LoopRotation.cpp

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//===- LoopRotation.cpp - Loop Rotation Pass ------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This file implements Loop Rotation Pass.
//
//===----------------------------------------------------------------------===//
 
#include "llvm/Transforms/Scalar/LoopRotation.h"
#include "llvm/Analysis/AssumptionCache.h"
#include "llvm/Analysis/InstructionSimplify.h"
#include "llvm/Analysis/LazyBlockFrequencyInfo.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/TargetTransformInfo.h"
#include "llvm/InitializePasses.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils/LoopRotationUtils.h"
#include "llvm/Transforms/Utils/LoopUtils.h"
#include <optional>
using namespace llvm;
 
#define DEBUG_TYPE "loop-rotate"
 
static cl::opt<unsigned> DefaultRotationThreshold(
    "rotation-max-header-size", cl::init(16), cl::Hidden,
    cl::desc("The default maximum header size for automatic loop rotation"));
 
static cl::opt<bool> PrepareForLTOOption(
    "rotation-prepare-for-lto", cl::init(false), cl::Hidden,
    cl::desc("Run loop-rotation in the prepare-for-lto stage. This option "
             "should be used for testing only."));
 
LoopRotatePass::LoopRotatePass(bool EnableHeaderDuplication, bool PrepareForLTO)
    : EnableHeaderDuplication(EnableHeaderDuplication),
      PrepareForLTO(PrepareForLTO) {}
 
PreservedAnalyses LoopRotatePass::run(Loop &L, LoopAnalysisManager &AM,
                                      LoopStandardAnalysisResults &AR,
                                      LPMUpdater &) {
  // Vectorization requires loop-rotation. Use default threshold for loops the
  // user explicitly marked for vectorization, even when header duplication is
  // disabled.
  int Threshold = EnableHeaderDuplication ||
                          hasVectorizeTransformation(&L) == TM_ForcedByUser
                      ? DefaultRotationThreshold
                      : 0;
  const DataLayout &DL = L.getHeader()->getModule()->getDataLayout();
  const SimplifyQuery SQ = getBestSimplifyQuery(AR, DL);
 
  std::optional<MemorySSAUpdater> MSSAU;
  if (AR.MSSA)
    MSSAU = MemorySSAUpdater(AR.MSSA);
  bool Changed = LoopRotation(&L, &AR.LI, &AR.TTI, &AR.AC, &AR.DT, &AR.SE,
                              MSSAU ? &*MSSAU : nullptr, SQ, false, Threshold,
                              false, PrepareForLTO || PrepareForLTOOption);
 
  if (!Changed)
    return PreservedAnalyses::all();
 
  if (AR.MSSA && VerifyMemorySSA)
    AR.MSSA->verifyMemorySSA();
 
  auto PA = getLoopPassPreservedAnalyses();
  if (AR.MSSA)
    PA.preserve<MemorySSAAnalysis>();
  return PA;
}
 
namespace {
 
class LoopRotateLegacyPass : public LoopPass {
  unsigned MaxHeaderSize;
  bool PrepareForLTO;
 
public:
  static char ID; // Pass ID, replacement for typeid
  LoopRotateLegacyPass(int SpecifiedMaxHeaderSize = -1,
                       bool PrepareForLTO = false)
      : LoopPass(ID), PrepareForLTO(PrepareForLTO) {
    initializeLoopRotateLegacyPassPass(*PassRegistry::getPassRegistry());
    if (SpecifiedMaxHeaderSize == -1)
      MaxHeaderSize = DefaultRotationThreshold;
    else
      MaxHeaderSize = unsigned(SpecifiedMaxHeaderSize);
  }
 
  // LCSSA form makes instruction renaming easier.
  void getAnalysisUsage(AnalysisUsage &AU) const override {
    AU.addRequired<AssumptionCacheTracker>();
    AU.addRequired<TargetTransformInfoWrapperPass>();
    AU.addPreserved<MemorySSAWrapperPass>();
    getLoopAnalysisUsage(AU);
 
    // Lazy BFI and BPI are marked as preserved here so LoopRotate
    // can remain part of the same loop pass manager as LICM.
    AU.addPreserved<LazyBlockFrequencyInfoPass>();
    AU.addPreserved<LazyBranchProbabilityInfoPass>();
  }
 
  bool runOnLoop(Loop *L, LPPassManager &LPM) override {
    if (skipLoop(L))
      return false;
    Function &F = *L->getHeader()->getParent();
 
    auto *LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
    const auto *TTI = &getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
    auto *AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
    auto &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
    auto &SE = getAnalysis<ScalarEvolutionWrapperPass>().getSE();
    const SimplifyQuery SQ = getBestSimplifyQuery(*this, F);
    std::optional<MemorySSAUpdater> MSSAU;
    // Not requiring MemorySSA and getting it only if available will split
    // the loop pass pipeline when LoopRotate is being run first.
    auto *MSSAA = getAnalysisIfAvailable<MemorySSAWrapperPass>();
    if (MSSAA)
      MSSAU = MemorySSAUpdater(&MSSAA->getMSSA());
    // Vectorization requires loop-rotation. Use default threshold for loops the
    // user explicitly marked for vectorization, even when header duplication is
    // disabled.
    int Threshold = hasVectorizeTransformation(L) == TM_ForcedByUser
                        ? DefaultRotationThreshold
                        : MaxHeaderSize;
 
    return LoopRotation(L, LI, TTI, AC, &DT, &SE, MSSAU ? &*MSSAU : nullptr, SQ,
                        false, Threshold, false,
                        PrepareForLTO || PrepareForLTOOption);
  }
};
} // end namespace
 
char LoopRotateLegacyPass::ID = 0;
INITIALIZE_PASS_BEGIN(LoopRotateLegacyPass, "loop-rotate", "Rotate Loops",
                      false, false)
INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
INITIALIZE_PASS_DEPENDENCY(LoopPass)
INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
INITIALIZE_PASS_DEPENDENCY(MemorySSAWrapperPass)
INITIALIZE_PASS_END(LoopRotateLegacyPass, "loop-rotate", "Rotate Loops", false,
                    false)
 
Pass *llvm::createLoopRotatePass(int MaxHeaderSize, bool PrepareForLTO) {
  return new LoopRotateLegacyPass(MaxHeaderSize, PrepareForLTO);
}