llvm/lib/Transforms/Scalar/Float2Int.cpp

//===- Float2Int.cpp - Demote floating point ops to work on integers ------===//
//
// 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 the Float2Int pass, which aims to demote floating
// point operations to work on integers, where that is losslessly possible.
//
//===----------------------------------------------------------------------===//

#include "llvm/Transforms/Scalar/Float2Int.h"
#include "ScalarOptions.h"
#include "llvm/ADT/APInt.h"
#include "llvm/ADT/APSInt.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/Analysis/GlobalsModRef.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/Module.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include <deque>

#define DEBUG_TYPE "float2int"

using namespace llvm;

// The algorithm is simple. Start at instructions that convert from the
// float to the int domain: fptoui, fptosi and fcmp. Walk up the def-use
// graph, using an equivalence datastructure to unify graphs that interfere.
//
// Mappable instructions are those with an integer corrollary that, given
// integer domain inputs, produce an integer output; fadd, for example.
//
// If a non-mappable instruction is seen, this entire def-use graph is marked
// as non-transformable. If we see an instruction that converts from the
// integer domain to FP domain (uitofp,sitofp), we terminate our walk.

// Given a FCmp predicate, return a matching ICmp predicate if one
// exists, otherwise return BAD_ICMP_PREDICATE.
static CmpInst::Predicate mapFCmpPred(CmpInst::Predicate P) {
  switch (P) {
  case CmpInst::FCMP_OEQ:
  case CmpInst::FCMP_UEQ:
    return CmpInst::ICMP_EQ;
  case CmpInst::FCMP_OGT:
  case CmpInst::FCMP_UGT:
    return CmpInst::ICMP_SGT;
  case CmpInst::FCMP_OGE:
  case CmpInst::FCMP_UGE:
    return CmpInst::ICMP_SGE;
  case CmpInst::FCMP_OLT:
  case CmpInst::FCMP_ULT:
    return CmpInst::ICMP_SLT;
  case CmpInst::FCMP_OLE:
  case CmpInst::FCMP_ULE:
    return CmpInst::ICMP_SLE;
  case CmpInst::FCMP_ONE:
  case CmpInst::FCMP_UNE:
    return CmpInst::ICMP_NE;
  default:
    return CmpInst::BAD_ICMP_PREDICATE;
  }
}

// Given a floating point binary operator, return the matching
// integer version.
static Instruction::BinaryOps mapBinOpcode(unsigned Opcode) {
  switch (Opcode) {
  default: llvm_unreachable("Unhandled opcode!");
  case Instruction::FAdd: return Instruction::Add;
  case Instruction::FSub: return Instruction::Sub;
  case Instruction::FMul: return Instruction::Mul;
  }
}

// Find the roots - instructions that convert from the FP domain to
// integer domain.
void Float2IntPass::findRoots(Function &F, const DominatorTree &DT) {
  for (BasicBlock &BB : F) {
    // Unreachable code can take on strange forms that we are not prepared to
    // handle. For example, an instruction may have itself as an operand.
    if (!DT.isReachableFromEntry(&BB))
      continue;

    for (Instruction &I : BB) {
      if (isa<VectorType>(I.getType()))
        continue;
      switch (I.getOpcode()) {
      default: break;
      case Instruction::FPToUI:
      case Instruction::FPToSI:
        Roots.insert(&I);
        break;
      case Instruction::FCmp:
        if (mapFCmpPred(cast<CmpInst>(&I)->getPredicate()) !=
            CmpInst::BAD_ICMP_PREDICATE)
          Roots.insert(&I);
        break;
      }
    }
  }
}

// Helper - mark I as having been traversed, having range R.
void Float2IntPass::seen(Instruction *I, ConstantRange R) {
  LLVM_DEBUG(dbgs() << "F2I: " << *I << ":" << R << "\n");
  SeenInsts.insert_or_assign(I, std::move(R));
}

// Helper - get a range representing a poison value.
ConstantRange Float2IntPass::badRange() {
  return ConstantRange::getFull(MaxIntegerBW + 1);
}
ConstantRange Float2IntPass::unknownRange() {
  return ConstantRange::getEmpty(MaxIntegerBW + 1);
}
ConstantRange Float2IntPass::validateRange(ConstantRange R) {
  if (R.getBitWidth() > MaxIntegerBW + 1)
    return badRange();
  return R;
}

// The most obvious way to structure the search is a depth-first, eager
// search from each root. However, that require direct recursion and so
// can only handle small instruction sequences. Instead, we split the search
// up into two phases:
//   - walkBackwards:  A breadth-first walk of the use-def graph starting from
//                     the roots. Populate "SeenInsts" with interesting
//                     instructions and poison values if they're obvious and
//                     cheap to compute. Calculate the equivalance set structure
//                     while we're here too.
//   - walkForwards:  Iterate over SeenInsts in reverse order, so we visit
//                     defs before their uses. Calculate the real range info.

// Breadth-first walk of the use-def graph; determine the set of nodes
// we care about and eagerly determine if some of them are poisonous.
void Float2IntPass::walkBackwards() {
  std::deque<Instruction*> Worklist(Roots.begin(), Roots.end());
  while (!Worklist.empty()) {
    Instruction *I = Worklist.back();
    Worklist.pop_back();

    if (SeenInsts.contains(I))
      // Seen already.
      continue;

    switch (I->getOpcode()) {
      // FIXME: Handle select and phi nodes.
    default:
      // Path terminated uncleanly.
      seen(I, badRange());
      break;

    case Instruction::UIToFP:
    case Instruction::SIToFP: {
      // Path terminated cleanly - use the type of the integer input to seed
      // the analysis.
      unsigned BW = I->getOperand(0)->getType()->getPrimitiveSizeInBits();
      auto Input = ConstantRange::getFull(BW);
      auto CastOp = (Instruction::CastOps)I->getOpcode();
      seen(I, validateRange(Input.castOp(CastOp, MaxIntegerBW+1)));
      continue;
    }

    case Instruction::FNeg:
    case Instruction::FAdd:
    case Instruction::FSub:
    case Instruction::FMul:
    case Instruction::FPToUI:
    case Instruction::FPToSI:
    case Instruction::FCmp:
      seen(I, unknownRange());
      break;
    }

    for (Value *O : I->operands()) {
      if (Instruction *OI = dyn_cast<Instruction>(O)) {
        // Unify def-use chains if they interfere.
        ECs.unionSets(I, OI);
        if (SeenInsts.find(I)->second != badRange())
          Worklist.push_back(OI);
      } else if (!isa<ConstantFP>(O)) {
        // Not an instruction or ConstantFP? we can't do anything.
        seen(I, badRange());
      }
    }
  }
}

// Calculate result range from operand ranges.
// Return std::nullopt if the range cannot be calculated yet.
std::optional<ConstantRange> Float2IntPass::calcRange(Instruction *I) {
  SmallVector<ConstantRange, 4> OpRanges;
  for (Value *O : I->operands()) {
    if (Instruction *OI = dyn_cast<Instruction>(O)) {
      auto OpIt = SeenInsts.find(OI);
      assert(OpIt != SeenInsts.end() && "def not seen before use!");
      if (OpIt->second == unknownRange())
        return std::nullopt; // Wait until operand range has been calculated.
      OpRanges.push_back(OpIt->second);
    } else if (ConstantFP *CF = dyn_cast<ConstantFP>(O)) {
      // Work out if the floating point number can be losslessly represented
      // as an integer.
      // APFloat::convertToInteger(&Exact) purports to do what we want, but
      // the exactness can be too precise. For example, negative zero can
      // never be exactly converted to an integer.
      //
      // Instead, we ask APFloat to round itself to an integral value - this
      // preserves sign-of-zero - then compare the result with the original.
      //
      const APFloat &F = CF->getValueAPF();

      // First, weed out obviously incorrect values. Non-finite numbers
      // can't be represented and neither can negative zero, unless
      // we're in fast math mode.
      if (!F.isFinite() ||
          (F.isZero() && F.isNegative() && isa<FPMathOperator>(I) &&
           !I->hasNoSignedZeros()))
        return badRange();

      APFloat NewF = F;
      auto Res = NewF.roundToIntegral(APFloat::rmNearestTiesToEven);
      if (Res != APFloat::opOK || NewF != F)
        return badRange();

      // OK, it's representable. Now get it.
      APSInt Int(MaxIntegerBW+1, false);
      bool Exact;
      APFloat::opStatus Status = CF->getValueAPF().convertToInteger(
          Int, APFloat::rmNearestTiesToEven, &Exact);
      // Although the round above is loseless, we still need to check if the
      // floating-point value can be represented in the integer type.
      if (Status == APFloat::opOK || Status == APFloat::opInexact)
        OpRanges.push_back(ConstantRange(Int));
      else
        return badRange();
    } else {
      llvm_unreachable("Should have already marked this as badRange!");
    }
  }

  switch (I->getOpcode()) {
  // FIXME: Handle select and phi nodes.
  default:
  case Instruction::UIToFP:
  case Instruction::SIToFP:
    llvm_unreachable("Should have been handled in walkForwards!");

  case Instruction::FNeg: {
    assert(OpRanges.size() == 1 && "FNeg is a unary operator!");
    unsigned Size = OpRanges[0].getBitWidth();
    auto Zero = ConstantRange(APInt::getZero(Size));
    return Zero.sub(OpRanges[0]);
  }

  case Instruction::FAdd:
  case Instruction::FSub:
  case Instruction::FMul: {
    assert(OpRanges.size() == 2 && "its a binary operator!");
    auto BinOp = (Instruction::BinaryOps) I->getOpcode();
    return OpRanges[0].binaryOp(BinOp, OpRanges[1]);
  }

  //
  // Root-only instructions - we'll only see these if they're the
  //                          first node in a walk.
  //
  case Instruction::FPToUI:
  case Instruction::FPToSI: {
    assert(OpRanges.size() == 1 && "FPTo[US]I is a unary operator!");
    // Note: We're ignoring the casts output size here as that's what the
    // caller expects.
    auto CastOp = (Instruction::CastOps)I->getOpcode();
    return OpRanges[0].castOp(CastOp, MaxIntegerBW+1);
  }

  case Instruction::FCmp:
    assert(OpRanges.size() == 2 && "FCmp is a binary operator!");
    return OpRanges[0].unionWith(OpRanges[1]);
  }
}

// Walk forwards down the list of seen instructions, so we visit defs before
// uses.
void Float2IntPass::walkForwards() {
  std::deque<Instruction *> Worklist;
  for (const auto &Pair : SeenInsts)
    if (Pair.second == unknownRange())
      Worklist.push_back(Pair.first);

  while (!Worklist.empty()) {
    Instruction *I = Worklist.back();
    Worklist.pop_back();

    if (std::optional<ConstantRange> Range = calcRange(I))
      seen(I, *Range);
    else
      Worklist.push_front(I); // Reprocess later.
  }
}

// If there is a valid transform to be done, do it.
bool Float2IntPass::validateAndTransform(const DataLayout &DL) {
  bool MadeChange = false;

  // Iterate over every disjoint partition of the def-use graph.
  for (const auto &E : ECs) {
    if (!E->isLeader())
      continue;

    ConstantRange R(MaxIntegerBW + 1, false);
    bool Fail = false;
    Type *ConvertedToTy = nullptr;

    // For every member of the partition, union all the ranges together.
    for (Instruction *I : ECs.members(*E)) {
      auto *SeenI = SeenInsts.find(I);
      if (SeenI == SeenInsts.end())
        continue;

      R = R.unionWith(SeenI->second);
      // We need to ensure I has no users that have not been seen.
      // If it does, transformation would be illegal.
      //
      // Don't count the roots, as they terminate the graphs.
      if (!Roots.contains(I)) {
        // Set the type of the conversion while we're here.
        if (!ConvertedToTy)
          ConvertedToTy = I->getType();
        for (User *U : I->users()) {
          Instruction *UI = dyn_cast<Instruction>(U);
          if (!UI || !SeenInsts.contains(UI)) {
            LLVM_DEBUG(dbgs() << "F2I: Failing because of " << *U << "\n");
            Fail = true;
            break;
          }
        }
      }
      if (Fail)
        break;
    }

    // If the set was empty, or we failed, or the range is poisonous,
    // bail out.
    if (ECs.member_begin(*E) == ECs.member_end() || Fail || R.isFullSet() ||
        R.isSignWrappedSet())
      continue;
    assert(ConvertedToTy && "Must have set the convertedtoty by this point!");

    // The number of bits required is the maximum of the upper and
    // lower limits, plus one so it can be signed.
    unsigned MinBW = R.getMinSignedBits() + 1;
    LLVM_DEBUG(dbgs() << "F2I: MinBitwidth=" << MinBW << ", R: " << R << "\n");

    // If we've run off the realms of the exactly representable integers,
    // the floating point result will differ from an integer approximation.

    // Do we need more bits than are in the mantissa of the type we converted
    // to? semanticsPrecision returns the number of mantissa bits plus one
    // for the sign bit.
    unsigned MaxRepresentableBits
      = APFloat::semanticsPrecision(ConvertedToTy->getFltSemantics()) - 1;
    if (MinBW > MaxRepresentableBits) {
      LLVM_DEBUG(dbgs() << "F2I: Value not guaranteed to be representable!\n");
      continue;
    }

    // OK, R is known to be representable.
    // Pick the smallest legal type that will fit.
    Type *Ty = DL.getSmallestLegalIntType(*Ctx, MinBW);
    if (!Ty) {
      // Every supported target supports 64-bit and 32-bit integers,
      // so fallback to a 32 or 64-bit integer if the value fits.
      if (MinBW <= 32) {
        Ty = Type::getInt32Ty(*Ctx);
      } else if (MinBW <= 64) {
        Ty = Type::getInt64Ty(*Ctx);
      } else {
        LLVM_DEBUG(dbgs() << "F2I: Value requires more bits to represent than "
                             "the target supports!\n");
        continue;
      }
    }

    for (Instruction *I : ECs.members(*E))
      convert(I, Ty);
    MadeChange = true;
  }

  return MadeChange;
}

Value *Float2IntPass::convert(Instruction *I, Type *ToTy) {
  if (auto It = ConvertedInsts.find(I); It != ConvertedInsts.end())
    // Already converted this instruction.
    return It->second;

  SmallVector<Value*,4> NewOperands;
  for (Value *V : I->operands()) {
    // Don't recurse if we're an instruction that terminates the path.
    if (I->getOpcode() == Instruction::UIToFP ||
        I->getOpcode() == Instruction::SIToFP) {
      NewOperands.push_back(V);
    } else if (Instruction *VI = dyn_cast<Instruction>(V)) {
      NewOperands.push_back(convert(VI, ToTy));
    } else if (ConstantFP *CF = dyn_cast<ConstantFP>(V)) {
      APSInt Val(ToTy->getPrimitiveSizeInBits(), /*isUnsigned=*/false);
      bool Exact;
      CF->getValueAPF().convertToInteger(Val,
                                         APFloat::rmNearestTiesToEven,
                                         &Exact);
      NewOperands.push_back(ConstantInt::get(ToTy, Val));
    } else {
      llvm_unreachable("Unhandled operand type?");
    }
  }

  // Now create a new instruction.
  IRBuilder<> IRB(I);
  Value *NewV = nullptr;
  switch (I->getOpcode()) {
  default: llvm_unreachable("Unhandled instruction!");

  case Instruction::FPToUI:
    NewV = IRB.CreateZExtOrTrunc(NewOperands[0], I->getType());
    break;

  case Instruction::FPToSI:
    NewV = IRB.CreateSExtOrTrunc(NewOperands[0], I->getType());
    break;

  case Instruction::FCmp: {
    CmpInst::Predicate P = mapFCmpPred(cast<CmpInst>(I)->getPredicate());
    assert(P != CmpInst::BAD_ICMP_PREDICATE && "Unhandled predicate!");
    NewV = IRB.CreateICmp(P, NewOperands[0], NewOperands[1], I->getName());
    break;
  }

  case Instruction::UIToFP:
    NewV = IRB.CreateZExtOrTrunc(NewOperands[0], ToTy);
    break;

  case Instruction::SIToFP:
    NewV = IRB.CreateSExtOrTrunc(NewOperands[0], ToTy);
    break;

  case Instruction::FNeg:
    NewV = IRB.CreateNeg(NewOperands[0], I->getName());
    break;

  case Instruction::FAdd:
  case Instruction::FSub:
  case Instruction::FMul:
    NewV = IRB.CreateBinOp(mapBinOpcode(I->getOpcode()),
                           NewOperands[0], NewOperands[1],
                           I->getName());
    break;
  }

  // If we're a root instruction, RAUW.
  if (Roots.count(I))
    I->replaceAllUsesWith(NewV);

  ConvertedInsts[I] = NewV;
  return NewV;
}

// Perform dead code elimination on the instructions we just modified.
void Float2IntPass::cleanup() {
  for (auto &I : reverse(ConvertedInsts))
    I.first->eraseFromParent();
}

bool Float2IntPass::runImpl(Function &F, const DominatorTree &DT) {
  LLVM_DEBUG(dbgs() << "F2I: Looking at function " << F.getName() << "\n");
  // Clear out all state.
  ECs = EquivalenceClasses<Instruction*>();
  SeenInsts.clear();
  ConvertedInsts.clear();
  Roots.clear();

  Ctx = &F.getParent()->getContext();
  MaxIntegerBW =
      Ctx->getLibraryOptions().get<ScalarOptions>().float2int_max_integer_bw;

  findRoots(F, DT);

  walkBackwards();
  walkForwards();

  const DataLayout &DL = F.getDataLayout();
  bool Modified = validateAndTransform(DL);
  if (Modified)
    cleanup();
  return Modified;
}

PreservedAnalyses Float2IntPass::run(Function &F, FunctionAnalysisManager &AM) {
  const DominatorTree &DT = AM.getResult<DominatorTreeAnalysis>(F);
  if (!runImpl(F, DT))
    return PreservedAnalyses::all();

  PreservedAnalyses PA;
  PA.preserveSet<CFGAnalyses>();
  return PA;
}