llvm-project/polly/lib/Transform/IndependentBlocks.cpp

//===------ IndependentBlocks.cpp - Create Independent Blocks in Regions --===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// Create independent blocks in the regions detected by ScopDetection.
//
//===----------------------------------------------------------------------===//
//
#include "polly/LinkAllPasses.h"
#include "polly/CodeGen/BlockGenerators.h"
#include "polly/Options.h"
#include "polly/ScopDetection.h"
#include "polly/Support/ScopHelper.h"
#include "llvm/Analysis/AliasAnalysis.h"
#include "llvm/Analysis/BasicAliasAnalysis.h"
#include "llvm/Analysis/DominanceFrontier.h"
#include "llvm/Analysis/GlobalsModRef.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/PostDominators.h"
#include "llvm/Analysis/RegionInfo.h"
#include "llvm/Analysis/ScalarEvolutionAliasAnalysis.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/IR/IntrinsicInst.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/Debug.h"
#include "llvm/Transforms/Utils/Local.h"
#include <vector>

using namespace polly;
using namespace llvm;

#define DEBUG_TYPE "polly-independent"

namespace {
struct IndependentBlocks : public FunctionPass {
  RegionInfo *RI;
  ScalarEvolution *SE;
  ScopDetection *SD;
  LoopInfo *LI;

  BasicBlock *AllocaBlock;

  static char ID;

  IndependentBlocks() : FunctionPass(ID) {}

  // Create new code for every instruction operator that can be expressed by a
  // SCEV.  Like this there are just two types of instructions left:
  //
  // 1. Instructions that only reference loop ivs or parameters outside the
  // region.
  //
  // 2. Instructions that are not used for any memory modification. (These
  //    will be ignored later on.)
  //
  // Blocks containing only these kind of instructions are called independent
  // blocks as they can be scheduled arbitrarily.
  bool createIndependentBlocks(BasicBlock *BB, const Region *R);
  bool createIndependentBlocks(const Region *R);

  // Elimination on the Scop to eliminate the scalar dependences come with
  // trivially dead instructions.
  bool eliminateDeadCode(const Region *R);

  //===--------------------------------------------------------------------===//
  /// Non trivial scalar dependences checking functions.
  /// Non trivial scalar dependences occur when the def and use are located in
  /// different BBs and we can not move them into the same one. This will
  /// prevent use from schedule BBs arbitrarily.
  ///
  /// @brief This function checks if a scalar value that is part of the
  ///        Scop is used outside of the Scop.
  ///
  /// @param Use  The use of the instruction.
  /// @param R    The maximum region in the Scop.
  ///
  /// @return Return true if the Use of an instruction and the instruction
  ///         itself form a non trivial scalar dependence.
  static bool isEscapeUse(const Value *Use, const Region *R);

  //===--------------------------------------------------------------------===//
  /// Operand tree moving functions.
  /// Trivial scalar dependences can eliminate by move the def to the same BB
  /// that containing use.
  ///
  /// @brief Check if the instruction can be moved to another place safely.
  ///
  /// @param Inst The instruction.
  ///
  /// @return Return true if the instruction can be moved safely, false
  ///         otherwise.
  static bool isSafeToMove(Instruction *Inst);

  typedef std::map<Instruction *, Instruction *> ReplacedMapType;

  /// @brief Move all safe to move instructions in the Operand Tree (DAG) to
  ///        eliminate trivial scalar dependences.
  ///
  /// @param Inst         The root of the operand Tree.
  /// @param R            The maximum region in the Scop.
  /// @param ReplacedMap  The map that mapping original instruction to the moved
  ///                     instruction.
  /// @param InsertPos    The insert position of the moved instructions.
  void moveOperandTree(Instruction *Inst, const Region *R,
                       ReplacedMapType &ReplacedMap, Instruction *InsertPos);

  bool isIndependentBlock(const Region *R, BasicBlock *BB) const;
  bool areAllBlocksIndependent(const Region *R) const;

  bool runOnFunction(Function &F);
  void verifyAnalysis() const;
  void verifyScop(const Region *R) const;
  void getAnalysisUsage(AnalysisUsage &AU) const;
};
}

bool IndependentBlocks::isSafeToMove(Instruction *Inst) {
  if (Inst->mayReadFromMemory() || Inst->mayWriteToMemory())
    return false;

  return isSafeToSpeculativelyExecute(Inst);
}

void IndependentBlocks::moveOperandTree(Instruction *Inst, const Region *R,
                                        ReplacedMapType &ReplacedMap,
                                        Instruction *InsertPos) {
  BasicBlock *CurBB = Inst->getParent();

  // Depth first traverse the operand tree (or operand dag, because we will
  // stop at PHINodes, so there are no cycle).
  typedef Instruction::op_iterator ChildIt;
  std::vector<std::pair<Instruction *, ChildIt>> WorkStack;

  WorkStack.push_back(std::make_pair(Inst, Inst->op_begin()));
  DenseSet<Instruction *> VisitedSet;

  while (!WorkStack.empty()) {
    Instruction *CurInst = WorkStack.back().first;
    ChildIt It = WorkStack.back().second;
    DEBUG(dbgs() << "Checking Operand of Node:\n" << *CurInst << "\n------>\n");
    if (It == CurInst->op_end()) {
      // Insert the new instructions in topological order.
      if (!CurInst->getParent()) {
        CurInst->insertBefore(InsertPos);
        SE->forgetValue(CurInst);
      }

      WorkStack.pop_back();
    } else {
      // for each node N,
      Instruction *Operand = dyn_cast<Instruction>(*It);
      ++WorkStack.back().second;

      // Can not move no instruction value.
      if (Operand == 0)
        continue;

      DEBUG(dbgs() << "For Operand:\n" << *Operand << "\n--->");

      // If the Scop Region does not contain N, skip it and all its operands and
      // continue: because we reach a "parameter".
      // FIXME: we must keep the predicate instruction inside the Scop,
      // otherwise it will be translated to a load instruction, and we can not
      // handle load as affine predicate at this moment.
      if (!R->contains(Operand) && !isa<TerminatorInst>(CurInst)) {
        DEBUG(dbgs() << "Out of region.\n");
        continue;
      }

      if (canSynthesize(Operand, LI, SE, R)) {
        DEBUG(dbgs() << "is IV.\n");
        continue;
      }

      // We can not move the operand, a non trivial scalar dependence found!
      if (!isSafeToMove(Operand)) {
        DEBUG(dbgs() << "Can not move!\n");
        continue;
      }

      // Do not need to move instruction if it is contained in the same BB with
      // the root instruction.
      if (Operand->getParent() == CurBB) {
        DEBUG(dbgs() << "No need to move.\n");
        // Try to move its operand, but do not visit an instuction twice.
        if (VisitedSet.insert(Operand).second)
          WorkStack.push_back(std::make_pair(Operand, Operand->op_begin()));
        continue;
      }

      // Now we need to move Operand to CurBB.
      // Check if we already moved it.
      ReplacedMapType::iterator At = ReplacedMap.find(Operand);
      if (At != ReplacedMap.end()) {
        DEBUG(dbgs() << "Moved.\n");
        Instruction *MovedOp = At->second;
        It->set(MovedOp);
        SE->forgetValue(MovedOp);
      } else {
        // Note that NewOp is not inserted in any BB now, we will insert it when
        // it popped form the work stack, so it will be inserted in topological
        // order.
        Instruction *NewOp = Operand->clone();
        NewOp->setName(Operand->getName() + ".moved.to." + CurBB->getName());
        DEBUG(dbgs() << "Move to " << *NewOp << "\n");
        It->set(NewOp);
        ReplacedMap.insert(std::make_pair(Operand, NewOp));
        SE->forgetValue(Operand);

        // Process its operands, but do not visit an instuction twice.
        if (VisitedSet.insert(NewOp).second)
          WorkStack.push_back(std::make_pair(NewOp, NewOp->op_begin()));
      }
    }
  }

  SE->forgetValue(Inst);
}

bool IndependentBlocks::createIndependentBlocks(BasicBlock *BB,
                                                const Region *R) {
  std::vector<Instruction *> WorkList;
  for (Instruction &Inst : *BB)
    if (!isSafeToMove(&Inst) && !canSynthesize(&Inst, LI, SE, R))
      WorkList.push_back(&Inst);

  ReplacedMapType ReplacedMap;
  Instruction *InsertPos = BB->getFirstNonPHIOrDbg();

  for (Instruction *Inst : WorkList)
    if (!isa<PHINode>(Inst))
      moveOperandTree(Inst, R, ReplacedMap, InsertPos);

  // The BB was changed if we replaced any operand.
  return !ReplacedMap.empty();
}

bool IndependentBlocks::createIndependentBlocks(const Region *R) {
  bool Changed = false;

  for (BasicBlock *BB : R->blocks())
    Changed |= createIndependentBlocks(BB, R);

  return Changed;
}

bool IndependentBlocks::eliminateDeadCode(const Region *R) {
  std::vector<Instruction *> WorkList;

  // Find all trivially dead instructions.
  for (BasicBlock *BB : R->blocks())
    for (Instruction &Inst : *BB)
      if (!isIgnoredIntrinsic(&Inst) && isInstructionTriviallyDead(&Inst))
        WorkList.push_back(&Inst);

  if (WorkList.empty())
    return false;

  // Delete them so the cross BB scalar dependences come with them will
  // also be eliminated.
  while (!WorkList.empty()) {
    RecursivelyDeleteTriviallyDeadInstructions(WorkList.back());
    WorkList.pop_back();
  }

  return true;
}

bool IndependentBlocks::isEscapeUse(const Value *Use, const Region *R) {
  // Non-instruction user will never escape.
  if (!isa<Instruction>(Use))
    return false;

  return !R->contains(cast<Instruction>(Use));
}

bool IndependentBlocks::isIndependentBlock(const Region *R,
                                           BasicBlock *BB) const {
  for (Instruction &Inst : *BB) {
    if (canSynthesize(&Inst, LI, SE, R))
      continue;
    if (isIgnoredIntrinsic(&Inst))
      continue;

    // A value inside the Scop is referenced outside.
    for (User *U : Inst.users()) {
      if (isEscapeUse(U, R)) {
        DEBUG(dbgs() << "Instruction not independent:\n");
        DEBUG(dbgs() << "Instruction used outside the Scop!\n");
        DEBUG(Inst.print(dbgs()));
        DEBUG(dbgs() << "\n");
        return false;
      }
    }
  }

  return true;
}

bool IndependentBlocks::areAllBlocksIndependent(const Region *R) const {
  for (BasicBlock *BB : R->blocks())
    if (!isIndependentBlock(R, BB))
      return false;

  return true;
}

void IndependentBlocks::getAnalysisUsage(AnalysisUsage &AU) const {
  // FIXME: If we set preserves cfg, the cfg only passes do not need to
  // be "addPreserved"?
  AU.addPreserved<AAResultsWrapperPass>();
  AU.addPreserved<BasicAAWrapperPass>();
  AU.addPreserved<DominatorTreeWrapperPass>();
  AU.addPreserved<DominanceFrontier>();
  AU.addPreserved<GlobalsAAWrapperPass>();
  AU.addPreserved<PostDominatorTree>();
  AU.addRequired<RegionInfoPass>();
  AU.addPreserved<RegionInfoPass>();
  AU.addRequired<LoopInfoWrapperPass>();
  AU.addPreserved<LoopInfoWrapperPass>();
  AU.addRequired<ScalarEvolutionWrapperPass>();
  AU.addPreserved<ScalarEvolutionWrapperPass>();
  AU.addPreserved<SCEVAAWrapperPass>();
  AU.addRequired<ScopDetection>();
  AU.addPreserved<ScopDetection>();
}

bool IndependentBlocks::runOnFunction(llvm::Function &F) {

  bool Changed = false;

  RI = &getAnalysis<RegionInfoPass>().getRegionInfo();
  LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
  SD = &getAnalysis<ScopDetection>();
  SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE();

  AllocaBlock = &F.getEntryBlock();

  DEBUG(dbgs() << "Run IndepBlock on " << F.getName() << '\n');

  for (const Region *R : *SD) {
    Changed |= createIndependentBlocks(R);
    Changed |= eliminateDeadCode(R);
  }

  verifyAnalysis();

  return Changed;
}

void IndependentBlocks::verifyAnalysis() const {}

void IndependentBlocks::verifyScop(const Region *R) const {
  assert(areAllBlocksIndependent(R) && "Cannot generate independent blocks");
}

char IndependentBlocks::ID = 0;
char &polly::IndependentBlocksID = IndependentBlocks::ID;

Pass *polly::createIndependentBlocksPass() { return new IndependentBlocks(); }

INITIALIZE_PASS_BEGIN(IndependentBlocks, "polly-independent",
                      "Polly - Create independent blocks", false, false);
INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass);
INITIALIZE_PASS_DEPENDENCY(RegionInfoPass);
INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass);
INITIALIZE_PASS_DEPENDENCY(ScopDetection);
INITIALIZE_PASS_END(IndependentBlocks, "polly-independent",
                    "Polly - Create independent blocks", false, false)