
ExitValue, L, SCEVCheapExpansionBudget, TTI, Inst);
// Note that we must not perform expansions until after
// we query *all* the costs, because if we perform temporary expansion
// inbetween, one that we might not intend to keep, said expansion
// *may* affect cost calculation of the the next SCEV's we'll query,
// and next SCEV may errneously get smaller cost.
// Collect all the candidate PHINodes to be rewritten.
RewritePhiSet.emplace_back(PN, i, ExitValue, Inst, HighCost);
// Now that we've done preliminary filtering and billed all the SCEV's,
// we can perform the last sanity check - the expansion must be valid.
for (RewritePhi &Phi : RewritePhiSet) {
Phi.Expansion = Rewriter.expandCodeFor(Phi.ExpansionSCEV, Phi.PN->getType(),
LLVM_DEBUG(dbgs() << "rewriteLoopExitValues: AfterLoopVal = "
<< *(Phi.Expansion) << '\n'
<< " LoopVal = " << *(Phi.ExpansionPoint) << "\n");
// FIXME: isValidRewrite() is a hack. it should be an assert, eventually.
Phi.ValidRewrite = isValidRewrite(SE, Phi.ExpansionPoint, Phi.Expansion);
if (!Phi.ValidRewrite) {
#ifndef NDEBUG
// If we reuse an instruction from a loop which is neither L nor one of
// its containing loops, we end up breaking LCSSA form for this loop by
// creating a new use of its instruction.
if (auto *ExitInsn = dyn_cast<Instruction>(Phi.Expansion))
if (auto *EVL = LI->getLoopFor(ExitInsn->getParent()))
if (EVL != L)
assert(EVL->contains(L) && "LCSSA breach detected!");
// TODO: after isValidRewrite() is an assertion, evaluate whether
// it is beneficial to change how we calculate high-cost:
// if we have SCEV 'A' which we know we will expand, should we calculate
// the cost of other SCEV's after expanding SCEV 'A',
// thus potentially giving cost bonus to those other SCEV's?
bool LoopCanBeDel = canLoopBeDeleted(L, RewritePhiSet);
int NumReplaced = 0;
// Transformation.
for (const RewritePhi &Phi : RewritePhiSet) {
if (!Phi.ValidRewrite)
PHINode *PN = Phi.PN;
Value *ExitVal = Phi.Expansion;
// Only do the rewrite when the ExitValue can be expanded cheaply.
// If LoopCanBeDel is true, rewrite exit value aggressively.
if (ReplaceExitValue == OnlyCheapRepl && !LoopCanBeDel && Phi.HighCost) {
Instruction *Inst = cast<Instruction>(PN->getIncomingValue(Phi.Ith));
PN->setIncomingValue(Phi.Ith, ExitVal);
// If this instruction is dead now, delete it. Don't do it now to avoid
// invalidating iterators.
if (isInstructionTriviallyDead(Inst, TLI))
// Replace PN with ExitVal if that is legal and does not break LCSSA.
if (PN->getNumIncomingValues() == 1 &&
LI->replacementPreservesLCSSAForm(PN, ExitVal)) {
// The insertion point instruction may have been deleted; clear it out
// so that the rewriter doesn't trip over it later.
return NumReplaced;
/// Set weights for \p UnrolledLoop and \p RemainderLoop based on weights for
/// \p OrigLoop.
void llvm::setProfileInfoAfterUnrolling(Loop *OrigLoop, Loop *UnrolledLoop,
Loop *RemainderLoop, uint64_t UF) {
assert(UF > 0 && "Zero unrolled factor is not supported");
assert(UnrolledLoop != RemainderLoop &&
"Unrolled and Remainder loops are expected to distinct");
// Get number of iterations in the original scalar loop.
unsigned OrigLoopInvocationWeight = 0;
Optional<unsigned> OrigAverageTripCount =
getLoopEstimatedTripCount(OrigLoop, &OrigLoopInvocationWeight);
if (!OrigAverageTripCount)
// Calculate number of iterations in unrolled loop.
unsigned UnrolledAverageTripCount = *OrigAverageTripCount / UF;
// Calculate number of iterations for remainder loop.
unsigned RemainderAverageTripCount = *OrigAverageTripCount % UF;
setLoopEstimatedTripCount(UnrolledLoop, UnrolledAverageTripCount,
setLoopEstimatedTripCount(RemainderLoop, RemainderAverageTripCount,
/// Utility that implements appending of loops onto a worklist.
/// Loops are added in preorder (analogous for reverse postorder for trees),
/// and the worklist is processed LIFO.
template <typename RangeT>
void llvm::appendReversedLoopsToWorklist(
RangeT &&Loops, SmallPriorityWorklist<Loop *, 4> &Worklist) {
// We use an internal worklist to build up the preorder traversal without
// recursion.
SmallVector<Loop *, 4> PreOrderLoops, PreOrderWorklist;
// We walk the initial sequence of loops in reverse because we generally want
// to visit defs before uses and the worklist is LIFO.
for (Loop *RootL : Loops) {
assert(PreOrderLoops.empty() && "Must start with an empty preorder walk.");
assert(PreOrderWorklist.empty() &&
"Must start with an empty preorder walk worklist.");
do {
Loop *L = PreOrderWorklist.pop_back_val();
PreOrderWorklist.append(L->begin(), L->end());
} while (!PreOrderWorklist.empty());
template <typename RangeT>
void llvm::appendLoopsToWorklist(RangeT &&Loops,
SmallPriorityWorklist<Loop *, 4> &Worklist) {
appendReversedLoopsToWorklist(reverse(Loops), Worklist);
template void llvm::appendLoopsToWorklist<ArrayRef<Loop *> &>(
ArrayRef<Loop *> &Loops, SmallPriorityWorklist<Loop *, 4> &Worklist);
template void
llvm::appendLoopsToWorklist<Loop &>(Loop &L,
SmallPriorityWorklist<Loop *, 4> &Worklist);
void llvm::appendLoopsToWorklist(LoopInfo &LI,
SmallPriorityWorklist<Loop *, 4> &Worklist) {
appendReversedLoopsToWorklist(LI, Worklist);
Loop *llvm::cloneLoop(Loop *L, Loop *PL, ValueToValueMapTy &VM,
LoopInfo *LI, LPPassManager *LPM) {
Loop &New = *LI->AllocateLoop();
if (PL)
if (LPM)
// Add all of the blocks in L to the new loop.
for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
I != E; ++I)
if (LI->getLoopFor(*I) == L)
New.addBasicBlockToLoop(cast<BasicBlock>(VM[*I]), *LI);
// Add all of the subloops to the new loop.
for (Loop *I : *L)
cloneLoop(I, &New, VM, LI, LPM);
return &New;
/// IR Values for the lower and upper bounds of a pointer evolution. We
/// need to use value-handles because SCEV expansion can invalidate previously
/// expanded values. Thus expansion of a pointer can invalidate the bounds for
/// a previous one.
struct PointerBounds {
TrackingVH<Value> Start;
TrackingVH<Value> End;
/// Expand code for the lower and upper bound of the pointer group \p CG
/// in \p TheLoop. \return the values for the bounds.
static PointerBounds expandBounds(const RuntimeCheckingPtrGroup *CG,
Loop *TheLoop, Instruction *Loc,
SCEVExpander &Exp) {
ScalarEvolution *SE = Exp.getSE();
// TODO: Add helper to retrieve pointers to CG.
Value *Ptr = CG->RtCheck.Pointers[CG->Members[0]].PointerValue;
const SCEV *Sc = SE->getSCEV(Ptr);
unsigned AS = Ptr->getType()->getPointerAddressSpace();
LLVMContext &Ctx = Loc->getContext();
// Use this type for pointer arithmetic.
Type *PtrArithTy = Type::getInt8PtrTy(Ctx, AS);
if (SE->isLoopInvariant(Sc, TheLoop)) {
LLVM_DEBUG(dbgs() << "LAA: Adding RT check for a loop invariant ptr:"
<< *Ptr << "\n");
// Ptr could be in the loop body. If so, expand a new one at the correct
// location.
Instruction *Inst = dyn_cast<Instruction>(Ptr);
Value *NewPtr = (Inst && TheLoop->contains(Inst))
? Exp.expandCodeFor(Sc, PtrArithTy, Loc)
: Ptr;
// We must return a half-open range, which means incrementing Sc.
const SCEV *ScPlusOne = SE->getAddExpr(Sc, SE->getOne(PtrArithTy));
Value *NewPtrPlusOne = Exp.expandCodeFor(ScPlusOne, PtrArithTy, Loc);
return {NewPtr, NewPtrPlusOne};
} else {
Value *Start = nullptr, *End = nullptr;
LLVM_DEBUG(dbgs() << "LAA: Adding RT check for range:\n");
Start = Exp.expandCodeFor(CG->Low, PtrArithTy, Loc);
End = Exp.expandCodeFor(CG->High, PtrArithTy, Loc);
LLVM_DEBUG(dbgs() << "Start: " << *CG->Low << " End: " << *CG->High
<< "\n");
return {Start, End};
/// Turns a collection of checks into a collection of expanded upper and
/// lower bounds for both pointers in the check.
static SmallVector<std::pair<PointerBounds, PointerBounds>, 4>
expandBounds(const SmallVectorImpl<RuntimePointerCheck> &PointerChecks, Loop *L,
Instruction *Loc, SCEVExpander &Exp) {
SmallVector<std::pair<PointerBounds, PointerBounds>, 4> ChecksWithBounds;
// Here we're relying on the SCEV Expander's cache to only emit code for the
// same bounds once.
transform(PointerChecks, std::back_inserter(ChecksWithBounds),
[&](const RuntimePointerCheck &Check) {
PointerBounds First = expandBounds(Check.first, L, Loc, Exp),
Second = expandBounds(Check.second, L, Loc, Exp);
return std::make_pair(First, Second);
return ChecksWithBounds;
std::pair<Instruction *, Instruction *> llvm::addRuntimeChecks(
Instruction *Loc, Loop *TheLoop,
const SmallVectorImpl<RuntimePointerCheck> &PointerChecks,
SCEVExpander &Exp) {
// TODO: Move noalias annotation code from LoopVersioning here and share with LV if possible.
// TODO: Pass RtPtrChecking instead of PointerChecks and SE separately, if possible
auto ExpandedChecks = expandBounds(PointerChecks, TheLoop, Loc, Exp);
LLVMContext &Ctx = Loc->getContext();
Instruction *FirstInst = nullptr;
IRBuilder<> ChkBuilder(Loc);
// Our instructions might fold to a constant.
Value *MemoryRuntimeCheck = nullptr;
// FIXME: this helper is currently a duplicate of the one in
// LoopVectorize.cpp.
auto GetFirstInst = [](Instruction *FirstInst, Value *V,
Instruction *Loc) -> Instruction * {
if (FirstInst)
return FirstInst;
if (Instruction *I = dyn_cast<Instruction>(V))
return I->getParent() == Loc->getParent() ? I : nullptr;
return nullptr;
for (const auto &Check : ExpandedChecks) {
const PointerBounds &A = Check.first, &B = Check.second;
// Check if two pointers (A and B) conflict where conflict is computed as:
// start(A) <= end(B) && start(B) <= end(A)
unsigned AS0 = A.Start->getType()->getPointerAddressSpace();
unsigned AS1 = B.Start->getType()->getPointerAddressSpace();
assert((AS0 == B.End->getType()->getPointerAddressSpace()) &&
(AS1 == A.End->getType()->getPointerAddressSpace()) &&
"Trying to bounds check pointers with different address spaces");
Type *PtrArithTy0 = Type::getInt8PtrTy(Ctx, AS0);
Type *PtrArithTy1 = Type::getInt8PtrTy(Ctx, AS1);
Value *Start0 = ChkBuilder.CreateBitCast(A.Start, PtrArithTy0, "bc");
Value *Start1 = ChkBuilder.CreateBitCast(B.Start, PtrArithTy1, "bc");
Value *End0 = ChkBuilder.CreateBitCast(A.End, PtrArithTy1, "bc");
Value *End1 = ChkBuilder.CreateBitCast(B.End, PtrArithTy0, "bc");
// [A|B].Start points to the first accessed byte under base [A|B].
// [A|B].End points to the last accessed byte, plus one.
// There is no conflict when the intervals are disjoint:
// NoConflict = (B.Start >= A.End) || (A.Start >= B.End)
// bound0 = (B.Start < A.End)
// bound1 = (A.Start < B.End)
// IsConflict = bound0 & bound1
Value *Cmp0 = ChkBuilder.CreateICmpULT(Start0, End1, "bound0");
FirstInst = GetFirstInst(FirstInst, Cmp0, Loc);
Value *Cmp1 = ChkBuilder.CreateICmpULT(Start1, End0, "bound1");
FirstInst = GetFirstInst(FirstInst, Cmp1, Loc);
Value *IsConflict = ChkBuilder.CreateAnd(Cmp0, Cmp1, "found.conflict");
FirstInst = GetFirstInst(FirstInst, IsConflict, Loc);
if (MemoryRuntimeCheck) {
IsConflict =
ChkBuilder.CreateOr(MemoryRuntimeCheck, IsConflict, "conflict.rdx");
FirstInst = GetFirstInst(FirstInst, IsConflict, Loc);
MemoryRuntimeCheck = IsConflict;
if (!MemoryRuntimeCheck)
return std::make_pair(nullptr, nullptr);
// We have to do this trickery because the IRBuilder might fold the check to a
// constant expression in which case there is no Instruction anchored in a
// the block.
Instruction *Check =
BinaryOperator::CreateAnd(MemoryRuntimeCheck, ConstantInt::getTrue(Ctx));
ChkBuilder.Insert(Check, "memcheck.conflict");
FirstInst = GetFirstInst(FirstInst, Check, Loc);
return std::make_pair(FirstInst, Check);
Optional<IVConditionInfo> llvm::hasPartialIVCondition(Loop &L,
unsigned MSSAThreshold,
MemorySSA &MSSA,
AAResults &AA) {
auto *TI = dyn_cast<BranchInst>(L.getHeader()->getTerminator());
if (!TI || !TI->isConditional())
return {};
auto *CondI = dyn_cast<CmpInst>(TI->getCondition());
// The case with the condition outside the loop should already be handled
// earlier.
if (!CondI || !L.contains(CondI))
return {};
SmallVector<Instruction *> InstToDuplicate;
SmallVector<Value *, 4> WorkList;
WorkList.append(CondI->op_begin(), CondI->op_end());
SmallVector<MemoryAccess *, 4> AccessesToCheck;
SmallVector<MemoryLocation, 4> AccessedLocs;
while (!WorkList.empty()) {
Instruction *I = dyn_cast<Instruction>(WorkList.pop_back_val());
if (!I || !L.contains(I))
// TODO: support additional instructions.
if (!isa<LoadInst>(I) && !isa<GetElementPtrInst>(I))
return {};
// Do not duplicate volatile and atomic loads.
if (auto *LI = dyn_cast<LoadInst>(I))
if (LI->isVolatile() || LI->isAtomic())
return {};
if (MemoryAccess *MA = MSSA.getMemoryAccess(I)) {
if (auto *MemUse = dyn_cast_or_null<MemoryUse>(MA)) {
// Queue the defining access to check for alias checks.
} else {
// MemoryDefs may clobber the location or may be atomic memory
// operations. Bail out.
return {};
WorkList.append(I->op_begin(), I->op_end());
if (InstToDuplicate.empty())
return {};
SmallVector<BasicBlock *, 4> ExitingBlocks;
auto HasNoClobbersOnPath =
[&L, &AA, &AccessedLocs, &ExitingBlocks, &InstToDuplicate,
MSSAThreshold](BasicBlock *Succ, BasicBlock *Header,
SmallVector<MemoryAccess *, 4> AccessesToCheck)
-> Optional<IVConditionInfo> {
IVConditionInfo Info;
// First, collect all blocks in the loop that are on a patch from Succ
// to the header.
SmallVector<BasicBlock *, 4> WorkList;
SmallPtrSet<BasicBlock *, 4> Seen;
Info.PathIsNoop &=
all_of(*Header, [](Instruction &I) { return !I.mayHaveSideEffects(); });
while (!WorkList.empty()) {
BasicBlock *Current = WorkList.pop_back_val();
if (!L.contains(Current))
const auto &SeenIns = Seen.insert(Current);
if (!SeenIns.second)
Info.PathIsNoop &= all_of(
*Current, [](Instruction &I) { return !I.mayHaveSideEffects(); });
WorkList.append(succ_begin(Current), succ_end(Current));
// Require at least 2 blocks on a path through the loop. This skips
// paths that directly exit the loop.
if (Seen.size() < 2)
return {};
// Next, check if there are any MemoryDefs that are on the path through
// the loop (in the Seen set) and they may-alias any of the locations in
// AccessedLocs. If that is the case, they may modify the condition and
// partial unswitching is not possible.
SmallPtrSet<MemoryAccess *, 4> SeenAccesses;
while (!AccessesToCheck.empty()) {
MemoryAccess *Current = AccessesToCheck.pop_back_val();
auto SeenI = SeenAccesses.insert(Current);
if (!SeenI.second || !Seen.contains(Current->getBlock()))
// Bail out if exceeded the threshold.
if (SeenAccesses.size() >= MSSAThreshold)
return {};
// MemoryUse are read-only accesses.
if (isa<MemoryUse>(Current))
// For a MemoryDef, check if is aliases any of the location feeding
// the original condition.
if (auto *CurrentDef = dyn_cast<MemoryDef>(Current)) {
if (any_of(AccessedLocs, [&AA, CurrentDef](MemoryLocation &Loc) {
return isModSet(
AA.getModRefInfo(CurrentDef->getMemoryInst(), Loc));
return {};
for (Use &U : Current->uses())
// We could also allow loops with known trip counts without mustprogress,
// but ScalarEvolution may not be available.
Info.PathIsNoop &=
L.getHeader()->getParent()->mustProgress() || hasMustProgress(&L);
// If the path is considered a no-op so far, check if it reaches a
// single exit block without any phis. This ensures no values from the
// loop are used outside of the loop.
if (Info.PathIsNoop) {
for (auto *Exiting : ExitingBlocks) {
if (!Seen.contains(Exiting))
for (auto *Succ : successors(Exiting)) {
if (L.contains(Succ))
Info.PathIsNoop &= llvm::empty(Succ->phis()) &&
(!Info.ExitForPath || Info.ExitForPath == Succ);
if (!Info.PathIsNoop)
assert((!Info.ExitForPath || Info.ExitForPath == Succ) &&
"cannot have multiple exit blocks");
Info.ExitForPath = Succ;
if (!Info.ExitForPath)
Info.PathIsNoop = false;
Info.InstToDuplicate = InstToDuplicate;
return Info;
// If we branch to the same successor, partial unswitching will not be
// beneficial.
if (TI->getSuccessor(0) == TI->getSuccessor(1))
return {};
if (auto Info = HasNoClobbersOnPath(TI->getSuccessor(0), L.getHeader(),
AccessesToCheck)) {
Info->KnownValue = ConstantInt::getTrue(TI->getContext());
return Info;
if (auto Info = HasNoClobbersOnPath(TI->getSuccessor(1), L.getHeader(),
AccessesToCheck)) {
Info->KnownValue = ConstantInt::getFalse(TI->getContext());
return Info;
return {};