LoopExtractor.cpp

//===- LoopExtractor.cpp - Extract each loop into a new function ----------===//
//
// 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
//
//===----------------------------------------------------------------------===//
//
// A pass wrapper around the ExtractLoop() scalar transformation to extract each
// top-level loop into its own new function. If the loop is the ONLY loop in a
// given function, it is not touched. This is a pass most useful for debugging
// via bugpoint.
//
//===----------------------------------------------------------------------===//

#include "llvm/Transforms/IPO/LoopExtractor.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/AssumptionCache.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Module.h"
#include "llvm/IR/PassManager.h"
#include "llvm/InitializePasses.h"
#include "llvm/Pass.h"
#include "llvm/Transforms/IPO.h"
#include "llvm/Transforms/Utils.h"
#include "llvm/Transforms/Utils/CodeExtractor.h"
using namespace llvm;

#define DEBUG_TYPE "loop-extract"

STATISTIC(NumExtracted, "Number of loops extracted");

namespace {
struct LoopExtractorLegacyPass : public ModulePass {
  static char ID; // Pass identification, replacement for typeid

  unsigned NumLoops;

  explicit LoopExtractorLegacyPass(unsigned NumLoops = ~0)
      : ModulePass(ID), NumLoops(NumLoops) {
    initializeLoopExtractorLegacyPassPass(*PassRegistry::getPassRegistry());
  }

  bool runOnModule(Module &M) override;

  void getAnalysisUsage(AnalysisUsage &AU) const override {
    AU.addRequiredID(BreakCriticalEdgesID);
    AU.addRequired<DominatorTreeWrapperPass>();
    AU.addRequired<LoopInfoWrapperPass>();
    AU.addPreserved<LoopInfoWrapperPass>();
    AU.addRequiredID(LoopSimplifyID);
    AU.addUsedIfAvailable<AssumptionCacheTracker>();
  }
};

struct LoopExtractor {
  explicit LoopExtractor(
      unsigned NumLoops,
      function_ref<DominatorTree &(Function &)> LookupDomTree,
      function_ref<LoopInfo &(Function &)> LookupLoopInfo,
      function_ref<AssumptionCache *(Function &)> LookupAssumptionCache)
      : NumLoops(NumLoops), LookupDomTree(LookupDomTree),
        LookupLoopInfo(LookupLoopInfo),
        LookupAssumptionCache(LookupAssumptionCache) {}
  bool runOnModule(Module &M);

private:
  // The number of natural loops to extract from the program into functions.
  unsigned NumLoops;

  function_ref<DominatorTree &(Function &)> LookupDomTree;
  function_ref<LoopInfo &(Function &)> LookupLoopInfo;
  function_ref<AssumptionCache *(Function &)> LookupAssumptionCache;

  bool runOnFunction(Function &F);

  bool extractLoops(Loop::iterator From, Loop::iterator To, LoopInfo &LI,
                    DominatorTree &DT);
  bool extractLoop(Loop *L, LoopInfo &LI, DominatorTree &DT);
};
} // namespace

char LoopExtractorLegacyPass::ID = 0;
INITIALIZE_PASS_BEGIN(LoopExtractorLegacyPass, "loop-extract",
                      "Extract loops into new functions", false, false)
INITIALIZE_PASS_DEPENDENCY(BreakCriticalEdges)
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
INITIALIZE_PASS_DEPENDENCY(LoopSimplify)
INITIALIZE_PASS_END(LoopExtractorLegacyPass, "loop-extract",
                    "Extract loops into new functions", false, false)

namespace {
  /// SingleLoopExtractor - For bugpoint.
struct SingleLoopExtractor : public LoopExtractorLegacyPass {
  static char ID; // Pass identification, replacement for typeid
  SingleLoopExtractor() : LoopExtractorLegacyPass(1) {}
};
} // End anonymous namespace

char SingleLoopExtractor::ID = 0;
INITIALIZE_PASS(SingleLoopExtractor, "loop-extract-single",
                "Extract at most one loop into a new function", false, false)

// createLoopExtractorPass - This pass extracts all natural loops from the
// program into a function if it can.
//
Pass *llvm::createLoopExtractorPass() { return new LoopExtractorLegacyPass(); }

bool LoopExtractorLegacyPass::runOnModule(Module &M) {
  if (skipModule(M))
    return false;

  bool Changed = false;
  auto LookupDomTree = [this](Function &F) -> DominatorTree & {
    return this->getAnalysis<DominatorTreeWrapperPass>(F).getDomTree();
  };
  auto LookupLoopInfo = [this, &Changed](Function &F) -> LoopInfo & {
    return this->getAnalysis<LoopInfoWrapperPass>(F, &Changed).getLoopInfo();
  };
  auto LookupACT = [this](Function &F) -> AssumptionCache * {
    if (auto *ACT = this->getAnalysisIfAvailable<AssumptionCacheTracker>())
      return ACT->lookupAssumptionCache(F);
    return nullptr;
  };
  return LoopExtractor(NumLoops, LookupDomTree, LookupLoopInfo, LookupACT)
             .runOnModule(M) ||
         Changed;
}

bool LoopExtractor::runOnModule(Module &M) {
  if (M.empty())
    return false;

  if (!NumLoops)
    return false;

  bool Changed = false;

  // The end of the function list may change (new functions will be added at the
  // end), so we run from the first to the current last.
  auto I = M.begin(), E = --M.end();
  while (true) {
    Function &F = *I;

    Changed |= runOnFunction(F);
    if (!NumLoops)
      break;

    // If this is the last function.
    if (I == E)
      break;

    ++I;
  }
  return Changed;
}

bool LoopExtractor::runOnFunction(Function &F) {
  // Do not modify `optnone` functions.
  if (F.hasOptNone())
    return false;

  if (F.empty())
    return false;

  bool Changed = false;
  LoopInfo &LI = LookupLoopInfo(F);

  // If there are no loops in the function.
  if (LI.empty())
    return Changed;

  DominatorTree &DT = LookupDomTree(F);

  // If there is more than one top-level loop in this function, extract all of
  // the loops.
  if (std::next(LI.begin()) != LI.end())
    return Changed | extractLoops(LI.begin(), LI.end(), LI, DT);

  // Otherwise there is exactly one top-level loop.
  Loop *TLL = *LI.begin();

  // If the loop is in LoopSimplify form, then extract it only if this function
  // is more than a minimal wrapper around the loop.
  if (TLL->isLoopSimplifyForm()) {
    bool ShouldExtractLoop = false;

    // Extract the loop if the entry block doesn't branch to the loop header.
    Instruction *EntryTI = F.getEntryBlock().getTerminator();
    if (!isa<BranchInst>(EntryTI) ||
        !cast<BranchInst>(EntryTI)->isUnconditional() ||
        EntryTI->getSuccessor(0) != TLL->getHeader()) {
      ShouldExtractLoop = true;
    } else {
      // Check to see if any exits from the loop are more than just return
      // blocks.
      SmallVector<BasicBlock *, 8> ExitBlocks;
      TLL->getExitBlocks(ExitBlocks);
      for (auto *ExitBlock : ExitBlocks)
        if (!isa<ReturnInst>(ExitBlock->getTerminator())) {
          ShouldExtractLoop = true;
          break;
        }
    }

    if (ShouldExtractLoop)
      return Changed | extractLoop(TLL, LI, DT);
  }

  // Okay, this function is a minimal container around the specified loop.
  // If we extract the loop, we will continue to just keep extracting it
  // infinitely... so don't extract it. However, if the loop contains any
  // sub-loops, extract them.
  return Changed | extractLoops(TLL->begin(), TLL->end(), LI, DT);
}

bool LoopExtractor::extractLoops(Loop::iterator From, Loop::iterator To,
                                 LoopInfo &LI, DominatorTree &DT) {
  bool Changed = false;
  SmallVector<Loop *, 8> Loops;

  // Save the list of loops, as it may change.
  Loops.assign(From, To);
  for (Loop *L : Loops) {
    // If LoopSimplify form is not available, stay out of trouble.
    if (!L->isLoopSimplifyForm())
      continue;

    Changed |= extractLoop(L, LI, DT);
    if (!NumLoops)
      break;
  }
  return Changed;
}

bool LoopExtractor::extractLoop(Loop *L, LoopInfo &LI, DominatorTree &DT) {
  assert(NumLoops != 0);
  Function &Func = *L->getHeader()->getParent();
  AssumptionCache *AC = LookupAssumptionCache(Func);
  CodeExtractorAnalysisCache CEAC(Func);
  CodeExtractor Extractor(DT, *L, false, nullptr, nullptr, AC);
  if (Extractor.extractCodeRegion(CEAC)) {
    LI.erase(L);
    --NumLoops;
    ++NumExtracted;
    return true;
  }
  return false;
}

// createSingleLoopExtractorPass - This pass extracts one natural loop from the
// program into a function if it can.  This is used by bugpoint.
//
Pass *llvm::createSingleLoopExtractorPass() {
  return new SingleLoopExtractor();
}

PreservedAnalyses LoopExtractorPass::run(Module &M, ModuleAnalysisManager &AM) {
  auto &FAM = AM.getResult<FunctionAnalysisManagerModuleProxy>(M).getManager();
  auto LookupDomTree = [&FAM](Function &F) -> DominatorTree & {
    return FAM.getResult<DominatorTreeAnalysis>(F);
  };
  auto LookupLoopInfo = [&FAM](Function &F) -> LoopInfo & {
    return FAM.getResult<LoopAnalysis>(F);
  };
  auto LookupAssumptionCache = [&FAM](Function &F) -> AssumptionCache * {
    return FAM.getCachedResult<AssumptionAnalysis>(F);
  };
  if (!LoopExtractor(NumLoops, LookupDomTree, LookupLoopInfo,
                     LookupAssumptionCache)
           .runOnModule(M))
    return PreservedAnalyses::all();

  PreservedAnalyses PA;
  PA.preserve<LoopAnalysis>();
  return PA;
}

void LoopExtractorPass::printPipeline(
    raw_ostream &OS, function_ref<StringRef(StringRef)> MapClassName2PassName) {
  static_cast<PassInfoMixin<LoopExtractorPass> *>(this)->printPipeline(
      OS, MapClassName2PassName);
  OS << '<';
  if (NumLoops == 1)
    OS << "single";
  OS << '>';
}