lib/Transforms/Scalar/LoopDeletion.cpp

   1 //===- LoopDeletion.cpp - Dead Loop Deletion Pass ---------------===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // This file implements the Dead Loop Deletion Pass. This pass is responsible
  10 // for eliminating loops with non-infinite computable trip counts that have no
  11 // side effects or volatile instructions, and do not contribute to the
  12 // computation of the function's return value.
  13 //
  14 //===----------------------------------------------------------------------===//
  15
  16 #include "llvm/Transforms/Scalar/LoopDeletion.h"
  17 #include "llvm/ADT/SmallVector.h"
  18 #include "llvm/ADT/Statistic.h"
  19 #include "llvm/Analysis/GlobalsModRef.h"
  20 #include "llvm/Analysis/LoopPass.h"
  21 #include "llvm/IR/Dominators.h"
  22 #include "llvm/IR/PatternMatch.h"
  23 #include "llvm/Transforms/Scalar.h"
  24 #include "llvm/Transforms/Scalar/LoopPassManager.h"
  25 #include "llvm/Transforms/Utils/LoopUtils.h"
  26 using namespace llvm;
  27
  28 #define DEBUG_TYPE "loop-delete"
  29
  30 STATISTIC(NumDeleted, "Number of loops deleted");
  31
  32 enum class LoopDeletionResult {
  33   Unmodified,
  34   Modified,
  35   Deleted,
  36 };
  37
  38 /// Determines if a loop is dead.
  39 ///
  40 /// This assumes that we've already checked for unique exit and exiting blocks,
  41 /// and that the code is in LCSSA form.
  42 static bool isLoopDead(Loop *L, ScalarEvolution &SE,
  43                        SmallVectorImpl<BasicBlock *> &ExitingBlocks,
  44                        BasicBlock *ExitBlock, bool &Changed,
  45                        BasicBlock *Preheader) {
  46   // Make sure that all PHI entries coming from the loop are loop invariant.
  47   // Because the code is in LCSSA form, any values used outside of the loop
  48   // must pass through a PHI in the exit block, meaning that this check is
  49   // sufficient to guarantee that no loop-variant values are used outside
  50   // of the loop.
  51   bool AllEntriesInvariant = true;
  52   bool AllOutgoingValuesSame = true;
  53   for (PHINode &P : ExitBlock->phis()) {
  54     Value *incoming = P.getIncomingValueForBlock(ExitingBlocks[0]);
  55
  56     // Make sure all exiting blocks produce the same incoming value for the exit
  57     // block.  If there are different incoming values for different exiting
  58     // blocks, then it is impossible to statically determine which value should
  59     // be used.
  60     AllOutgoingValuesSame =
  61         all_of(makeArrayRef(ExitingBlocks).slice(1), [&](BasicBlock *BB) {
  62           return incoming == P.getIncomingValueForBlock(BB);
  63         });
  64
  65     if (!AllOutgoingValuesSame)
  66       break;
  67
  68     if (Instruction *I = dyn_cast<Instruction>(incoming))
  69       if (!L->makeLoopInvariant(I, Changed, Preheader->getTerminator())) {
  70         AllEntriesInvariant = false;
  71         break;
  72       }
  73   }
  74
  75   if (Changed)
  76     SE.forgetLoopDispositions(L);
  77
  78   if (!AllEntriesInvariant || !AllOutgoingValuesSame)
  79     return false;
  80
  81   // Make sure that no instructions in the block have potential side-effects.
  82   // This includes instructions that could write to memory, and loads that are
  83   // marked volatile.
  84   for (auto &I : L->blocks())
  85     if (any_of(*I, [](Instruction &I) { return I.mayHaveSideEffects(); }))
  86       return false;
  87   return true;
  88 }
  89
  90 /// This function returns true if there is no viable path from the
  91 /// entry block to the header of \p L. Right now, it only does
  92 /// a local search to save compile time.
  93 static bool isLoopNeverExecuted(Loop *L) {
  94   using namespace PatternMatch;
  95
  96   auto *Preheader = L->getLoopPreheader();
  97   // TODO: We can relax this constraint, since we just need a loop
  98   // predecessor.
  99   assert(Preheader && "Needs preheader!");
 100
 101   if (Preheader == &Preheader->getParent()->getEntryBlock())
 102     return false;
 103   // All predecessors of the preheader should have a constant conditional
 104   // branch, with the loop's preheader as not-taken.
 105   for (auto *Pred: predecessors(Preheader)) {
 106     BasicBlock *Taken, *NotTaken;
 107     ConstantInt *Cond;
 108     if (!match(Pred->getTerminator(),
 109                m_Br(m_ConstantInt(Cond), Taken, NotTaken)))
 110       return false;
 111     if (!Cond->getZExtValue())
 112       std::swap(Taken, NotTaken);
 113     if (Taken == Preheader)
 114       return false;
 115   }
 116   assert(!pred_empty(Preheader) &&
 117          "Preheader should have predecessors at this point!");
 118   // All the predecessors have the loop preheader as not-taken target.
 119   return true;
 120 }
 121
 122 /// Remove a loop if it is dead.
 123 ///
 124 /// A loop is considered dead if it does not impact the observable behavior of
 125 /// the program other than finite running time. This never removes a loop that
 126 /// might be infinite (unless it is never executed), as doing so could change
 127 /// the halting/non-halting nature of a program.
 128 ///
 129 /// This entire process relies pretty heavily on LoopSimplify form and LCSSA in
 130 /// order to make various safety checks work.
 131 ///
 132 /// \returns true if any changes were made. This may mutate the loop even if it
 133 /// is unable to delete it due to hoisting trivially loop invariant
 134 /// instructions out of the loop.
 135 static LoopDeletionResult deleteLoopIfDead(Loop *L, DominatorTree &DT,
 136                                            ScalarEvolution &SE, LoopInfo &LI) {
 137   assert(L->isLCSSAForm(DT) && "Expected LCSSA!");
 138
 139   // We can only remove the loop if there is a preheader that we can branch from
 140   // after removing it. Also, if LoopSimplify form is not available, stay out
 141   // of trouble.
 142   BasicBlock *Preheader = L->getLoopPreheader();
 143   if (!Preheader || !L->hasDedicatedExits()) {
 144     LLVM_DEBUG(
 145         dbgs()
 146         << "Deletion requires Loop with preheader and dedicated exits.\n");
 147     return LoopDeletionResult::Unmodified;
 148   }
 149   // We can't remove loops that contain subloops.  If the subloops were dead,
 150   // they would already have been removed in earlier executions of this pass.
 151   if (L->begin() != L->end()) {
 152     LLVM_DEBUG(dbgs() << "Loop contains subloops.\n");
 153     return LoopDeletionResult::Unmodified;
 154   }
 155
 156
 157   BasicBlock *ExitBlock = L->getUniqueExitBlock();
 158
 159   if (ExitBlock && isLoopNeverExecuted(L)) {
 160     LLVM_DEBUG(dbgs() << "Loop is proven to never execute, delete it!");
 161     // Set incoming value to undef for phi nodes in the exit block.
 162     for (PHINode &P : ExitBlock->phis()) {
 163       std::fill(P.incoming_values().begin(), P.incoming_values().end(),
 164                 UndefValue::get(P.getType()));
 165     }
 166     deleteDeadLoop(L, &DT, &SE, &LI);
 167     ++NumDeleted;
 168     return LoopDeletionResult::Deleted;
 169   }
 170
 171   // The remaining checks below are for a loop being dead because all statements
 172   // in the loop are invariant.
 173   SmallVector<BasicBlock *, 4> ExitingBlocks;
 174   L->getExitingBlocks(ExitingBlocks);
 175
 176   // We require that the loop only have a single exit block.  Otherwise, we'd
 177   // be in the situation of needing to be able to solve statically which exit
 178   // block will be branched to, or trying to preserve the branching logic in
 179   // a loop invariant manner.
 180   if (!ExitBlock) {
 181     LLVM_DEBUG(dbgs() << "Deletion requires single exit block\n");
 182     return LoopDeletionResult::Unmodified;
 183   }
 184   // Finally, we have to check that the loop really is dead.
 185   bool Changed = false;
 186   if (!isLoopDead(L, SE, ExitingBlocks, ExitBlock, Changed, Preheader)) {
 187     LLVM_DEBUG(dbgs() << "Loop is not invariant, cannot delete.\n");
 188     return Changed ? LoopDeletionResult::Modified
 189                    : LoopDeletionResult::Unmodified;
 190   }
 191
 192   // Don't remove loops for which we can't solve the trip count.
 193   // They could be infinite, in which case we'd be changing program behavior.
 194   const SCEV *S = SE.getConstantMaxBackedgeTakenCount(L);
 195   if (isa<SCEVCouldNotCompute>(S)) {
 196     LLVM_DEBUG(dbgs() << "Could not compute SCEV MaxBackedgeTakenCount.\n");
 197     return Changed ? LoopDeletionResult::Modified
 198                    : LoopDeletionResult::Unmodified;
 199   }
 200
 201   LLVM_DEBUG(dbgs() << "Loop is invariant, delete it!");
 202   deleteDeadLoop(L, &DT, &SE, &LI);
 203   ++NumDeleted;
 204
 205   return LoopDeletionResult::Deleted;
 206 }
 207
 208 PreservedAnalyses LoopDeletionPass::run(Loop &L, LoopAnalysisManager &AM,
 209                                         LoopStandardAnalysisResults &AR,
 210                                         LPMUpdater &Updater) {
 211
 212   LLVM_DEBUG(dbgs() << "Analyzing Loop for deletion: ");
 213   LLVM_DEBUG(L.dump());
 214   std::string LoopName = L.getName();
 215   auto Result = deleteLoopIfDead(&L, AR.DT, AR.SE, AR.LI);
 216   if (Result == LoopDeletionResult::Unmodified)
 217     return PreservedAnalyses::all();
 218
 219   if (Result == LoopDeletionResult::Deleted)
 220     Updater.markLoopAsDeleted(L, LoopName);
 221
 222   return getLoopPassPreservedAnalyses();
 223 }
 224
 225 namespace {
 226 class LoopDeletionLegacyPass : public LoopPass {
 227 public:
 228   static char ID; // Pass ID, replacement for typeid
 229   LoopDeletionLegacyPass() : LoopPass(ID) {
 230     initializeLoopDeletionLegacyPassPass(*PassRegistry::getPassRegistry());
 231   }
 232
 233   // Possibly eliminate loop L if it is dead.
 234   bool runOnLoop(Loop *L, LPPassManager &) override;
 235
 236   void getAnalysisUsage(AnalysisUsage &AU) const override {
 237     getLoopAnalysisUsage(AU);
 238   }
 239 };
 240 }
 241
 242 char LoopDeletionLegacyPass::ID = 0;
 243 INITIALIZE_PASS_BEGIN(LoopDeletionLegacyPass, "loop-deletion",
 244                       "Delete dead loops", false, false)
 245 INITIALIZE_PASS_DEPENDENCY(LoopPass)
 246 INITIALIZE_PASS_END(LoopDeletionLegacyPass, "loop-deletion",
 247                     "Delete dead loops", false, false)
 248
 249 Pass *llvm::createLoopDeletionPass() { return new LoopDeletionLegacyPass(); }
 250
 251 bool LoopDeletionLegacyPass::runOnLoop(Loop *L, LPPassManager &LPM) {
 252   if (skipLoop(L))
 253     return false;
 254   DominatorTree &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
 255   ScalarEvolution &SE = getAnalysis<ScalarEvolutionWrapperPass>().getSE();
 256   LoopInfo &LI = getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
 257
 258   LLVM_DEBUG(dbgs() << "Analyzing Loop for deletion: ");
 259   LLVM_DEBUG(L->dump());
 260
 261   LoopDeletionResult Result = deleteLoopIfDead(L, DT, SE, LI);
 262
 263   if (Result == LoopDeletionResult::Deleted)
 264     LPM.markLoopAsDeleted(*L);
 265
 266   return Result != LoopDeletionResult::Unmodified;
 267 }