forked from OSchip/llvm-project
279 lines
10 KiB
C++
279 lines
10 KiB
C++
//===- LoopDeletion.cpp - Dead Loop Deletion Pass ---------------===//
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//
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// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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// See https://llvm.org/LICENSE.txt for license information.
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// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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//
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//===----------------------------------------------------------------------===//
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//
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// This file implements the Dead Loop Deletion Pass. This pass is responsible
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// for eliminating loops with non-infinite computable trip counts that have no
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// side effects or volatile instructions, and do not contribute to the
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// computation of the function's return value.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Transforms/Scalar/LoopDeletion.h"
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#include "llvm/ADT/SmallVector.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/Analysis/GlobalsModRef.h"
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#include "llvm/Analysis/LoopPass.h"
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#include "llvm/Analysis/MemorySSA.h"
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#include "llvm/IR/Dominators.h"
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#include "llvm/IR/PatternMatch.h"
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#include "llvm/InitializePasses.h"
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/Transforms/Scalar/LoopPassManager.h"
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#include "llvm/Transforms/Utils/LoopUtils.h"
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using namespace llvm;
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#define DEBUG_TYPE "loop-delete"
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STATISTIC(NumDeleted, "Number of loops deleted");
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enum class LoopDeletionResult {
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Unmodified,
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Modified,
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Deleted,
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};
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/// Determines if a loop is dead.
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///
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/// This assumes that we've already checked for unique exit and exiting blocks,
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/// and that the code is in LCSSA form.
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static bool isLoopDead(Loop *L, ScalarEvolution &SE,
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SmallVectorImpl<BasicBlock *> &ExitingBlocks,
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BasicBlock *ExitBlock, bool &Changed,
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BasicBlock *Preheader) {
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// Make sure that all PHI entries coming from the loop are loop invariant.
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// Because the code is in LCSSA form, any values used outside of the loop
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// must pass through a PHI in the exit block, meaning that this check is
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// sufficient to guarantee that no loop-variant values are used outside
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// of the loop.
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bool AllEntriesInvariant = true;
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bool AllOutgoingValuesSame = true;
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for (PHINode &P : ExitBlock->phis()) {
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Value *incoming = P.getIncomingValueForBlock(ExitingBlocks[0]);
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// Make sure all exiting blocks produce the same incoming value for the exit
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// block. If there are different incoming values for different exiting
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// blocks, then it is impossible to statically determine which value should
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// be used.
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AllOutgoingValuesSame =
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all_of(makeArrayRef(ExitingBlocks).slice(1), [&](BasicBlock *BB) {
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return incoming == P.getIncomingValueForBlock(BB);
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});
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if (!AllOutgoingValuesSame)
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break;
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if (Instruction *I = dyn_cast<Instruction>(incoming))
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if (!L->makeLoopInvariant(I, Changed, Preheader->getTerminator())) {
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AllEntriesInvariant = false;
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break;
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}
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}
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if (Changed)
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SE.forgetLoopDispositions(L);
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if (!AllEntriesInvariant || !AllOutgoingValuesSame)
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return false;
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// Make sure that no instructions in the block have potential side-effects.
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// This includes instructions that could write to memory, and loads that are
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// marked volatile.
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for (auto &I : L->blocks())
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if (any_of(*I, [](Instruction &I) { return I.mayHaveSideEffects(); }))
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return false;
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return true;
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}
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/// This function returns true if there is no viable path from the
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/// entry block to the header of \p L. Right now, it only does
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/// a local search to save compile time.
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static bool isLoopNeverExecuted(Loop *L) {
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using namespace PatternMatch;
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auto *Preheader = L->getLoopPreheader();
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// TODO: We can relax this constraint, since we just need a loop
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// predecessor.
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assert(Preheader && "Needs preheader!");
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if (Preheader == &Preheader->getParent()->getEntryBlock())
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return false;
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// All predecessors of the preheader should have a constant conditional
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// branch, with the loop's preheader as not-taken.
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for (auto *Pred: predecessors(Preheader)) {
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BasicBlock *Taken, *NotTaken;
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ConstantInt *Cond;
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if (!match(Pred->getTerminator(),
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m_Br(m_ConstantInt(Cond), Taken, NotTaken)))
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return false;
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if (!Cond->getZExtValue())
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std::swap(Taken, NotTaken);
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if (Taken == Preheader)
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return false;
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}
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assert(!pred_empty(Preheader) &&
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"Preheader should have predecessors at this point!");
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// All the predecessors have the loop preheader as not-taken target.
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return true;
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}
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/// Remove a loop if it is dead.
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///
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/// A loop is considered dead if it does not impact the observable behavior of
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/// the program other than finite running time. This never removes a loop that
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/// might be infinite (unless it is never executed), as doing so could change
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/// the halting/non-halting nature of a program.
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///
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/// This entire process relies pretty heavily on LoopSimplify form and LCSSA in
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/// order to make various safety checks work.
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///
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/// \returns true if any changes were made. This may mutate the loop even if it
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/// is unable to delete it due to hoisting trivially loop invariant
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/// instructions out of the loop.
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static LoopDeletionResult deleteLoopIfDead(Loop *L, DominatorTree &DT,
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ScalarEvolution &SE, LoopInfo &LI,
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MemorySSA *MSSA) {
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assert(L->isLCSSAForm(DT) && "Expected LCSSA!");
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// We can only remove the loop if there is a preheader that we can branch from
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// after removing it. Also, if LoopSimplify form is not available, stay out
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// of trouble.
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BasicBlock *Preheader = L->getLoopPreheader();
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if (!Preheader || !L->hasDedicatedExits()) {
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LLVM_DEBUG(
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dbgs()
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<< "Deletion requires Loop with preheader and dedicated exits.\n");
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return LoopDeletionResult::Unmodified;
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}
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// We can't remove loops that contain subloops. If the subloops were dead,
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// they would already have been removed in earlier executions of this pass.
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if (L->begin() != L->end()) {
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LLVM_DEBUG(dbgs() << "Loop contains subloops.\n");
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return LoopDeletionResult::Unmodified;
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}
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BasicBlock *ExitBlock = L->getUniqueExitBlock();
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if (ExitBlock && isLoopNeverExecuted(L)) {
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LLVM_DEBUG(dbgs() << "Loop is proven to never execute, delete it!");
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// Set incoming value to undef for phi nodes in the exit block.
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for (PHINode &P : ExitBlock->phis()) {
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std::fill(P.incoming_values().begin(), P.incoming_values().end(),
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UndefValue::get(P.getType()));
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}
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deleteDeadLoop(L, &DT, &SE, &LI, MSSA);
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++NumDeleted;
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return LoopDeletionResult::Deleted;
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}
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// The remaining checks below are for a loop being dead because all statements
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// in the loop are invariant.
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SmallVector<BasicBlock *, 4> ExitingBlocks;
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L->getExitingBlocks(ExitingBlocks);
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// We require that the loop only have a single exit block. Otherwise, we'd
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// be in the situation of needing to be able to solve statically which exit
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// block will be branched to, or trying to preserve the branching logic in
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// a loop invariant manner.
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if (!ExitBlock) {
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LLVM_DEBUG(dbgs() << "Deletion requires single exit block\n");
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return LoopDeletionResult::Unmodified;
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}
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// Finally, we have to check that the loop really is dead.
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bool Changed = false;
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if (!isLoopDead(L, SE, ExitingBlocks, ExitBlock, Changed, Preheader)) {
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LLVM_DEBUG(dbgs() << "Loop is not invariant, cannot delete.\n");
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return Changed ? LoopDeletionResult::Modified
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: LoopDeletionResult::Unmodified;
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}
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// Don't remove loops for which we can't solve the trip count.
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// They could be infinite, in which case we'd be changing program behavior.
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const SCEV *S = SE.getConstantMaxBackedgeTakenCount(L);
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if (isa<SCEVCouldNotCompute>(S)) {
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LLVM_DEBUG(dbgs() << "Could not compute SCEV MaxBackedgeTakenCount.\n");
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return Changed ? LoopDeletionResult::Modified
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: LoopDeletionResult::Unmodified;
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}
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LLVM_DEBUG(dbgs() << "Loop is invariant, delete it!");
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deleteDeadLoop(L, &DT, &SE, &LI, MSSA);
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++NumDeleted;
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return LoopDeletionResult::Deleted;
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}
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PreservedAnalyses LoopDeletionPass::run(Loop &L, LoopAnalysisManager &AM,
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LoopStandardAnalysisResults &AR,
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LPMUpdater &Updater) {
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LLVM_DEBUG(dbgs() << "Analyzing Loop for deletion: ");
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LLVM_DEBUG(L.dump());
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std::string LoopName = L.getName();
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auto Result = deleteLoopIfDead(&L, AR.DT, AR.SE, AR.LI, AR.MSSA);
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if (Result == LoopDeletionResult::Unmodified)
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return PreservedAnalyses::all();
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if (Result == LoopDeletionResult::Deleted)
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Updater.markLoopAsDeleted(L, LoopName);
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auto PA = getLoopPassPreservedAnalyses();
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if (AR.MSSA)
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PA.preserve<MemorySSAAnalysis>();
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return PA;
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}
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namespace {
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class LoopDeletionLegacyPass : public LoopPass {
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public:
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static char ID; // Pass ID, replacement for typeid
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LoopDeletionLegacyPass() : LoopPass(ID) {
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initializeLoopDeletionLegacyPassPass(*PassRegistry::getPassRegistry());
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}
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// Possibly eliminate loop L if it is dead.
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bool runOnLoop(Loop *L, LPPassManager &) override;
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void getAnalysisUsage(AnalysisUsage &AU) const override {
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AU.addPreserved<MemorySSAWrapperPass>();
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getLoopAnalysisUsage(AU);
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}
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};
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}
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char LoopDeletionLegacyPass::ID = 0;
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INITIALIZE_PASS_BEGIN(LoopDeletionLegacyPass, "loop-deletion",
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"Delete dead loops", false, false)
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INITIALIZE_PASS_DEPENDENCY(LoopPass)
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INITIALIZE_PASS_END(LoopDeletionLegacyPass, "loop-deletion",
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"Delete dead loops", false, false)
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Pass *llvm::createLoopDeletionPass() { return new LoopDeletionLegacyPass(); }
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bool LoopDeletionLegacyPass::runOnLoop(Loop *L, LPPassManager &LPM) {
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if (skipLoop(L))
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return false;
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DominatorTree &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
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ScalarEvolution &SE = getAnalysis<ScalarEvolutionWrapperPass>().getSE();
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LoopInfo &LI = getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
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auto *MSSAAnalysis = getAnalysisIfAvailable<MemorySSAWrapperPass>();
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MemorySSA *MSSA = nullptr;
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if (MSSAAnalysis)
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MSSA = &MSSAAnalysis->getMSSA();
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LLVM_DEBUG(dbgs() << "Analyzing Loop for deletion: ");
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LLVM_DEBUG(L->dump());
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LoopDeletionResult Result = deleteLoopIfDead(L, DT, SE, LI, MSSA);
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if (Result == LoopDeletionResult::Deleted)
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LPM.markLoopAsDeleted(*L);
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return Result != LoopDeletionResult::Unmodified;
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}
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