Transforms/IPO/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/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/InitializePasses.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 ModulePass {
    static char ID; // Pass identification, replacement for typeid

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

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

    bool runOnModule(Module &M) override;
    bool runOnFunction(Function &F);

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

    void getAnalysisUsage(AnalysisUsage &AU) const override {
      AU.addRequiredID(BreakCriticalEdgesID);
      AU.addRequired<DominatorTreeWrapperPass>();
      AU.addRequired<LoopInfoWrapperPass>();
      AU.addPreserved<LoopInfoWrapperPass>();
      AU.addRequiredID(LoopSimplifyID);
      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(DominatorTreeWrapperPass)
INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)
INITIALIZE_PASS_DEPENDENCY(LoopSimplify)
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::runOnModule(Module &M) {
  if (skipModule(M))
    return false;

  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 = getAnalysis<LoopInfoWrapperPass>(F, &Changed).getLoopInfo();

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

  DominatorTree &DT = getAnalysis<DominatorTreeWrapperPass>(F).getDomTree();

  // 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);
  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)) {
    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();
}