LoopExtractor.cpp

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//===- 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/ADT/Statistic.h"
#include "llvm/Analysis/AssumptionCache.h"
#include "llvm/Analysis/LoopPass.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Module.h"
#include "llvm/Pass.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Transforms/IPO.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Utils.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include "llvm/Transforms/Utils/CodeExtractor.h"
#include <fstream>
#include <set>
using namespace llvm;

#define DEBUG_TYPE "loop-extract"

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

namespace {
  struct LoopExtractor : public LoopPass {
    static char ID; // Pass identification, replacement for typeid
    unsigned NumLoops;

    explicit LoopExtractor(unsigned numLoops = ~0)
      : LoopPass(ID), NumLoops(numLoops) {
        initializeLoopExtractorPass(*PassRegistry::getPassRegistry());
      }

    bool runOnLoop(Loop *L, LPPassManager &) override;

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

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

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

bool LoopExtractor::runOnLoop(Loop *L, LPPassManager &LPM) {
  if (skipLoop(L))
    return false;

  // Only visit top-level loops.
  if (L->getParentLoop())
    return false;

  // If LoopSimplify form is not available, stay out of trouble.
  if (!L->isLoopSimplifyForm())
    return false;

  DominatorTree &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
  LoopInfo &LI = getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
  bool Changed = false;

  // If there is more than one top-level loop in this function, extract all of
  // the loops. Otherwise there is exactly one top-level loop; in this case if
  // this function is more than a minimal wrapper around the loop, extract
  // the loop.
  bool ShouldExtractLoop = false;

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

  if (ShouldExtractLoop) {
    // We must omit EH pads. EH pads must accompany the invoke
    // instruction. But this would result in a loop in the extracted
    // function. An infinite cycle occurs when it tries to extract that loop as
    // well.
    SmallVector<BasicBlock*, 8> ExitBlocks;
    L->getExitBlocks(ExitBlocks);
    for (unsigned i = 0, e = ExitBlocks.size(); i != e; ++i)
      if (ExitBlocks[i]->isEHPad()) {
        ShouldExtractLoop = false;
        break;
      }
  }

  if (ShouldExtractLoop) {
    if (NumLoops == 0) return Changed;
    --NumLoops;
    AssumptionCache *AC = nullptr;
    Function &Func = *L->getHeader()->getParent();
    if (auto *ACT = getAnalysisIfAvailable<AssumptionCacheTracker>())
      AC = ACT->lookupAssumptionCache(Func);
    CodeExtractorAnalysisCache CEAC(Func);
    CodeExtractor Extractor(DT, *L, false, nullptr, nullptr, AC);
    if (Extractor.extractCodeRegion(CEAC) != nullptr) {
      Changed = true;
      // After extraction, the loop is replaced by a function call, so
      // we shouldn't try to run any more loop passes on it.
      LPM.markLoopAsDeleted(*L);
      LI.erase(L);
    }
    ++NumExtracted;
  }

  return Changed;
}

// 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();
}