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//===- LoopDeletion.cpp - Dead Loop Deletion Pass ---------------===//
// The LLVM Compiler Infrastructure
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
// This file implements the Dead Loop Deletion Pass. This pass is responsible
// for eliminating loops with non-infinite computable trip counts that have no
// side effects or volatile instructions, and do not contribute to the
// computation of the function's return value.
#include "llvm/Transforms/Scalar/LoopDeletion.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/GlobalsModRef.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/PatternMatch.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Scalar/LoopPassManager.h"
#include "llvm/Transforms/Utils/LoopUtils.h"
using namespace llvm;
#define DEBUG_TYPE "loop-delete"
STATISTIC(NumDeleted, "Number of loops deleted");
enum class LoopDeletionResult {
/// Determines if a loop is dead.
/// This assumes that we've already checked for unique exit and exiting blocks,
/// and that the code is in LCSSA form.
static bool isLoopDead(Loop *L, ScalarEvolution &SE,
SmallVectorImpl<BasicBlock *> &ExitingBlocks,
BasicBlock *ExitBlock, bool &Changed,
BasicBlock *Preheader) {
// Make sure that all PHI entries coming from the loop are loop invariant.
// Because the code is in LCSSA form, any values used outside of the loop
// must pass through a PHI in the exit block, meaning that this check is
// sufficient to guarantee that no loop-variant values are used outside
// of the loop.
bool AllEntriesInvariant = true;
bool AllOutgoingValuesSame = true;
for (PHINode &P : ExitBlock->phis()) {
Value *incoming = P.getIncomingValueForBlock(ExitingBlocks[0]);
// Make sure all exiting blocks produce the same incoming value for the exit
// block. If there are different incoming values for different exiting
// blocks, then it is impossible to statically determine which value should
// be used.
AllOutgoingValuesSame =
all_of(makeArrayRef(ExitingBlocks).slice(1), [&](BasicBlock *BB) {
return incoming == P.getIncomingValueForBlock(BB);
if (!AllOutgoingValuesSame)
if (Instruction *I = dyn_cast<Instruction>(incoming))
if (!L->makeLoopInvariant(I, Changed, Preheader->getTerminator())) {
AllEntriesInvariant = false;
if (Changed)
if (!AllEntriesInvariant || !AllOutgoingValuesSame)
return false;
// Make sure that no instructions in the block have potential side-effects.
// This includes instructions that could write to memory, and loads that are
// marked volatile.
for (auto &I : L->blocks())
if (any_of(*I, [](Instruction &I) { return I.mayHaveSideEffects(); }))
return false;
return true;
/// This function returns true if there is no viable path from the
/// entry block to the header of \p L. Right now, it only does
/// a local search to save compile time.
static bool isLoopNeverExecuted(Loop *L) {
using namespace PatternMatch;
auto *Preheader = L->getLoopPreheader();
// TODO: We can relax this constraint, since we just need a loop
// predecessor.
assert(Preheader && "Needs preheader!");
if (Preheader == &Preheader->getParent()->getEntryBlock())
return false;
// All predecessors of the preheader should have a constant conditional
// branch, with the loop's preheader as not-taken.
for (auto *Pred: predecessors(Preheader)) {
BasicBlock *Taken, *NotTaken;
ConstantInt *Cond;
if (!match(Pred->getTerminator(),
m_Br(m_ConstantInt(Cond), Taken, NotTaken)))
return false;
if (!Cond->getZExtValue())
std::swap(Taken, NotTaken);
if (Taken == Preheader)
return false;
assert(!pred_empty(Preheader) &&
"Preheader should have predecessors at this point!");
// All the predecessors have the loop preheader as not-taken target.
return true;
/// Remove a loop if it is dead.
/// A loop is considered dead if it does not impact the observable behavior of
/// the program other than finite running time. This never removes a loop that
/// might be infinite (unless it is never executed), as doing so could change
/// the halting/non-halting nature of a program.
/// This entire process relies pretty heavily on LoopSimplify form and LCSSA in
/// order to make various safety checks work.
/// \returns true if any changes were made. This may mutate the loop even if it
/// is unable to delete it due to hoisting trivially loop invariant
/// instructions out of the loop.
static LoopDeletionResult deleteLoopIfDead(Loop *L, DominatorTree &DT,
ScalarEvolution &SE, LoopInfo &LI) {
assert(L->isLCSSAForm(DT) && "Expected LCSSA!");
// We can only remove the loop if there is a preheader that we can branch from
// after removing it. Also, if LoopSimplify form is not available, stay out
// of trouble.
BasicBlock *Preheader = L->getLoopPreheader();
if (!Preheader || !L->hasDedicatedExits()) {
<< "Deletion requires Loop with preheader and dedicated exits.\n");
return LoopDeletionResult::Unmodified;
// We can't remove loops that contain subloops. If the subloops were dead,
// they would already have been removed in earlier executions of this pass.
if (L->begin() != L->end()) {
DEBUG(dbgs() << "Loop contains subloops.\n");
return LoopDeletionResult::Unmodified;
BasicBlock *ExitBlock = L->getUniqueExitBlock();
if (ExitBlock && isLoopNeverExecuted(L)) {
DEBUG(dbgs() << "Loop is proven to never execute, delete it!");
// Set incoming value to undef for phi nodes in the exit block.
for (PHINode &P : ExitBlock->phis()) {
std::fill(P.incoming_values().begin(), P.incoming_values().end(),
deleteDeadLoop(L, &DT, &SE, &LI);
return LoopDeletionResult::Deleted;
// The remaining checks below are for a loop being dead because all statements
// in the loop are invariant.
SmallVector<BasicBlock *, 4> ExitingBlocks;
// We require that the loop only have a single exit block. Otherwise, we'd
// be in the situation of needing to be able to solve statically which exit
// block will be branched to, or trying to preserve the branching logic in
// a loop invariant manner.
if (!ExitBlock) {
DEBUG(dbgs() << "Deletion requires single exit block\n");
return LoopDeletionResult::Unmodified;
// Finally, we have to check that the loop really is dead.
bool Changed = false;
if (!isLoopDead(L, SE, ExitingBlocks, ExitBlock, Changed, Preheader)) {
DEBUG(dbgs() << "Loop is not invariant, cannot delete.\n");
return Changed ? LoopDeletionResult::Modified
: LoopDeletionResult::Unmodified;
// Don't remove loops for which we can't solve the trip count.
// They could be infinite, in which case we'd be changing program behavior.
const SCEV *S = SE.getMaxBackedgeTakenCount(L);
if (isa<SCEVCouldNotCompute>(S)) {
DEBUG(dbgs() << "Could not compute SCEV MaxBackedgeTakenCount.\n");
return Changed ? LoopDeletionResult::Modified
: LoopDeletionResult::Unmodified;
DEBUG(dbgs() << "Loop is invariant, delete it!");
deleteDeadLoop(L, &DT, &SE, &LI);
return LoopDeletionResult::Deleted;
PreservedAnalyses LoopDeletionPass::run(Loop &L, LoopAnalysisManager &AM,
LoopStandardAnalysisResults &AR,
LPMUpdater &Updater) {
DEBUG(dbgs() << "Analyzing Loop for deletion: ");
std::string LoopName = L.getName();
auto Result = deleteLoopIfDead(&L, AR.DT, AR.SE, AR.LI);
if (Result == LoopDeletionResult::Unmodified)
return PreservedAnalyses::all();
if (Result == LoopDeletionResult::Deleted)
Updater.markLoopAsDeleted(L, LoopName);
return getLoopPassPreservedAnalyses();
namespace {
class LoopDeletionLegacyPass : public LoopPass {
static char ID; // Pass ID, replacement for typeid
LoopDeletionLegacyPass() : LoopPass(ID) {
// Possibly eliminate loop L if it is dead.
bool runOnLoop(Loop *L, LPPassManager &) override;
void getAnalysisUsage(AnalysisUsage &AU) const override {
char LoopDeletionLegacyPass::ID = 0;
INITIALIZE_PASS_BEGIN(LoopDeletionLegacyPass, "loop-deletion",
"Delete dead loops", false, false)
INITIALIZE_PASS_END(LoopDeletionLegacyPass, "loop-deletion",
"Delete dead loops", false, false)
Pass *llvm::createLoopDeletionPass() { return new LoopDeletionLegacyPass(); }
bool LoopDeletionLegacyPass::runOnLoop(Loop *L, LPPassManager &LPM) {
if (skipLoop(L))
return false;
DominatorTree &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
ScalarEvolution &SE = getAnalysis<ScalarEvolutionWrapperPass>().getSE();
LoopInfo &LI = getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
DEBUG(dbgs() << "Analyzing Loop for deletion: ");
LoopDeletionResult Result = deleteLoopIfDead(L, DT, SE, LI);
if (Result == LoopDeletionResult::Deleted)
return Result != LoopDeletionResult::Unmodified;