//===- IVUsers.cpp - Induction Variable Users -------------------*- C++ -*-===//

//

// 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 bookkeeping for "interesting" users of expressions

// computed from induction variables.

//

//===----------------------------------------------------------------------===//

#include "llvm/Analysis/IVUsers.h"

#include "llvm/Analysis/AssumptionCache.h"

#include "llvm/Analysis/CodeMetrics.h"

#include "llvm/Analysis/LoopAnalysisManager.h"

#include "llvm/Analysis/LoopInfo.h"

#include "llvm/Analysis/LoopPass.h"

#include "llvm/Analysis/ScalarEvolutionExpressions.h"

#include "llvm/Analysis/ValueTracking.h"

#include "llvm/Config/llvm-config.h"

#include "llvm/IR/DataLayout.h"

#include "llvm/IR/Dominators.h"

#include "llvm/IR/Instructions.h"

#include "llvm/IR/Module.h"

#include "llvm/InitializePasses.h"

#include "llvm/Support/Debug.h"

#include "llvm/Support/raw_ostream.h"

using namespace llvm;

#define DEBUG_TYPE "iv-users"

AnalysisKey IVUsersAnalysis::Key;

IVUsers IVUsersAnalysis::run(Loop &L, LoopAnalysisManager &AM,

                             LoopStandardAnalysisResults &AR) {

  return IVUsers(&L, &AR.AC, &AR.LI, &AR.DT, &AR.SE);

}

char IVUsersWrapperPass::ID = 0;

INITIALIZE_PASS_BEGIN(IVUsersWrapperPass, "iv-users",

                      "Induction Variable Users", false, true)

INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)

INITIALIZE_PASS_DEPENDENCY(LoopInfoWrapperPass)

INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)

INITIALIZE_PASS_DEPENDENCY(ScalarEvolutionWrapperPass)

INITIALIZE_PASS_END(IVUsersWrapperPass, "iv-users", "Induction Variable Users",

                    false, true)

Pass *llvm::createIVUsersPass() { return new IVUsersWrapperPass(); }

/// isInteresting - Test whether the given expression is "interesting" when

/// used by the given expression, within the context of analyzing the

/// given loop.

static bool isInteresting(const SCEV *S, const Instruction *I, const Loop *L,

                          ScalarEvolution *SE, LoopInfo *LI) {

  // An addrec is interesting if it's affine or if it has an interesting start.

  if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {

    // Keep things simple. Don't touch loop-variant strides unless they're

    // only used outside the loop and we can simplify them.

    if (AR->getLoop() == L)

      return AR->isAffine() ||

             (!L->contains(I) &&

              SE->getSCEVAtScope(AR, LI->getLoopFor(I->getParent())) != AR);

    // Otherwise recurse to see if the start value is interesting, and that

    // the step value is not interesting, since we don't yet know how to

    // do effective SCEV expansions for addrecs with interesting steps.

    return isInteresting(AR->getStart(), I, L, SE, LI) &&

          !isInteresting(AR->getStepRecurrence(*SE), I, L, SE, LI);

  }

  // An add is interesting if exactly one of its operands is interesting.

  if (const SCEVAddExpr *Add = dyn_cast<SCEVAddExpr>(S)) {

    bool AnyInterestingYet = false;

    for (const auto *Op : Add->operands())

      if (isInteresting(Op, I, L, SE, LI)) {

        if (AnyInterestingYet)

          return false;

        AnyInterestingYet = true;

      }

    return AnyInterestingYet;

  }

  // Nothing else is interesting here.

  return false;

}

/// IVUseShouldUsePostIncValue - We have discovered a "User" of an IV expression

/// and now we need to decide whether the user should use the preinc or post-inc

/// value.  If this user should use the post-inc version of the IV, return true.

///

/// Choosing wrong here can break dominance properties (if we choose to use the

/// post-inc value when we cannot) or it can end up adding extra live-ranges to

/// the loop, resulting in reg-reg copies (if we use the pre-inc value when we

/// should use the post-inc value).

static bool IVUseShouldUsePostIncValue(Instruction *User, Value *Operand,

                                       const Loop *L, DominatorTree *DT) {

  // If the user is in the loop, use the preinc value.

  if (L->contains(User))

    return false;

  BasicBlock *LatchBlock = L->getLoopLatch();

  if (!LatchBlock)

    return false;

  // Ok, the user is outside of the loop.  If it is dominated by the latch

  // block, use the post-inc value.

  if (DT->dominates(LatchBlock, User->getParent()))

    return true;

  // There is one case we have to be careful of: PHI nodes.  These little guys

  // can live in blocks that are not dominated by the latch block, but (since

  // their uses occur in the predecessor block, not the block the PHI lives in)

  // should still use the post-inc value.  Check for this case now.

  PHINode *PN = dyn_cast<PHINode>(User);

  if (!PN || !Operand)

    return false; // not a phi, not dominated by latch block.

  // Look at all of the uses of Operand by the PHI node.  If any use corresponds

  // to a block that is not dominated by the latch block, give up and use the

  // preincremented value.

  for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)

    if (PN->getIncomingValue(i) == Operand &&

        !DT->dominates(LatchBlock, PN->getIncomingBlock(i)))

      return false;

  // Okay, all uses of Operand by PN are in predecessor blocks that really are

  // dominated by the latch block.  Use the post-incremented value.

  return true;

}

/// Inspect the specified instruction.  If it is a reducible SCEV, recursively

/// add its users to the IVUsesByStride set and return true.  Otherwise, return

/// false.

bool IVUsers::AddUsersIfInteresting(Instruction *I) {

  const DataLayout &DL = I->getModule()->getDataLayout();

  // Add this IV user to the Processed set before returning false to ensure that

  // all IV users are members of the set. See IVUsers::isIVUserOrOperand.

  if (!Processed.insert(I).second)

    return true;    // Instruction already handled.

  if (!SE->isSCEVable(I->getType()))

    return false;   // Void and FP expressions cannot be reduced.

  // IVUsers is used by LSR which assumes that all SCEV expressions are safe to

  // pass to SCEVExpander. Expressions are not safe to expand if they represent

  // operations that are not safe to speculate, namely integer division.

  if (!isa<PHINode>(I) && !isSafeToSpeculativelyExecute(I))

    return false;

  // LSR is not APInt clean, do not touch integers bigger than 64-bits.

  // Also avoid creating IVs of non-native types. For example, we don't want a

  // 64-bit IV in 32-bit code just because the loop has one 64-bit cast.

  uint64_t Width = SE->getTypeSizeInBits(I->getType());

  if (Width > 64 || !DL.isLegalInteger(Width))

    return false;

  // Don't attempt to promote ephemeral values to indvars. They will be removed

  // later anyway.

  if (EphValues.count(I))

    return false;

  // Get the symbolic expression for this instruction.

  const SCEV *ISE = SE->getSCEV(I);

  // If we've come to an uninteresting expression, stop the traversal and

  // call this a user.

  if (!isInteresting(ISE, I, L, SE, LI))

    return false;

  SmallPtrSet<Instruction *, 4> UniqueUsers;

  for (Use &U : I->uses()) {

    Instruction *User = cast<Instruction>(U.getUser());

    if (!UniqueUsers.insert(User).second)

      continue;

    // Do not infinitely recurse on PHI nodes.

    if (isa<PHINode>(User) && Processed.count(User))

      continue;

    // Descend recursively, but not into PHI nodes outside the current loop.

    // It's important to see the entire expression outside the loop to get

    // choices that depend on addressing mode use right, although we won't

    // consider references outside the loop in all cases.

    // If User is already in Processed, we don't want to recurse into it again,

    // but do want to record a second reference in the same instruction.

    bool AddUserToIVUsers = false;

    if (LI->getLoopFor(User->getParent()) != L) {

      if (isa<PHINode>(User) || Processed.count(User) ||

          !AddUsersIfInteresting(User)) {

        LLVM_DEBUG(dbgs() << "FOUND USER in other loop: " << *User << '\n'

                          << "   OF SCEV: " << *ISE << '\n');

        AddUserToIVUsers = true;

      }

    } else if (Processed.count(User) || !AddUsersIfInteresting(User)) {

      LLVM_DEBUG(dbgs() << "FOUND USER: " << *User << '\n'

                        << "   OF SCEV: " << *ISE << '\n');

      AddUserToIVUsers = true;

    }

    if (AddUserToIVUsers) {

      // Okay, we found a user that we cannot reduce.

      IVStrideUse &NewUse = AddUser(User, I);

      // Autodetect the post-inc loop set, populating NewUse.PostIncLoops.

      // The regular return value here is discarded; instead of recording

      // it, we just recompute it when we need it.

      const SCEV *OriginalISE = ISE;

      auto NormalizePred = [&](const SCEVAddRecExpr *AR) {

        auto *L = AR->getLoop();

        bool Result = IVUseShouldUsePostIncValue(User, I, L, DT);

        if (Result)

          NewUse.PostIncLoops.insert(L);

        return Result;

      };

      ISE = normalizeForPostIncUseIf(ISE, NormalizePred, *SE);

      // PostIncNormalization effectively simplifies the expression under

      // pre-increment assumptions. Those assumptions (no wrapping) might not

      // hold for the post-inc value. Catch such cases by making sure the

      // transformation is invertible.

      if (OriginalISE != ISE) {

        const SCEV *DenormalizedISE =

            denormalizeForPostIncUse(ISE, NewUse.PostIncLoops, *SE);

        // If we normalized the expression, but denormalization doesn't give the

        // original one, discard this user.

        if (OriginalISE != DenormalizedISE) {

          LLVM_DEBUG(dbgs()

                     << "   DISCARDING (NORMALIZATION ISN'T INVERTIBLE): "

                     << *ISE << '\n');

          IVUses.pop_back();

          return false;

        }

      }

      LLVM_DEBUG(if (SE->getSCEV(I) != ISE) dbgs()

                 << "   NORMALIZED TO: " << *ISE << '\n');

    }

  }

  return true;

}

IVStrideUse &IVUsers::AddUser(Instruction *User, Value *Operand) {

  IVUses.push_back(new IVStrideUse(this, User, Operand));

  return IVUses.back();

}

IVUsers::IVUsers(Loop *L, AssumptionCache *AC, LoopInfo *LI, DominatorTree *DT,

                 ScalarEvolution *SE)

    : L(L), AC(AC), LI(LI), DT(DT), SE(SE) {

  // Collect ephemeral values so that AddUsersIfInteresting skips them.

  EphValues.clear();

  CodeMetrics::collectEphemeralValues(L, AC, EphValues);

  // Find all uses of induction variables in this loop, and categorize

  // them by stride.  Start by finding all of the PHI nodes in the header for

  // this loop.  If they are induction variables, inspect their uses.

  for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ++I)

    (void)AddUsersIfInteresting(&*I);

}

void IVUsers::print(raw_ostream &OS, const Module *M) const {

  OS << "IV Users for loop ";

  L->getHeader()->printAsOperand(OS, false);

  if (SE->hasLoopInvariantBackedgeTakenCount(L)) {

    OS << " with backedge-taken count " << *SE->getBackedgeTakenCount(L);

  }

  OS << ":\n";

  for (const IVStrideUse &IVUse : IVUses) {

    OS << "  ";

    IVUse.getOperandValToReplace()->printAsOperand(OS, false);

    OS << " = " << *getReplacementExpr(IVUse);

    for (const auto *PostIncLoop : IVUse.PostIncLoops) {

      OS << " (post-inc with loop ";

      PostIncLoop->getHeader()->printAsOperand(OS, false);

      OS << ")";

    }

    OS << " in  ";

    if (IVUse.getUser())

      IVUse.getUser()->print(OS);

    else

      OS << "Printing <null> User";

    OS << '\n';

  }

}

#if !defined(NDEBUG) || defined(LLVM_ENABLE_DUMP)

LLVM_DUMP_METHOD void IVUsers::dump() const { print(dbgs()); }

#endif

void IVUsers::releaseMemory() {

  Processed.clear();

  IVUses.clear();

}

IVUsersWrapperPass::IVUsersWrapperPass() : LoopPass(ID) {

  initializeIVUsersWrapperPassPass(*PassRegistry::getPassRegistry());

}

void IVUsersWrapperPass::getAnalysisUsage(AnalysisUsage &AU) const {

  AU.addRequired<AssumptionCacheTracker>();

  AU.addRequired<LoopInfoWrapperPass>();

  AU.addRequired<DominatorTreeWrapperPass>();

  AU.addRequired<ScalarEvolutionWrapperPass>();

  AU.setPreservesAll();

}

bool IVUsersWrapperPass::runOnLoop(Loop *L, LPPassManager &LPM) {

  auto *AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(

      *L->getHeader()->getParent());

  auto *LI = &getAnalysis<LoopInfoWrapperPass>().getLoopInfo();

  auto *DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();

  auto *SE = &getAnalysis<ScalarEvolutionWrapperPass>().getSE();

  IU.reset(new IVUsers(L, AC, LI, DT, SE));

  return false;

}

void IVUsersWrapperPass::print(raw_ostream &OS, const Module *M) const {

  IU->print(OS, M);

}

void IVUsersWrapperPass::releaseMemory() { IU->releaseMemory(); }

/// getReplacementExpr - Return a SCEV expression which computes the

/// value of the OperandValToReplace.

const SCEV *IVUsers::getReplacementExpr(const IVStrideUse &IU) const {

  return SE->getSCEV(IU.getOperandValToReplace());

}

/// getExpr - Return the expression for the use.

const SCEV *IVUsers::getExpr(const IVStrideUse &IU) const {

  const SCEV *Replacement = getReplacementExpr(IU);

  return normalizeForPostIncUse(Replacement, IU.getPostIncLoops(), *SE);

}

static const SCEVAddRecExpr *findAddRecForLoop(const SCEV *S, const Loop *L) {

  if (const SCEVAddRecExpr *AR = dyn_cast<SCEVAddRecExpr>(S)) {

    if (AR->getLoop() == L)

      return AR;

    return findAddRecForLoop(AR->getStart(), L);

  }

  if (const SCEVAddExpr *Add = dyn_cast<SCEVAddExpr>(S)) {

    for (const auto *Op : Add->operands())

      if (const SCEVAddRecExpr *AR = findAddRecForLoop(Op, L))

        return AR;

    return nullptr;

  }

  return nullptr;

}

const SCEV *IVUsers::getStride(const IVStrideUse &IU, const Loop *L) const {

  const SCEV *Expr = getExpr(IU);

  if (!Expr)

    return nullptr;

  if (const SCEVAddRecExpr *AR = findAddRecForLoop(Expr, L))

    return AR->getStepRecurrence(*SE);

  return nullptr;

}

void IVStrideUse::transformToPostInc(const Loop *L) {

  PostIncLoops.insert(L);

}

void IVStrideUse::deleted() {

  // Remove this user from the list.

  Parent->Processed.erase(this->getUser());

  Parent->IVUses.erase(this);

  // this now dangles!

}