lib/Analysis/LoopUnrollAnalyzer.cpp

//===- LoopUnrollAnalyzer.cpp - Unrolling Effect Estimation -----*- 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 UnrolledInstAnalyzer class. It's used for predicting
// potential effects that loop unrolling might have, such as enabling constant
// propagation and other optimizations.
//
//===----------------------------------------------------------------------===//

#include "llvm/Analysis/LoopUnrollAnalyzer.h"
#include "llvm/Analysis/LoopInfo.h"

using namespace llvm;

/// Try to simplify instruction \param I using its SCEV expression.
///
/// The idea is that some AddRec expressions become constants, which then
/// could trigger folding of other instructions. However, that only happens
/// for expressions whose start value is also constant, which isn't always the
/// case. In another common and important case the start value is just some
/// address (i.e. SCEVUnknown) - in this case we compute the offset and save
/// it along with the base address instead.
bool UnrolledInstAnalyzer::simplifyInstWithSCEV(Instruction *I) {
  if (!SE.isSCEVable(I->getType()))
    return false;

  const SCEV *S = SE.getSCEV(I);
  if (auto *SC = dyn_cast<SCEVConstant>(S)) {
    SimplifiedValues[I] = SC->getValue();
    return true;
  }

  // If we have a loop invariant computation, we only need to compute it once.
  // Given that, all but the first occurance are free.
  if (!IterationNumber->isZero() && SE.isLoopInvariant(S, L))
    return true;

  auto *AR = dyn_cast<SCEVAddRecExpr>(S);
  if (!AR || AR->getLoop() != L)
    return false;

  const SCEV *ValueAtIteration = AR->evaluateAtIteration(IterationNumber, SE);
  // Check if the AddRec expression becomes a constant.
  if (auto *SC = dyn_cast<SCEVConstant>(ValueAtIteration)) {
    SimplifiedValues[I] = SC->getValue();
    return true;
  }

  // Check if the offset from the base address becomes a constant.
  auto *Base = dyn_cast<SCEVUnknown>(SE.getPointerBase(S));
  if (!Base)
    return false;
  auto *Offset =
      dyn_cast<SCEVConstant>(SE.getMinusSCEV(ValueAtIteration, Base));
  if (!Offset)
    return false;
  SimplifiedAddress Address;
  Address.Base = Base->getValue();
  Address.Offset = Offset->getValue();
  SimplifiedAddresses[I] = Address;
  return false;
}

/// Try to simplify binary operator I.
///
/// TODO: Probably it's worth to hoist the code for estimating the
/// simplifications effects to a separate class, since we have a very similar
/// code in InlineCost already.
bool UnrolledInstAnalyzer::visitBinaryOperator(BinaryOperator &I) {
  Value *LHS = I.getOperand(0), *RHS = I.getOperand(1);
  if (!isa<Constant>(LHS))
    if (Constant *SimpleLHS = SimplifiedValues.lookup(LHS))
      LHS = SimpleLHS;
  if (!isa<Constant>(RHS))
    if (Constant *SimpleRHS = SimplifiedValues.lookup(RHS))
      RHS = SimpleRHS;

  Value *SimpleV = nullptr;
  const DataLayout &DL = I.getModule()->getDataLayout();
  if (auto FI = dyn_cast<FPMathOperator>(&I))
    SimpleV =
        SimplifyBinOp(I.getOpcode(), LHS, RHS, FI->getFastMathFlags(), DL);
  else
    SimpleV = SimplifyBinOp(I.getOpcode(), LHS, RHS, DL);

  if (Constant *C = dyn_cast_or_null<Constant>(SimpleV))
    SimplifiedValues[&I] = C;

  if (SimpleV)
    return true;
  return Base::visitBinaryOperator(I);
}

/// Try to fold load I.
bool UnrolledInstAnalyzer::visitLoad(LoadInst &I) {
  Value *AddrOp = I.getPointerOperand();

  auto AddressIt = SimplifiedAddresses.find(AddrOp);
  if (AddressIt == SimplifiedAddresses.end())
    return false;
  ConstantInt *SimplifiedAddrOp = AddressIt->second.Offset;

  auto *GV = dyn_cast<GlobalVariable>(AddressIt->second.Base);
  // We're only interested in loads that can be completely folded to a
  // constant.
  if (!GV || !GV->hasDefinitiveInitializer() || !GV->isConstant())
    return false;

  ConstantDataSequential *CDS =
      dyn_cast<ConstantDataSequential>(GV->getInitializer());
  if (!CDS)
    return false;

  // We might have a vector load from an array. FIXME: for now we just bail
  // out in this case, but we should be able to resolve and simplify such
  // loads.
  if (CDS->getElementType() != I.getType())
    return false;

  unsigned ElemSize = CDS->getElementType()->getPrimitiveSizeInBits() / 8U;
  if (SimplifiedAddrOp->getValue().getActiveBits() > 64)
    return false;
  int64_t SimplifiedAddrOpV = SimplifiedAddrOp->getSExtValue();
  if (SimplifiedAddrOpV < 0) {
    // FIXME: For now we conservatively ignore out of bound accesses, but
    // we're allowed to perform the optimization in this case.
    return false;
  }
  uint64_t Index = static_cast<uint64_t>(SimplifiedAddrOpV) / ElemSize;
  if (Index >= CDS->getNumElements()) {
    // FIXME: For now we conservatively ignore out of bound accesses, but
    // we're allowed to perform the optimization in this case.
    return false;
  }

  Constant *CV = CDS->getElementAsConstant(Index);
  assert(CV && "Constant expected.");
  SimplifiedValues[&I] = CV;

  return true;
}

/// Try to simplify cast instruction.
bool UnrolledInstAnalyzer::visitCastInst(CastInst &I) {
  // Propagate constants through casts.
  Constant *COp = dyn_cast<Constant>(I.getOperand(0));
  if (!COp)
    COp = SimplifiedValues.lookup(I.getOperand(0));

  // If we know a simplified value for this operand and cast is valid, save the
  // result to SimplifiedValues.
  // The cast can be invalid, because SimplifiedValues contains results of SCEV
  // analysis, which operates on integers (and, e.g., might convert i8* null to
  // i32 0).
  if (COp && CastInst::castIsValid(I.getOpcode(), COp, I.getType())) {
    if (Constant *C =
            ConstantExpr::getCast(I.getOpcode(), COp, I.getType())) {
      SimplifiedValues[&I] = C;
      return true;
    }
  }

  return Base::visitCastInst(I);
}

/// Try to simplify cmp instruction.
bool UnrolledInstAnalyzer::visitCmpInst(CmpInst &I) {
  Value *LHS = I.getOperand(0), *RHS = I.getOperand(1);

  // First try to handle simplified comparisons.
  if (!isa<Constant>(LHS))
    if (Constant *SimpleLHS = SimplifiedValues.lookup(LHS))
      LHS = SimpleLHS;
  if (!isa<Constant>(RHS))
    if (Constant *SimpleRHS = SimplifiedValues.lookup(RHS))
      RHS = SimpleRHS;

  if (!isa<Constant>(LHS) && !isa<Constant>(RHS)) {
    auto SimplifiedLHS = SimplifiedAddresses.find(LHS);
    if (SimplifiedLHS != SimplifiedAddresses.end()) {
      auto SimplifiedRHS = SimplifiedAddresses.find(RHS);
      if (SimplifiedRHS != SimplifiedAddresses.end()) {
        SimplifiedAddress &LHSAddr = SimplifiedLHS->second;
        SimplifiedAddress &RHSAddr = SimplifiedRHS->second;
        if (LHSAddr.Base == RHSAddr.Base) {
          LHS = LHSAddr.Offset;
          RHS = RHSAddr.Offset;
        }
      }
    }
  }

  if (Constant *CLHS = dyn_cast<Constant>(LHS)) {
    if (Constant *CRHS = dyn_cast<Constant>(RHS)) {
      if (CLHS->getType() == CRHS->getType()) {
        if (Constant *C = ConstantExpr::getCompare(I.getPredicate(), CLHS, CRHS)) {
          SimplifiedValues[&I] = C;
          return true;
        }
      }
    }
  }

  return Base::visitCmpInst(I);
}

bool UnrolledInstAnalyzer::visitPHINode(PHINode &PN) {
  // Run base visitor first. This way we can gather some useful for later
  // analysis information.
  if (Base::visitPHINode(PN))
    return true;

  // The loop induction PHI nodes are definitionally free.
  return PN.getParent() == L->getHeader();
}