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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/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/InstSimplifyFolder.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/ScalarEvolution.h"
#include "llvm/Analysis/ScalarEvolutionAliasAnalysis.h"
#include "llvm/Analysis/ScalarEvolutionExpressions.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/PatternMatch.h"
#include "llvm/IR/ProfDataUtils.h"
#include "llvm/IR/ValueHandle.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/Support/Debug.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";
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;
} 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() == 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);
llvm::getOptionalElementCountLoopAttribute(const Loop *TheLoop) {
std::optional<int> Width =
getOptionalIntLoopAttribute(TheLoop, "llvm.loop.vectorize.width");
if (Width) {
std::optional<int> IsScalable = getOptionalIntLoopAttribute(
TheLoop, "llvm.loop.vectorize.scalable.enable");
return ElementCount::get(*Width, IsScalable.value_or(false));
return std::nullopt;
std::optional<MDNode *> llvm::makeFollowupLoopID(
MDNode *OrigLoopID, ArrayRef<StringRef> FollowupOptions,
const char *InheritOptionsExceptPrefix, bool AlwaysNew) {
if (!OrigLoopID) {
if (AlwaysNew)
return nullptr;
return std::nullopt;
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.starts_with(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 std::nullopt;
// 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);
TransformationMode llvm::hasUnrollTransformation(const Loop *L) {
if (getBooleanLoopAttribute(L, "llvm.loop.unroll.disable"))
return TM_SuppressedByUser;
std::optional<int> Count =
getOptionalIntLoopAttribute(L, "llvm.loop.unroll.count");
if (Count)
return *Count == 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(const Loop *L) {
if (getBooleanLoopAttribute(L, "llvm.loop.unroll_and_jam.disable"))
return TM_SuppressedByUser;
std::optional<int> Count =
getOptionalIntLoopAttribute(L, "llvm.loop.unroll_and_jam.count");
if (Count)
return *Count == 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(const Loop *L) {
std::optional<bool> Enable =
getOptionalBoolLoopAttribute(L, "llvm.loop.vectorize.enable");
if (Enable == false)
return TM_SuppressedByUser;
std::optional<ElementCount> VectorizeWidth =
std::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(const Loop *L) {
if (getBooleanLoopAttribute(L, "llvm.loop.distribute.enable"))
return TM_ForcedByUser;
if (hasDisableAllTransformsHint(L))
return TM_Disable;
return TM_Unspecified;
TransformationMode llvm::hasLICMVersioningTransformation(const 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;
bool llvm::isAlmostDeadIV(PHINode *PN, BasicBlock *LatchBlock, Value *Cond) {
int LatchIdx = PN->getBasicBlockIndex(LatchBlock);
assert(LatchIdx != -1 && "LatchBlock is not a case in this PHINode");
Value *IncV = PN->getIncomingValue(LatchIdx);
for (User *U : PN->users())
if (U != Cond && U != IncV) return false;
for (User *U : IncV->users())
if (U != Cond && U != PN) return false;
return true;
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) {
Instruction *OldTerm = Preheader->getTerminator();
assert(!OldTerm->mayHaveSideEffects() &&
"Preheader must end with a side-effect-free terminator");
assert(OldTerm->getNumSuccessors() == 1 &&
"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(OldTerm);
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.
P.removeIncomingValueIf([](unsigned Idx) { return Idx != 0; },
/* DeletePHIIfEmpty */ 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<DebugVariable, 4> DeadDebugSet;
llvm::SmallVector<DbgVariableIntrinsic *, 4> DeadDebugInst;
llvm::SmallVector<DbgVariableRecord *, 4> DeadDbgVariableRecords;
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 poison value of corresponding type first.
for (auto *Block : L->blocks())
for (Instruction &I : *Block) {
auto *Poison = PoisonValue::get(I.getType());
for (Use &U : llvm::make_early_inc_range(I.uses())) {
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");
// RemoveDIs: do the same as below for DbgVariableRecords.
if (Block->IsNewDbgInfoFormat) {
for (DbgVariableRecord &DVR : llvm::make_early_inc_range(
filterDbgVars(I.getDbgRecordRange()))) {
DebugVariable Key(DVR.getVariable(), DVR.getExpression(),
if (!DeadDebugSet.insert(Key).second)
// Unlinks the DVR from it's container, for later insertion.
// For one of each variable encountered, preserve a debug intrinsic (set
// to Poison) and transfer it to the loop exit. This terminates any
// variable locations that were set during the loop.
auto *DVI = dyn_cast<DbgVariableIntrinsic>(&I);
if (!DVI)
if (!DeadDebugSet.insert(DebugVariable(DVI)).second)
// After the loop has been deleted all the values defined and modified
// inside the loop are going to be unavailable. Values computed in the
// loop will have been deleted, automatically causing their debug uses
// be be replaced with undef. Loop invariant values will still be available.
// Move dbg.values out the loop so that earlier location ranges are still
// terminated and loop invariant assignments are preserved.
DIBuilder DIB(*ExitBlock->getModule());
BasicBlock::iterator InsertDbgValueBefore =
assert(InsertDbgValueBefore != ExitBlock->end() &&
"There should be a non-PHI instruction in exit block, else these "
"instructions will have no parent.");
for (auto *DVI : DeadDebugInst)
DVI->moveBefore(*ExitBlock, InsertDbgValueBefore);
// Due to the "head" bit in BasicBlock::iterator, we're going to insert
// each DbgVariableRecord right at the start of the block, wheras dbg.values
// would be repeatedly inserted before the first instruction. To replicate
// this behaviour, do it backwards.
for (DbgVariableRecord *DVR : llvm::reverse(DeadDbgVariableRecords))
ExitBlock->insertDbgRecordBefore(DVR, 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 (BasicBlock *BB : L->blocks())
// 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");
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 = L->getOutermostLoop();
std::unique_ptr<MemorySSAUpdater> MSSAU;
if (MSSA)
MSSAU = std::make_unique<MemorySSAUpdater>(MSSA);
// Update the CFG and domtree. We chose to special case a couple of
// of common cases for code quality and test readability reasons.
[&]() -> void {
if (auto *BI = dyn_cast<BranchInst>(Latch->getTerminator())) {
if (!BI->isConditional()) {
DomTreeUpdater DTU(&DT, DomTreeUpdater::UpdateStrategy::Eager);
(void)changeToUnreachable(BI, /*PreserveLCSSA*/ true, &DTU,
// Conditional latch/exit - note that latch can be shared by inner
// and outer loop so the other target doesn't need to an exit
if (L->isLoopExiting(Latch)) {
// TODO: Generalize ConstantFoldTerminator so that it can be used
// here without invalidating LCSSA or MemorySSA. (Tricky case for
// LCSSA: header is an exit block of a preceeding sibling loop w/o
// dedicated exits.)
const unsigned ExitIdx = L->contains(BI->getSuccessor(0)) ? 1 : 0;
BasicBlock *ExitBB = BI->getSuccessor(ExitIdx);
DomTreeUpdater DTU(&DT, DomTreeUpdater::UpdateStrategy::Eager);
Header->removePredecessor(Latch, true);
IRBuilder<> Builder(BI);
auto *NewBI = Builder.CreateBr(ExitBB);
// Transfer the metadata to the new branch instruction (minus the
// loop info since this is no longer a loop)
NewBI->copyMetadata(*BI, {LLVMContext::MD_dbg,
DTU.applyUpdates({{DominatorTree::Delete, Latch, Header}});
if (MSSA)
MSSAU->applyUpdates({{DominatorTree::Delete, Latch, Header}}, DT);
// General case. By splitting the backedge, and then explicitly making it
// unreachable we gracefully handle corner cases such as switch and invoke
// termiantors.
auto *BackedgeBB = SplitEdge(Latch, Header, &DT, &LI, MSSAU.get());
DomTreeUpdater DTU(&DT, DomTreeUpdater::UpdateStrategy::Eager);
/*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 an exiting latch branch. There may also be other
/// exiting blocks. 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");
return LatchBR;
/// Return the estimated trip count for any exiting branch which dominates
/// the loop latch.
static std::optional<uint64_t> getEstimatedTripCount(BranchInst *ExitingBranch,
Loop *L,
uint64_t &OrigExitWeight) {
// 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 LoopWeight, ExitWeight;
if (!extractBranchWeights(*ExitingBranch, LoopWeight, ExitWeight))
return std::nullopt;
if (L->contains(ExitingBranch->getSuccessor(1)))
std::swap(LoopWeight, ExitWeight);
if (!ExitWeight)
// Don't have a way to return predicated infinite
return std::nullopt;
OrigExitWeight = ExitWeight;
// Estimated exit count is a ratio of the loop weight by the weight of the
// edge exiting the loop, rounded to nearest.
uint64_t ExitCount = llvm::divideNearest(LoopWeight, ExitWeight);
// Estimated trip count is one plus estimated exit count.
return ExitCount + 1;
llvm::getLoopEstimatedTripCount(Loop *L,
unsigned *EstimatedLoopInvocationWeight) {
// Currently we take the estimate exit count only from the loop latch,
// ignoring other exiting blocks. This can overestimate the trip count
// if we exit through another exit, but can never underestimate it.
// TODO: incorporate information from other exits
if (BranchInst *LatchBranch = getExpectedExitLoopLatchBranch(L)) {
uint64_t ExitWeight;
if (std::optional<uint64_t> EstTripCount =
getEstimatedTripCount(LatchBranch, L, ExitWeight)) {
if (EstimatedLoopInvocationWeight)
*EstimatedLoopInvocationWeight = ExitWeight;
return *EstTripCount;
return std::nullopt;
bool llvm::setLoopEstimatedTripCount(Loop *L, unsigned EstimatedTripCount,
unsigned EstimatedloopInvocationWeight) {
// At the moment, we currently support changing the estimate trip count of
// the latch branch only. We could extend this API to manipulate estimated
// trip counts for any exit.
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;
unsigned llvm::getArithmeticReductionInstruction(Intrinsic::ID RdxID) {
switch (RdxID) {
case Intrinsic::vector_reduce_fadd:
return Instruction::FAdd;
case Intrinsic::vector_reduce_fmul:
return Instruction::FMul;
case Intrinsic::vector_reduce_add:
return Instruction::Add;
case Intrinsic::vector_reduce_mul:
return Instruction::Mul;
case Intrinsic::vector_reduce_and:
return Instruction::And;
case Intrinsic::vector_reduce_or:
return Instruction::Or;
case Intrinsic::vector_reduce_xor:
return Instruction::Xor;
case Intrinsic::vector_reduce_smax:
case Intrinsic::vector_reduce_smin:
case Intrinsic::vector_reduce_umax:
case Intrinsic::vector_reduce_umin:
return Instruction::ICmp;
case Intrinsic::vector_reduce_fmax:
case Intrinsic::vector_reduce_fmin:
return Instruction::FCmp;
llvm_unreachable("Unexpected ID");
Intrinsic::ID llvm::getMinMaxReductionIntrinsicOp(Intrinsic::ID RdxID) {
switch (RdxID) {
llvm_unreachable("Unknown min/max recurrence kind");
case Intrinsic::vector_reduce_umin:
return Intrinsic::umin;
case Intrinsic::vector_reduce_umax:
return Intrinsic::umax;
case Intrinsic::vector_reduce_smin:
return Intrinsic::smin;
case Intrinsic::vector_reduce_smax:
return Intrinsic::smax;
case Intrinsic::vector_reduce_fmin:
return Intrinsic::minnum;
case Intrinsic::vector_reduce_fmax:
return Intrinsic::maxnum;
case Intrinsic::vector_reduce_fminimum:
return Intrinsic::minimum;
case Intrinsic::vector_reduce_fmaximum:
return Intrinsic::maximum;
Intrinsic::ID llvm::getMinMaxReductionIntrinsicOp(RecurKind RK) {
switch (RK) {
llvm_unreachable("Unknown min/max recurrence kind");
case RecurKind::UMin:
return Intrinsic::umin;
case RecurKind::UMax:
return Intrinsic::umax;
case RecurKind::SMin:
return Intrinsic::smin;
case RecurKind::SMax:
return Intrinsic::smax;
case RecurKind::FMin:
return Intrinsic::minnum;
case RecurKind::FMax:
return Intrinsic::maxnum;
case RecurKind::FMinimum:
return Intrinsic::minimum;
case RecurKind::FMaximum:
return Intrinsic::maximum;
RecurKind llvm::getMinMaxReductionRecurKind(Intrinsic::ID RdxID) {
switch (RdxID) {
case Intrinsic::vector_reduce_smax:
return RecurKind::SMax;
case Intrinsic::vector_reduce_smin:
return RecurKind::SMin;
case Intrinsic::vector_reduce_umax:
return RecurKind::UMax;
case Intrinsic::vector_reduce_umin:
return RecurKind::UMin;
case Intrinsic::vector_reduce_fmax:
return RecurKind::FMax;
case Intrinsic::vector_reduce_fmin:
return RecurKind::FMin;
return RecurKind::None;
CmpInst::Predicate llvm::getMinMaxReductionPredicate(RecurKind RK) {
switch (RK) {
llvm_unreachable("Unknown min/max recurrence kind");
case RecurKind::UMin:
return CmpInst::ICMP_ULT;
case RecurKind::UMax:
return CmpInst::ICMP_UGT;
case RecurKind::SMin:
return CmpInst::ICMP_SLT;
case RecurKind::SMax:
return CmpInst::ICMP_SGT;
case RecurKind::FMin:
return CmpInst::FCMP_OLT;
case RecurKind::FMax:
return CmpInst::FCMP_OGT;
// We do not add FMinimum/FMaximum recurrence kind here since there is no
// equivalent predicate which compares signed zeroes according to the
// semantics of the intrinsics (llvm.minimum/maximum).
Value *llvm::createMinMaxOp(IRBuilderBase &Builder, RecurKind RK, Value *Left,
Value *Right) {
Type *Ty = Left->getType();
if (Ty->isIntOrIntVectorTy() ||
(RK == RecurKind::FMinimum || RK == RecurKind::FMaximum)) {
// TODO: Add float minnum/maxnum support when FMF nnan is set.
Intrinsic::ID Id = getMinMaxReductionIntrinsicOp(RK);
return Builder.CreateIntrinsic(Ty, Id, {Left, Right}, nullptr,
CmpInst::Predicate Pred = getMinMaxReductionPredicate(RK);
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) {
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);
return Result;
// Helper to generate a log2 shuffle reduction.
Value *llvm::getShuffleReduction(IRBuilderBase &Builder, Value *Src,
unsigned Op, RecurKind RdxKind) {
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!");
// Note: fast-math-flags flags are controlled by the builder configuration
// and are assumed to apply to all generated arithmetic instructions. Other
// poison generating flags (nsw/nuw/inbounds/inrange/exact) are not part
// of the builder configuration, and since they're not passed explicitly,
// will never be relevant here. Note that it would be generally unsound to
// propagate these from an intrinsic call to the expansion anyways as we/
// change the order of operations.
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) {
TmpVec = Builder.CreateBinOp((Instruction::BinaryOps)Op, TmpVec, Shuf,
} else {
assert(RecurrenceDescriptor::isMinMaxRecurrenceKind(RdxKind) &&
"Invalid min/max");
TmpVec = createMinMaxOp(Builder, RdxKind, TmpVec, Shuf);
// The result is in the first element of the vector.
return Builder.CreateExtractElement(TmpVec, Builder.getInt32(0));
Value *llvm::createAnyOfTargetReduction(IRBuilderBase &Builder, Value *Src,
const RecurrenceDescriptor &Desc,
PHINode *OrigPhi) {
RecurrenceDescriptor::isAnyOfRecurrenceKind(Desc.getRecurrenceKind()) &&
"Unexpected reduction kind");
Value *InitVal = Desc.getRecurrenceStartValue();
Value *NewVal = nullptr;
// First use the original phi to determine the new value we're trying to
// select from in the loop.
SelectInst *SI = nullptr;
for (auto *U : OrigPhi->users()) {
if ((SI = dyn_cast<SelectInst>(U)))
assert(SI && "One user of the original phi should be a select");
if (SI->getTrueValue() == OrigPhi)
NewVal = SI->getFalseValue();
else {
assert(SI->getFalseValue() == OrigPhi &&
"At least one input to the select should be the original Phi");
NewVal = SI->getTrueValue();
// If any predicate is true it means that we want to select the new value.
Value *AnyOf =
Src->getType()->isVectorTy() ? Builder.CreateOrReduce(Src) : Src;
// The compares in the loop may yield poison, which propagates through the
// bitwise ORs. Freeze it here before the condition is used.
AnyOf = Builder.CreateFreeze(AnyOf);
return Builder.CreateSelect(AnyOf, NewVal, InitVal, "");
Value *llvm::createSimpleTargetReduction(IRBuilderBase &Builder, Value *Src,
RecurKind RdxKind) {
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::FMulAdd:
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);
case RecurKind::FMinimum:
return Builder.CreateFPMinimumReduce(Src);
case RecurKind::FMaximum:
return Builder.CreateFPMaximumReduce(Src);
llvm_unreachable("Unhandled opcode");
Value *llvm::createTargetReduction(IRBuilderBase &B,
const RecurrenceDescriptor &Desc, Value *Src,
PHINode *OrigPhi) {
// 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);
RecurKind RK = Desc.getRecurrenceKind();
if (RecurrenceDescriptor::isAnyOfRecurrenceKind(RK))
return createAnyOfTargetReduction(B, Src, Desc, OrigPhi);
return createSimpleTargetReduction(B, Src, RK);
Value *llvm::createOrderedReduction(IRBuilderBase &B,
const RecurrenceDescriptor &Desc,
Value *Src, Value *Start) {
assert((Desc.getRecurrenceKind() == RecurKind::FAdd ||
Desc.getRecurrenceKind() == RecurKind::FMulAdd) &&
"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,
bool IncludeWrapFlags) {
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();
VecOp->copyIRFlags(Intersection, IncludeWrapFlags);
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::isKnownPositiveInLoop(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_SGT, S, Zero);
bool llvm::isKnownNonPositiveInLoop(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_SLE, 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.
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 (const 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?
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) {
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;
/// Checks if it is safe to call InductionDescriptor::isInductionPHI for \p Phi,
/// and returns true if this Phi is an induction phi in the loop. When
/// isInductionPHI returns true, \p ID will be also be set by isInductionPHI.
static bool checkIsIndPhi(PHINode *Phi, Loop *L, ScalarEvolution *SE,
InductionDescriptor &ID) {
if (!Phi)
return false;
if (!L->getLoopPreheader())
return false;
if (Phi->getParent() != L->getHeader())
return false;
return InductionDescriptor::isInductionPHI(Phi, L, SE, ID);
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()))
// 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))
// Find exit values which are induction variables in the loop, and are
// unused in the loop, with the only use being the exit block PhiNode,
// and the induction variable update binary operator.
// The exit value can be replaced with the final value when it is cheap
// to do so.
if (ReplaceExitValue == UnusedIndVarInLoop) {
InductionDescriptor ID;
PHINode *IndPhi = dyn_cast<PHINode>(Inst);
if (IndPhi) {
if (!checkIsIndPhi(IndPhi, L, SE, ID))
// This is an induction PHI. Check that the only users are PHI
// nodes, and induction variable update binary operators.
if (llvm::any_of(Inst->users(), [&](User *U) {
if (!isa<PHINode>(U) && !isa<BinaryOperator>(U))
return true;
BinaryOperator *B = dyn_cast<BinaryOperator>(U);
if (B && B != ID.getInductionBinOp())
return true;
return false;
} else {
// If it is not an induction phi, it must be an induction update
// binary operator with an induction phi user.
BinaryOperator *B = dyn_cast<BinaryOperator>(Inst);
if (!B)
if (llvm::any_of(Inst->users(), [&](User *U) {
PHINode *Phi = dyn_cast<PHINode>(U);
if (Phi != PN && !checkIsIndPhi(Phi, L, SE, ID))
return true;
return false;
if (B != ID.getInductionBinOp())
// 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) ||
!Rewriter.isSafeToExpand(ExitValue)) {
// 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) ||
// 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 next SCEV's we'll query,
// and next SCEV may errneously get smaller cost.
// Collect all the candidate PHINodes to be rewritten.
Instruction *InsertPt =
(isa<PHINode>(Inst) || isa<LandingPadInst>(Inst)) ?
&*Inst->getParent()->getFirstInsertionPt() : Inst;
RewritePhiSet.emplace_back(PN, i, ExitValue, InsertPt, HighCost);
// TODO: 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) {
PHINode *PN = Phi.PN;
// Only do the rewrite when the ExitValue can be expanded cheaply.
// If LoopCanBeDel is true, rewrite exit value aggressively.
if ((ReplaceExitValue == OnlyCheapRepl ||
ReplaceExitValue == UnusedIndVarInLoop) &&
!LoopCanBeDel && Phi.HighCost)
Value *ExitVal = Rewriter.expandCodeFor(
Phi.ExpansionSCEV, Phi.PN->getType(), Phi.ExpansionPoint);
LLVM_DEBUG(dbgs() << "rewriteLoopExitValues: AfterLoopVal = " << *ExitVal
<< '\n'
<< " LoopVal = " << *(Phi.ExpansionPoint) << "\n");
#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>(ExitVal))
if (auto *EVL = LI->getLoopFor(ExitInsn->getParent()))
if (EVL != L)
assert(EVL->contains(L) && "LCSSA breach detected!");
Instruction *Inst = cast<Instruction>(PN->getIncomingValue(Phi.Ith));
PN->setIncomingValue(Phi.Ith, ExitVal);
// 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.
// 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;
std::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 (BasicBlock *BB : L->blocks())
if (LI->getLoopFor(BB) == L)
New.addBasicBlockToLoop(cast<BasicBlock>(VM[BB]), *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;
Value *StrideToCheck;
/// 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, bool HoistRuntimeChecks) {
LLVMContext &Ctx = Loc->getContext();
Type *PtrArithTy = PointerType::get(Ctx, CG->AddressSpace);
Value *Start = nullptr, *End = nullptr;
LLVM_DEBUG(dbgs() << "LAA: Adding RT check for range:\n");
const SCEV *Low = CG->Low, *High = CG->High, *Stride = nullptr;
// If the Low and High values are themselves loop-variant, then we may want
// to expand the range to include those covered by the outer loop as well.
// There is a trade-off here with the advantage being that creating checks
// using the expanded range permits the runtime memory checks to be hoisted
// out of the outer loop. This reduces the cost of entering the inner loop,
// which can be significant for low trip counts. The disadvantage is that
// there is a chance we may now never enter the vectorized inner loop,
// whereas using a restricted range check could have allowed us to enter at
// least once. This is why the behaviour is not currently the default and is
// controlled by the parameter 'HoistRuntimeChecks'.
if (HoistRuntimeChecks && TheLoop->getParentLoop() &&
isa<SCEVAddRecExpr>(High) && isa<SCEVAddRecExpr>(Low)) {
auto *HighAR = cast<SCEVAddRecExpr>(High);
auto *LowAR = cast<SCEVAddRecExpr>(Low);
const Loop *OuterLoop = TheLoop->getParentLoop();
const SCEV *Recur = LowAR->getStepRecurrence(*Exp.getSE());
if (Recur == HighAR->getStepRecurrence(*Exp.getSE()) &&
HighAR->getLoop() == OuterLoop && LowAR->getLoop() == OuterLoop) {
BasicBlock *OuterLoopLatch = OuterLoop->getLoopLatch();
const SCEV *OuterExitCount =
Exp.getSE()->getExitCount(OuterLoop, OuterLoopLatch);
if (!isa<SCEVCouldNotCompute>(OuterExitCount) &&
OuterExitCount->getType()->isIntegerTy()) {
const SCEV *NewHigh = cast<SCEVAddRecExpr>(High)->evaluateAtIteration(
OuterExitCount, *Exp.getSE());
if (!isa<SCEVCouldNotCompute>(NewHigh)) {
LLVM_DEBUG(dbgs() << "LAA: Expanded RT check for range to include "
"outer loop in order to permit hoisting\n");
High = NewHigh;
Low = cast<SCEVAddRecExpr>(Low)->getStart();
// If there is a possibility that the stride is negative then we have
// to generate extra checks to ensure the stride is positive.
if (!Exp.getSE()->isKnownNonNegative(Recur)) {
Stride = Recur;
LLVM_DEBUG(dbgs() << "LAA: ... but need to check stride is "
"positive: "
<< *Stride << '\n');
Start = Exp.expandCodeFor(Low, PtrArithTy, Loc);
End = Exp.expandCodeFor(High, PtrArithTy, Loc);
if (CG->NeedsFreeze) {
IRBuilder<> Builder(Loc);
Start = Builder.CreateFreeze(Start, Start->getName() + ".fr");
End = Builder.CreateFreeze(End, End->getName() + ".fr");
Value *StrideVal =
Stride ? Exp.expandCodeFor(Stride, Stride->getType(), Loc) : nullptr;
LLVM_DEBUG(dbgs() << "Start: " << *Low << " End: " << *High << "\n");
return {Start, End, StrideVal};
/// 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, bool HoistRuntimeChecks) {
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;
Value *llvm::addRuntimeChecks(
Instruction *Loc, Loop *TheLoop,
const SmallVectorImpl<RuntimePointerCheck> &PointerChecks,
SCEVExpander &Exp, bool HoistRuntimeChecks) {
// 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, HoistRuntimeChecks);
LLVMContext &Ctx = Loc->getContext();
IRBuilder<InstSimplifyFolder> ChkBuilder(Ctx,
// Our instructions might fold to a constant.
Value *MemoryRuntimeCheck = 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)
assert((A.Start->getType()->getPointerAddressSpace() ==
B.End->getType()->getPointerAddressSpace()) &&
(B.Start->getType()->getPointerAddressSpace() ==
A.End->getType()->getPointerAddressSpace()) &&
"Trying to bounds check pointers with different address spaces");
// [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(A.Start, B.End, "bound0");
Value *Cmp1 = ChkBuilder.CreateICmpULT(B.Start, A.End, "bound1");
Value *IsConflict = ChkBuilder.CreateAnd(Cmp0, Cmp1, "found.conflict");
if (A.StrideToCheck) {
Value *IsNegativeStride = ChkBuilder.CreateICmpSLT(
A.StrideToCheck, ConstantInt::get(A.StrideToCheck->getType(), 0),
IsConflict = ChkBuilder.CreateOr(IsConflict, IsNegativeStride);
if (B.StrideToCheck) {
Value *IsNegativeStride = ChkBuilder.CreateICmpSLT(
B.StrideToCheck, ConstantInt::get(B.StrideToCheck->getType(), 0),
IsConflict = ChkBuilder.CreateOr(IsConflict, IsNegativeStride);
if (MemoryRuntimeCheck) {
IsConflict =
ChkBuilder.CreateOr(MemoryRuntimeCheck, IsConflict, "conflict.rdx");
MemoryRuntimeCheck = IsConflict;
return MemoryRuntimeCheck;
Value *llvm::addDiffRuntimeChecks(
Instruction *Loc, ArrayRef<PointerDiffInfo> Checks, SCEVExpander &Expander,
function_ref<Value *(IRBuilderBase &, unsigned)> GetVF, unsigned IC) {
LLVMContext &Ctx = Loc->getContext();
IRBuilder<InstSimplifyFolder> ChkBuilder(Ctx,
// Our instructions might fold to a constant.
Value *MemoryRuntimeCheck = nullptr;
auto &SE = *Expander.getSE();
// Map to keep track of created compares, The key is the pair of operands for
// the compare, to allow detecting and re-using redundant compares.
DenseMap<std::pair<Value *, Value *>, Value *> SeenCompares;
for (const auto &C : Checks) {
Type *Ty = C.SinkStart->getType();
// Compute VF * IC * AccessSize.
auto *VFTimesUFTimesSize =
ChkBuilder.CreateMul(GetVF(ChkBuilder, Ty->getScalarSizeInBits()),
ConstantInt::get(Ty, IC * C.AccessSize));
Value *Diff = Expander.expandCodeFor(
SE.getMinusSCEV(C.SinkStart, C.SrcStart), Ty, Loc);
// Check if the same compare has already been created earlier. In that case,
// there is no need to check it again.
Value *IsConflict = SeenCompares.lookup({Diff, VFTimesUFTimesSize});
if (IsConflict)
IsConflict =
ChkBuilder.CreateICmpULT(Diff, VFTimesUFTimesSize, "diff.check");
SeenCompares.insert({{Diff, VFTimesUFTimesSize}, IsConflict});
if (C.NeedsFreeze)
IsConflict =
ChkBuilder.CreateFreeze(IsConflict, IsConflict->getName() + ".fr");
if (MemoryRuntimeCheck) {
IsConflict =
ChkBuilder.CreateOr(MemoryRuntimeCheck, IsConflict, "conflict.rdx");
MemoryRuntimeCheck = IsConflict;
return MemoryRuntimeCheck;
llvm::hasPartialIVCondition(const Loop &L, unsigned MSSAThreshold,
const MemorySSA &MSSA, AAResults &AA) {
auto *TI = dyn_cast<BranchInst>(L.getHeader()->getTerminator());
if (!TI || !TI->isConditional())
return {};
auto *CondI = dyn_cast<Instruction>(TI->getCondition());
// The case with the condition outside the loop should already be handled
// earlier.
// Allow CmpInst and TruncInsts as they may be users of load instructions
// and have potential for partial unswitching
if (!CondI || !isa<CmpInst, TruncInst>(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)
-> std::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 &= isMustProgress(&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 &= Succ->phis().empty() &&
(!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 {};