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//===- LoopInfo.cpp - Natural Loop Calculator -----------------------------===//
// The LLVM Compiler Infrastructure
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
// This file defines the LoopInfo class that is used to identify natural loops
// and determine the loop depth of various nodes of the CFG. Note that the
// loops identified may actually be several natural loops that share the same
// header node... not just a single natural loop.
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/ADT/DepthFirstIterator.h"
#include "llvm/ADT/ScopeExit.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/Analysis/LoopInfoImpl.h"
#include "llvm/Analysis/LoopIterator.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/Config/llvm-config.h"
#include "llvm/IR/CFG.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DebugLoc.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/Metadata.h"
#include "llvm/IR/PassManager.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include <algorithm>
using namespace llvm;
// Explicitly instantiate methods in LoopInfoImpl.h for IR-level Loops.
template class llvm::LoopBase<BasicBlock, Loop>;
template class llvm::LoopInfoBase<BasicBlock, Loop>;
// Always verify loopinfo if expensive checking is enabled.
bool llvm::VerifyLoopInfo = true;
bool llvm::VerifyLoopInfo = false;
static cl::opt<bool, true>
VerifyLoopInfoX("verify-loop-info", cl::location(VerifyLoopInfo),
cl::Hidden, cl::desc("Verify loop info (time consuming)"));
// Loop implementation
bool Loop::isLoopInvariant(const Value *V) const {
if (const Instruction *I = dyn_cast<Instruction>(V))
return !contains(I);
return true; // All non-instructions are loop invariant
bool Loop::hasLoopInvariantOperands(const Instruction *I) const {
return all_of(I->operands(), [this](Value *V) { return isLoopInvariant(V); });
bool Loop::makeLoopInvariant(Value *V, bool &Changed,
Instruction *InsertPt) const {
if (Instruction *I = dyn_cast<Instruction>(V))
return makeLoopInvariant(I, Changed, InsertPt);
return true; // All non-instructions are loop-invariant.
bool Loop::makeLoopInvariant(Instruction *I, bool &Changed,
Instruction *InsertPt) const {
// Test if the value is already loop-invariant.
if (isLoopInvariant(I))
return true;
if (!isSafeToSpeculativelyExecute(I))
return false;
if (I->mayReadFromMemory())
return false;
// EH block instructions are immobile.
if (I->isEHPad())
return false;
// Determine the insertion point, unless one was given.
if (!InsertPt) {
BasicBlock *Preheader = getLoopPreheader();
// Without a preheader, hoisting is not feasible.
if (!Preheader)
return false;
InsertPt = Preheader->getTerminator();
// Don't hoist instructions with loop-variant operands.
for (Value *Operand : I->operands())
if (!makeLoopInvariant(Operand, Changed, InsertPt))
return false;
// Hoist.
// There is possibility of hoisting this instruction above some arbitrary
// condition. Any metadata defined on it can be control dependent on this
// condition. Conservatively strip it here so that we don't give any wrong
// information to the optimizer.
Changed = true;
return true;
PHINode *Loop::getCanonicalInductionVariable() const {
BasicBlock *H = getHeader();
BasicBlock *Incoming = nullptr, *Backedge = nullptr;
pred_iterator PI = pred_begin(H);
assert(PI != pred_end(H) && "Loop must have at least one backedge!");
Backedge = *PI++;
if (PI == pred_end(H))
return nullptr; // dead loop
Incoming = *PI++;
if (PI != pred_end(H))
return nullptr; // multiple backedges?
if (contains(Incoming)) {
if (contains(Backedge))
return nullptr;
std::swap(Incoming, Backedge);
} else if (!contains(Backedge))
return nullptr;
// Loop over all of the PHI nodes, looking for a canonical indvar.
for (BasicBlock::iterator I = H->begin(); isa<PHINode>(I); ++I) {
PHINode *PN = cast<PHINode>(I);
if (ConstantInt *CI =
if (CI->isZero())
if (Instruction *Inc =
if (Inc->getOpcode() == Instruction::Add && Inc->getOperand(0) == PN)
if (ConstantInt *CI = dyn_cast<ConstantInt>(Inc->getOperand(1)))
if (CI->isOne())
return PN;
return nullptr;
// Check that 'BB' doesn't have any uses outside of the 'L'
static bool isBlockInLCSSAForm(const Loop &L, const BasicBlock &BB,
DominatorTree &DT) {
for (const Instruction &I : BB) {
// Tokens can't be used in PHI nodes and live-out tokens prevent loop
// optimizations, so for the purposes of considered LCSSA form, we
// can ignore them.
if (I.getType()->isTokenTy())
for (const Use &U : I.uses()) {
const Instruction *UI = cast<Instruction>(U.getUser());
const BasicBlock *UserBB = UI->getParent();
if (const PHINode *P = dyn_cast<PHINode>(UI))
UserBB = P->getIncomingBlock(U);
// Check the current block, as a fast-path, before checking whether
// the use is anywhere in the loop. Most values are used in the same
// block they are defined in. Also, blocks not reachable from the
// entry are special; uses in them don't need to go through PHIs.
if (UserBB != &BB && !L.contains(UserBB) &&
return false;
return true;
bool Loop::isLCSSAForm(DominatorTree &DT) const {
// For each block we check that it doesn't have any uses outside of this loop.
return all_of(this->blocks(), [&](const BasicBlock *BB) {
return isBlockInLCSSAForm(*this, *BB, DT);
bool Loop::isRecursivelyLCSSAForm(DominatorTree &DT, const LoopInfo &LI) const {
// For each block we check that it doesn't have any uses outside of its
// innermost loop. This process will transitively guarantee that the current
// loop and all of the nested loops are in LCSSA form.
return all_of(this->blocks(), [&](const BasicBlock *BB) {
return isBlockInLCSSAForm(*LI.getLoopFor(BB), *BB, DT);
bool Loop::isLoopSimplifyForm() const {
// Normal-form loops have a preheader, a single backedge, and all of their
// exits have all their predecessors inside the loop.
return getLoopPreheader() && getLoopLatch() && hasDedicatedExits();
// Routines that reform the loop CFG and split edges often fail on indirectbr.
bool Loop::isSafeToClone() const {
// Return false if any loop blocks contain indirectbrs, or there are any calls
// to noduplicate functions.
for (BasicBlock *BB : this->blocks()) {
if (isa<IndirectBrInst>(BB->getTerminator()))
return false;
for (Instruction &I : *BB)
if (auto CS = CallSite(&I))
if (CS.cannotDuplicate())
return false;
return true;
MDNode *Loop::getLoopID() const {
MDNode *LoopID = nullptr;
if (BasicBlock *Latch = getLoopLatch()) {
LoopID = Latch->getTerminator()->getMetadata(LLVMContext::MD_loop);
} else {
assert(!getLoopLatch() &&
"The loop should have no single latch at this point");
// Go through each predecessor of the loop header and check the
// terminator for the metadata.
BasicBlock *H = getHeader();
for (BasicBlock *BB : this->blocks()) {
TerminatorInst *TI = BB->getTerminator();
MDNode *MD = nullptr;
// Check if this terminator branches to the loop header.
for (BasicBlock *Successor : TI->successors()) {
if (Successor == H) {
MD = TI->getMetadata(LLVMContext::MD_loop);
if (!MD)
return nullptr;
if (!LoopID)
LoopID = MD;
else if (MD != LoopID)
return nullptr;
if (!LoopID || LoopID->getNumOperands() == 0 ||
LoopID->getOperand(0) != LoopID)
return nullptr;
return LoopID;
void Loop::setLoopID(MDNode *LoopID) const {
assert(LoopID && "Loop ID should not be null");
assert(LoopID->getNumOperands() > 0 && "Loop ID needs at least one operand");
assert(LoopID->getOperand(0) == LoopID && "Loop ID should refer to itself");
if (BasicBlock *Latch = getLoopLatch()) {
Latch->getTerminator()->setMetadata(LLVMContext::MD_loop, LoopID);
assert(!getLoopLatch() &&
"The loop should have no single latch at this point");
BasicBlock *H = getHeader();
for (BasicBlock *BB : this->blocks()) {
TerminatorInst *TI = BB->getTerminator();
for (BasicBlock *Successor : TI->successors()) {
if (Successor == H)
TI->setMetadata(LLVMContext::MD_loop, LoopID);
void Loop::setLoopAlreadyUnrolled() {
MDNode *LoopID = getLoopID();
// First remove any existing loop unrolling metadata.
SmallVector<Metadata *, 4> MDs;
// Reserve first location for self reference to the LoopID metadata node.
if (LoopID) {
for (unsigned i = 1, ie = LoopID->getNumOperands(); i < ie; ++i) {
bool IsUnrollMetadata = false;
MDNode *MD = dyn_cast<MDNode>(LoopID->getOperand(i));
if (MD) {
const MDString *S = dyn_cast<MDString>(MD->getOperand(0));
IsUnrollMetadata = S && S->getString().startswith("llvm.loop.unroll.");
if (!IsUnrollMetadata)
// Add unroll(disable) metadata to disable future unrolling.
LLVMContext &Context = getHeader()->getContext();
SmallVector<Metadata *, 1> DisableOperands;
DisableOperands.push_back(MDString::get(Context, "llvm.loop.unroll.disable"));
MDNode *DisableNode = MDNode::get(Context, DisableOperands);
MDNode *NewLoopID = MDNode::get(Context, MDs);
// Set operand 0 to refer to the loop id itself.
NewLoopID->replaceOperandWith(0, NewLoopID);
bool Loop::isAnnotatedParallel() const {
MDNode *DesiredLoopIdMetadata = getLoopID();
if (!DesiredLoopIdMetadata)
return false;
// The loop branch contains the parallel loop metadata. In order to ensure
// that any parallel-loop-unaware optimization pass hasn't added loop-carried
// dependencies (thus converted the loop back to a sequential loop), check
// that all the memory instructions in the loop contain parallelism metadata
// that point to the same unique "loop id metadata" the loop branch does.
for (BasicBlock *BB : this->blocks()) {
for (Instruction &I : *BB) {
if (!I.mayReadOrWriteMemory())
// The memory instruction can refer to the loop identifier metadata
// directly or indirectly through another list metadata (in case of
// nested parallel loops). The loop identifier metadata refers to
// itself so we can check both cases with the same routine.
MDNode *LoopIdMD =
if (!LoopIdMD)
return false;
bool LoopIdMDFound = false;
for (const MDOperand &MDOp : LoopIdMD->operands()) {
if (MDOp == DesiredLoopIdMetadata) {
LoopIdMDFound = true;
if (!LoopIdMDFound)
return false;
return true;
DebugLoc Loop::getStartLoc() const { return getLocRange().getStart(); }
Loop::LocRange Loop::getLocRange() const {
// If we have a debug location in the loop ID, then use it.
if (MDNode *LoopID = getLoopID()) {
DebugLoc Start;
// We use the first DebugLoc in the header as the start location of the loop
// and if there is a second DebugLoc in the header we use it as end location
// of the loop.
for (unsigned i = 1, ie = LoopID->getNumOperands(); i < ie; ++i) {
if (DILocation *L = dyn_cast<DILocation>(LoopID->getOperand(i))) {
if (!Start)
Start = DebugLoc(L);
return LocRange(Start, DebugLoc(L));
if (Start)
return LocRange(Start);
// Try the pre-header first.
if (BasicBlock *PHeadBB = getLoopPreheader())
if (DebugLoc DL = PHeadBB->getTerminator()->getDebugLoc())
return LocRange(DL);
// If we have no pre-header or there are no instructions with debug
// info in it, try the header.
if (BasicBlock *HeadBB = getHeader())
return LocRange(HeadBB->getTerminator()->getDebugLoc());
return LocRange();
#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)
LLVM_DUMP_METHOD void Loop::dump() const { print(dbgs()); }
LLVM_DUMP_METHOD void Loop::dumpVerbose() const {
print(dbgs(), /*Depth=*/0, /*Verbose=*/true);
// UnloopUpdater implementation
namespace {
/// Find the new parent loop for all blocks within the "unloop" whose last
/// backedges has just been removed.
class UnloopUpdater {
Loop &Unloop;
LoopInfo *LI;
LoopBlocksDFS DFS;
// Map unloop's immediate subloops to their nearest reachable parents. Nested
// loops within these subloops will not change parents. However, an immediate
// subloop's new parent will be the nearest loop reachable from either its own
// exits *or* any of its nested loop's exits.
DenseMap<Loop *, Loop *> SubloopParents;
// Flag the presence of an irreducible backedge whose destination is a block
// directly contained by the original unloop.
bool FoundIB;
UnloopUpdater(Loop *UL, LoopInfo *LInfo)
: Unloop(*UL), LI(LInfo), DFS(UL), FoundIB(false) {}
void updateBlockParents();
void removeBlocksFromAncestors();
void updateSubloopParents();
Loop *getNearestLoop(BasicBlock *BB, Loop *BBLoop);
} // end anonymous namespace
/// Update the parent loop for all blocks that are directly contained within the
/// original "unloop".
void UnloopUpdater::updateBlockParents() {
if (Unloop.getNumBlocks()) {
// Perform a post order CFG traversal of all blocks within this loop,
// propagating the nearest loop from successors to predecessors.
LoopBlocksTraversal Traversal(DFS, LI);
for (BasicBlock *POI : Traversal) {
Loop *L = LI->getLoopFor(POI);
Loop *NL = getNearestLoop(POI, L);
if (NL != L) {
// For reducible loops, NL is now an ancestor of Unloop.
assert((NL != &Unloop && (!NL || NL->contains(&Unloop))) &&
"uninitialized successor");
LI->changeLoopFor(POI, NL);
} else {
// Or the current block is part of a subloop, in which case its parent
// is unchanged.
assert((FoundIB || Unloop.contains(L)) && "uninitialized successor");
// Each irreducible loop within the unloop induces a round of iteration using
// the DFS result cached by Traversal.
bool Changed = FoundIB;
for (unsigned NIters = 0; Changed; ++NIters) {
assert(NIters < Unloop.getNumBlocks() && "runaway iterative algorithm");
// Iterate over the postorder list of blocks, propagating the nearest loop
// from successors to predecessors as before.
Changed = false;
for (LoopBlocksDFS::POIterator POI = DFS.beginPostorder(),
POE = DFS.endPostorder();
POI != POE; ++POI) {
Loop *L = LI->getLoopFor(*POI);
Loop *NL = getNearestLoop(*POI, L);
if (NL != L) {
assert(NL != &Unloop && (!NL || NL->contains(&Unloop)) &&
"uninitialized successor");
LI->changeLoopFor(*POI, NL);
Changed = true;
/// Remove unloop's blocks from all ancestors below their new parents.
void UnloopUpdater::removeBlocksFromAncestors() {
// Remove all unloop's blocks (including those in nested subloops) from
// ancestors below the new parent loop.
for (Loop::block_iterator BI = Unloop.block_begin(), BE = Unloop.block_end();
BI != BE; ++BI) {
Loop *OuterParent = LI->getLoopFor(*BI);
if (Unloop.contains(OuterParent)) {
while (OuterParent->getParentLoop() != &Unloop)
OuterParent = OuterParent->getParentLoop();
OuterParent = SubloopParents[OuterParent];
// Remove blocks from former Ancestors except Unloop itself which will be
// deleted.
for (Loop *OldParent = Unloop.getParentLoop(); OldParent != OuterParent;
OldParent = OldParent->getParentLoop()) {
assert(OldParent && "new loop is not an ancestor of the original");
/// Update the parent loop for all subloops directly nested within unloop.
void UnloopUpdater::updateSubloopParents() {
while (!Unloop.empty()) {
Loop *Subloop = *std::prev(Unloop.end());
assert(SubloopParents.count(Subloop) && "DFS failed to visit subloop");
if (Loop *Parent = SubloopParents[Subloop])
/// Return the nearest parent loop among this block's successors. If a successor
/// is a subloop header, consider its parent to be the nearest parent of the
/// subloop's exits.
/// For subloop blocks, simply update SubloopParents and return NULL.
Loop *UnloopUpdater::getNearestLoop(BasicBlock *BB, Loop *BBLoop) {
// Initially for blocks directly contained by Unloop, NearLoop == Unloop and
// is considered uninitialized.
Loop *NearLoop = BBLoop;
Loop *Subloop = nullptr;
if (NearLoop != &Unloop && Unloop.contains(NearLoop)) {
Subloop = NearLoop;
// Find the subloop ancestor that is directly contained within Unloop.
while (Subloop->getParentLoop() != &Unloop) {
Subloop = Subloop->getParentLoop();
assert(Subloop && "subloop is not an ancestor of the original loop");
// Get the current nearest parent of the Subloop exits, initially Unloop.
NearLoop = SubloopParents.insert({Subloop, &Unloop}).first->second;
succ_iterator I = succ_begin(BB), E = succ_end(BB);
if (I == E) {
assert(!Subloop && "subloop blocks must have a successor");
NearLoop = nullptr; // unloop blocks may now exit the function.
for (; I != E; ++I) {
if (*I == BB)
continue; // self loops are uninteresting
Loop *L = LI->getLoopFor(*I);
if (L == &Unloop) {
// This successor has not been processed. This path must lead to an
// irreducible backedge.
assert((FoundIB || !DFS.hasPostorder(*I)) && "should have seen IB");
FoundIB = true;
if (L != &Unloop && Unloop.contains(L)) {
// Successor is in a subloop.
if (Subloop)
continue; // Branching within subloops. Ignore it.
// BB branches from the original into a subloop header.
assert(L->getParentLoop() == &Unloop && "cannot skip into nested loops");
// Get the current nearest parent of the Subloop's exits.
L = SubloopParents[L];
// L could be Unloop if the only exit was an irreducible backedge.
if (L == &Unloop) {
// Handle critical edges from Unloop into a sibling loop.
if (L && !L->contains(&Unloop)) {
L = L->getParentLoop();
// Remember the nearest parent loop among successors or subloop exits.
if (NearLoop == &Unloop || !NearLoop || NearLoop->contains(L))
NearLoop = L;
if (Subloop) {
SubloopParents[Subloop] = NearLoop;
return BBLoop;
return NearLoop;
LoopInfo::LoopInfo(const DomTreeBase<BasicBlock> &DomTree) { analyze(DomTree); }
bool LoopInfo::invalidate(Function &F, const PreservedAnalyses &PA,
FunctionAnalysisManager::Invalidator &) {
// Check whether the analysis, all analyses on functions, or the function's
// CFG have been preserved.
auto PAC = PA.getChecker<LoopAnalysis>();
return !(PAC.preserved() || PAC.preservedSet<AllAnalysesOn<Function>>() ||
void LoopInfo::erase(Loop *Unloop) {
assert(!Unloop->isInvalid() && "Loop has already been erased!");
auto InvalidateOnExit = make_scope_exit([&]() { destroy(Unloop); });
// First handle the special case of no parent loop to simplify the algorithm.
if (!Unloop->getParentLoop()) {
// Since BBLoop had no parent, Unloop blocks are no longer in a loop.
for (Loop::block_iterator I = Unloop->block_begin(),
E = Unloop->block_end();
I != E; ++I) {
// Don't reparent blocks in subloops.
if (getLoopFor(*I) != Unloop)
// Blocks no longer have a parent but are still referenced by Unloop until
// the Unloop object is deleted.
changeLoopFor(*I, nullptr);
// Remove the loop from the top-level LoopInfo object.
for (iterator I = begin();; ++I) {
assert(I != end() && "Couldn't find loop");
if (*I == Unloop) {
// Move all of the subloops to the top-level.
while (!Unloop->empty())
// Update the parent loop for all blocks within the loop. Blocks within
// subloops will not change parents.
UnloopUpdater Updater(Unloop, this);
// Remove blocks from former ancestor loops.
// Add direct subloops as children in their new parent loop.
// Remove unloop from its parent loop.
Loop *ParentLoop = Unloop->getParentLoop();
for (Loop::iterator I = ParentLoop->begin();; ++I) {
assert(I != ParentLoop->end() && "Couldn't find loop");
if (*I == Unloop) {
AnalysisKey LoopAnalysis::Key;
LoopInfo LoopAnalysis::run(Function &F, FunctionAnalysisManager &AM) {
// FIXME: Currently we create a LoopInfo from scratch for every function.
// This may prove to be too wasteful due to deallocating and re-allocating
// memory each time for the underlying map and vector datastructures. At some
// point it may prove worthwhile to use a freelist and recycle LoopInfo
// objects. I don't want to add that kind of complexity until the scope of
// the problem is better understood.
LoopInfo LI;
return LI;
PreservedAnalyses LoopPrinterPass::run(Function &F,
FunctionAnalysisManager &AM) {
return PreservedAnalyses::all();
void llvm::printLoop(Loop &L, raw_ostream &OS, const std::string &Banner) {
if (forcePrintModuleIR()) {
// handling -print-module-scope
OS << Banner << " (loop: ";
L.getHeader()->printAsOperand(OS, false);
OS << ")\n";
// printing whole module
OS << *L.getHeader()->getModule();
OS << Banner;
auto *PreHeader = L.getLoopPreheader();
if (PreHeader) {
OS << "\n; Preheader:";
OS << "\n; Loop:";
for (auto *Block : L.blocks())
if (Block)
OS << "Printing <null> block";
SmallVector<BasicBlock *, 8> ExitBlocks;
if (!ExitBlocks.empty()) {
OS << "\n; Exit blocks";
for (auto *Block : ExitBlocks)
if (Block)
OS << "Printing <null> block";
// LoopInfo implementation
char LoopInfoWrapperPass::ID = 0;
INITIALIZE_PASS_BEGIN(LoopInfoWrapperPass, "loops", "Natural Loop Information",
true, true)
INITIALIZE_PASS_END(LoopInfoWrapperPass, "loops", "Natural Loop Information",
true, true)
bool LoopInfoWrapperPass::runOnFunction(Function &) {
return false;
void LoopInfoWrapperPass::verifyAnalysis() const {
// LoopInfoWrapperPass is a FunctionPass, but verifying every loop in the
// function each time verifyAnalysis is called is very expensive. The
// -verify-loop-info option can enable this. In order to perform some
// checking by default, LoopPass has been taught to call verifyLoop manually
// during loop pass sequences.
if (VerifyLoopInfo) {
auto &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
void LoopInfoWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const {
void LoopInfoWrapperPass::print(raw_ostream &OS, const Module *) const {
PreservedAnalyses LoopVerifierPass::run(Function &F,
FunctionAnalysisManager &AM) {
LoopInfo &LI = AM.getResult<LoopAnalysis>(F);
auto &DT = AM.getResult<DominatorTreeAnalysis>(F);
return PreservedAnalyses::all();
// LoopBlocksDFS implementation
/// Traverse the loop blocks and store the DFS result.
/// Useful for clients that just want the final DFS result and don't need to
/// visit blocks during the initial traversal.
void LoopBlocksDFS::perform(LoopInfo *LI) {
LoopBlocksTraversal Traversal(*this, LI);
for (LoopBlocksTraversal::POTIterator POI = Traversal.begin(),
POE = Traversal.end();
POI != POE; ++POI)