Target/LLVMIR/ModuleTranslation.cpp

//===- ModuleTranslation.cpp - MLIR to LLVM conversion --------------------===//
//
// 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 translation between an MLIR LLVM dialect module and
// the corresponding LLVMIR module. It only handles core LLVM IR operations.
//
//===----------------------------------------------------------------------===//

#include "mlir/Target/LLVMIR/ModuleTranslation.h"

#include "DebugTranslation.h"
#include "mlir/Dialect/LLVMIR/LLVMDialect.h"
#include "mlir/Dialect/OpenMP/OpenMPDialect.h"
#include "mlir/IR/Attributes.h"
#include "mlir/IR/Module.h"
#include "mlir/IR/StandardTypes.h"
#include "mlir/Support/LLVM.h"
#include "llvm/ADT/TypeSwitch.h"

#include "llvm/ADT/SetVector.h"
#include "llvm/Frontend/OpenMP/OMPIRBuilder.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/DerivedTypes.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/LLVMContext.h"
#include "llvm/IR/Module.h"
#include "llvm/Transforms/Utils/Cloning.h"

using namespace mlir;
using namespace mlir::LLVM;
using namespace mlir::LLVM::detail;

#include "mlir/Dialect/LLVMIR/LLVMConversionEnumsToLLVM.inc"

/// Builds a constant of a sequential LLVM type `type`, potentially containing
/// other sequential types recursively, from the individual constant values
/// provided in `constants`. `shape` contains the number of elements in nested
/// sequential types. Reports errors at `loc` and returns nullptr on error.
static llvm::Constant *
buildSequentialConstant(ArrayRef<llvm::Constant *> &constants,
                        ArrayRef<int64_t> shape, llvm::Type *type,
                        Location loc) {
  if (shape.empty()) {
    llvm::Constant *result = constants.front();
    constants = constants.drop_front();
    return result;
  }

  llvm::Type *elementType;
  if (auto *arrayTy = dyn_cast<llvm::ArrayType>(type)) {
    elementType = arrayTy->getElementType();
  } else if (auto *vectorTy = dyn_cast<llvm::VectorType>(type)) {
    elementType = vectorTy->getElementType();
  } else {
    emitError(loc) << "expected sequential LLVM types wrapping a scalar";
    return nullptr;
  }

  SmallVector<llvm::Constant *, 8> nested;
  nested.reserve(shape.front());
  for (int64_t i = 0; i < shape.front(); ++i) {
    nested.push_back(buildSequentialConstant(constants, shape.drop_front(),
                                             elementType, loc));
    if (!nested.back())
      return nullptr;
  }

  if (shape.size() == 1 && type->isVectorTy())
    return llvm::ConstantVector::get(nested);
  return llvm::ConstantArray::get(
      llvm::ArrayType::get(elementType, shape.front()), nested);
}

/// Returns the first non-sequential type nested in sequential types.
static llvm::Type *getInnermostElementType(llvm::Type *type) {
  do {
    if (auto *arrayTy = dyn_cast<llvm::ArrayType>(type)) {
      type = arrayTy->getElementType();
    } else if (auto *vectorTy = dyn_cast<llvm::VectorType>(type)) {
      type = vectorTy->getElementType();
    } else {
      return type;
    }
  } while (1);
}

/// Create an LLVM IR constant of `llvmType` from the MLIR attribute `attr`.
/// This currently supports integer, floating point, splat and dense element
/// attributes and combinations thereof.  In case of error, report it to `loc`
/// and return nullptr.
llvm::Constant *ModuleTranslation::getLLVMConstant(llvm::Type *llvmType,
                                                   Attribute attr,
                                                   Location loc) {
  if (!attr)
    return llvm::UndefValue::get(llvmType);
  if (llvmType->isStructTy()) {
    emitError(loc, "struct types are not supported in constants");
    return nullptr;
  }
  // For integer types, we allow a mismatch in sizes as the index type in
  // MLIR might have a different size than the index type in the LLVM module.
  if (auto intAttr = attr.dyn_cast<IntegerAttr>())
    return llvm::ConstantInt::get(
        llvmType,
        intAttr.getValue().sextOrTrunc(llvmType->getIntegerBitWidth()));
  if (auto floatAttr = attr.dyn_cast<FloatAttr>())
    return llvm::ConstantFP::get(llvmType, floatAttr.getValue());
  if (auto funcAttr = attr.dyn_cast<FlatSymbolRefAttr>())
    return llvm::ConstantExpr::getBitCast(
        functionMapping.lookup(funcAttr.getValue()), llvmType);
  if (auto splatAttr = attr.dyn_cast<SplatElementsAttr>()) {
    llvm::Type *elementType;
    uint64_t numElements;
    if (auto *arrayTy = dyn_cast<llvm::ArrayType>(llvmType)) {
      elementType = arrayTy->getElementType();
      numElements = arrayTy->getNumElements();
    } else {
      auto *vectorTy = cast<llvm::FixedVectorType>(llvmType);
      elementType = vectorTy->getElementType();
      numElements = vectorTy->getNumElements();
    }
    // Splat value is a scalar. Extract it only if the element type is not
    // another sequence type. The recursion terminates because each step removes
    // one outer sequential type.
    bool elementTypeSequential =
        isa<llvm::ArrayType, llvm::VectorType>(elementType);
    llvm::Constant *child = getLLVMConstant(
        elementType,
        elementTypeSequential ? splatAttr : splatAttr.getSplatValue(), loc);
    if (!child)
      return nullptr;
    if (llvmType->isVectorTy())
      return llvm::ConstantVector::getSplat(
          llvm::ElementCount(numElements, /*Scalable=*/false), child);
    if (llvmType->isArrayTy()) {
      auto *arrayType = llvm::ArrayType::get(elementType, numElements);
      SmallVector<llvm::Constant *, 8> constants(numElements, child);
      return llvm::ConstantArray::get(arrayType, constants);
    }
  }

  if (auto elementsAttr = attr.dyn_cast<ElementsAttr>()) {
    assert(elementsAttr.getType().hasStaticShape());
    assert(elementsAttr.getNumElements() != 0 &&
           "unexpected empty elements attribute");
    assert(!elementsAttr.getType().getShape().empty() &&
           "unexpected empty elements attribute shape");

    SmallVector<llvm::Constant *, 8> constants;
    constants.reserve(elementsAttr.getNumElements());
    llvm::Type *innermostType = getInnermostElementType(llvmType);
    for (auto n : elementsAttr.getValues<Attribute>()) {
      constants.push_back(getLLVMConstant(innermostType, n, loc));
      if (!constants.back())
        return nullptr;
    }
    ArrayRef<llvm::Constant *> constantsRef = constants;
    llvm::Constant *result = buildSequentialConstant(
        constantsRef, elementsAttr.getType().getShape(), llvmType, loc);
    assert(constantsRef.empty() && "did not consume all elemental constants");
    return result;
  }

  if (auto stringAttr = attr.dyn_cast<StringAttr>()) {
    return llvm::ConstantDataArray::get(
        llvmModule->getContext(), ArrayRef<char>{stringAttr.getValue().data(),
                                                 stringAttr.getValue().size()});
  }
  emitError(loc, "unsupported constant value");
  return nullptr;
}

/// Convert MLIR integer comparison predicate to LLVM IR comparison predicate.
static llvm::CmpInst::Predicate getLLVMCmpPredicate(ICmpPredicate p) {
  switch (p) {
  case LLVM::ICmpPredicate::eq:
    return llvm::CmpInst::Predicate::ICMP_EQ;
  case LLVM::ICmpPredicate::ne:
    return llvm::CmpInst::Predicate::ICMP_NE;
  case LLVM::ICmpPredicate::slt:
    return llvm::CmpInst::Predicate::ICMP_SLT;
  case LLVM::ICmpPredicate::sle:
    return llvm::CmpInst::Predicate::ICMP_SLE;
  case LLVM::ICmpPredicate::sgt:
    return llvm::CmpInst::Predicate::ICMP_SGT;
  case LLVM::ICmpPredicate::sge:
    return llvm::CmpInst::Predicate::ICMP_SGE;
  case LLVM::ICmpPredicate::ult:
    return llvm::CmpInst::Predicate::ICMP_ULT;
  case LLVM::ICmpPredicate::ule:
    return llvm::CmpInst::Predicate::ICMP_ULE;
  case LLVM::ICmpPredicate::ugt:
    return llvm::CmpInst::Predicate::ICMP_UGT;
  case LLVM::ICmpPredicate::uge:
    return llvm::CmpInst::Predicate::ICMP_UGE;
  }
  llvm_unreachable("incorrect comparison predicate");
}

static llvm::CmpInst::Predicate getLLVMCmpPredicate(FCmpPredicate p) {
  switch (p) {
  case LLVM::FCmpPredicate::_false:
    return llvm::CmpInst::Predicate::FCMP_FALSE;
  case LLVM::FCmpPredicate::oeq:
    return llvm::CmpInst::Predicate::FCMP_OEQ;
  case LLVM::FCmpPredicate::ogt:
    return llvm::CmpInst::Predicate::FCMP_OGT;
  case LLVM::FCmpPredicate::oge:
    return llvm::CmpInst::Predicate::FCMP_OGE;
  case LLVM::FCmpPredicate::olt:
    return llvm::CmpInst::Predicate::FCMP_OLT;
  case LLVM::FCmpPredicate::ole:
    return llvm::CmpInst::Predicate::FCMP_OLE;
  case LLVM::FCmpPredicate::one:
    return llvm::CmpInst::Predicate::FCMP_ONE;
  case LLVM::FCmpPredicate::ord:
    return llvm::CmpInst::Predicate::FCMP_ORD;
  case LLVM::FCmpPredicate::ueq:
    return llvm::CmpInst::Predicate::FCMP_UEQ;
  case LLVM::FCmpPredicate::ugt:
    return llvm::CmpInst::Predicate::FCMP_UGT;
  case LLVM::FCmpPredicate::uge:
    return llvm::CmpInst::Predicate::FCMP_UGE;
  case LLVM::FCmpPredicate::ult:
    return llvm::CmpInst::Predicate::FCMP_ULT;
  case LLVM::FCmpPredicate::ule:
    return llvm::CmpInst::Predicate::FCMP_ULE;
  case LLVM::FCmpPredicate::une:
    return llvm::CmpInst::Predicate::FCMP_UNE;
  case LLVM::FCmpPredicate::uno:
    return llvm::CmpInst::Predicate::FCMP_UNO;
  case LLVM::FCmpPredicate::_true:
    return llvm::CmpInst::Predicate::FCMP_TRUE;
  }
  llvm_unreachable("incorrect comparison predicate");
}

static llvm::AtomicRMWInst::BinOp getLLVMAtomicBinOp(AtomicBinOp op) {
  switch (op) {
  case LLVM::AtomicBinOp::xchg:
    return llvm::AtomicRMWInst::BinOp::Xchg;
  case LLVM::AtomicBinOp::add:
    return llvm::AtomicRMWInst::BinOp::Add;
  case LLVM::AtomicBinOp::sub:
    return llvm::AtomicRMWInst::BinOp::Sub;
  case LLVM::AtomicBinOp::_and:
    return llvm::AtomicRMWInst::BinOp::And;
  case LLVM::AtomicBinOp::nand:
    return llvm::AtomicRMWInst::BinOp::Nand;
  case LLVM::AtomicBinOp::_or:
    return llvm::AtomicRMWInst::BinOp::Or;
  case LLVM::AtomicBinOp::_xor:
    return llvm::AtomicRMWInst::BinOp::Xor;
  case LLVM::AtomicBinOp::max:
    return llvm::AtomicRMWInst::BinOp::Max;
  case LLVM::AtomicBinOp::min:
    return llvm::AtomicRMWInst::BinOp::Min;
  case LLVM::AtomicBinOp::umax:
    return llvm::AtomicRMWInst::BinOp::UMax;
  case LLVM::AtomicBinOp::umin:
    return llvm::AtomicRMWInst::BinOp::UMin;
  case LLVM::AtomicBinOp::fadd:
    return llvm::AtomicRMWInst::BinOp::FAdd;
  case LLVM::AtomicBinOp::fsub:
    return llvm::AtomicRMWInst::BinOp::FSub;
  }
  llvm_unreachable("incorrect atomic binary operator");
}

static llvm::AtomicOrdering getLLVMAtomicOrdering(AtomicOrdering ordering) {
  switch (ordering) {
  case LLVM::AtomicOrdering::not_atomic:
    return llvm::AtomicOrdering::NotAtomic;
  case LLVM::AtomicOrdering::unordered:
    return llvm::AtomicOrdering::Unordered;
  case LLVM::AtomicOrdering::monotonic:
    return llvm::AtomicOrdering::Monotonic;
  case LLVM::AtomicOrdering::acquire:
    return llvm::AtomicOrdering::Acquire;
  case LLVM::AtomicOrdering::release:
    return llvm::AtomicOrdering::Release;
  case LLVM::AtomicOrdering::acq_rel:
    return llvm::AtomicOrdering::AcquireRelease;
  case LLVM::AtomicOrdering::seq_cst:
    return llvm::AtomicOrdering::SequentiallyConsistent;
  }
  llvm_unreachable("incorrect atomic ordering");
}

ModuleTranslation::ModuleTranslation(Operation *module,
                                     std::unique_ptr<llvm::Module> llvmModule)
    : mlirModule(module), llvmModule(std::move(llvmModule)),
      debugTranslation(
          std::make_unique<DebugTranslation>(module, *this->llvmModule)),
      ompDialect(
          module->getContext()->getRegisteredDialect<omp::OpenMPDialect>()),
      llvmDialect(module->getContext()->getRegisteredDialect<LLVMDialect>()) {
  assert(satisfiesLLVMModule(mlirModule) &&
         "mlirModule should honor LLVM's module semantics.");
}
ModuleTranslation::~ModuleTranslation() {}

/// Given an OpenMP MLIR operation, create the corresponding LLVM IR
/// (including OpenMP runtime calls).
LogicalResult
ModuleTranslation::convertOmpOperation(Operation &opInst,
                                       llvm::IRBuilder<> &builder) {
  if (!ompBuilder) {
    ompBuilder = std::make_unique<llvm::OpenMPIRBuilder>(*llvmModule);
    ompBuilder->initialize();
  }
  return llvm::TypeSwitch<Operation *, LogicalResult>(&opInst)
      .Case([&](omp::BarrierOp) {
        ompBuilder->CreateBarrier(builder.saveIP(), llvm::omp::OMPD_barrier);
        return success();
      })
      .Case([&](omp::TaskwaitOp) {
        ompBuilder->CreateTaskwait(builder.saveIP());
        return success();
      })
      .Case([&](omp::TaskyieldOp) {
        ompBuilder->CreateTaskyield(builder.saveIP());
        return success();
      })
      .Case([&](omp::FlushOp) {
        // No support in Openmp runtime funciton (__kmpc_flush) to accept
        // the argument list.
        // OpenMP standard states the following:
        //  "An implementation may implement a flush with a list by ignoring
        //   the list, and treating it the same as a flush without a list."
        //
        // The argument list is discarded so that, flush with a list is treated
        // same as a flush without a list.
        ompBuilder->CreateFlush(builder.saveIP());
        return success();
      })
      .Default([&](Operation *inst) {
        return inst->emitError("unsupported OpenMP operation: ")
               << inst->getName();
      });
}

/// Given a single MLIR operation, create the corresponding LLVM IR operation
/// using the `builder`.  LLVM IR Builder does not have a generic interface so
/// this has to be a long chain of `if`s calling different functions with a
/// different number of arguments.
LogicalResult ModuleTranslation::convertOperation(Operation &opInst,
                                                  llvm::IRBuilder<> &builder) {
  auto extractPosition = [](ArrayAttr attr) {
    SmallVector<unsigned, 4> position;
    position.reserve(attr.size());
    for (Attribute v : attr)
      position.push_back(v.cast<IntegerAttr>().getValue().getZExtValue());
    return position;
  };

#include "mlir/Dialect/LLVMIR/LLVMConversions.inc"

  // Emit function calls.  If the "callee" attribute is present, this is a
  // direct function call and we also need to look up the remapped function
  // itself.  Otherwise, this is an indirect call and the callee is the first
  // operand, look it up as a normal value.  Return the llvm::Value representing
  // the function result, which may be of llvm::VoidTy type.
  auto convertCall = [this, &builder](Operation &op) -> llvm::Value * {
    auto operands = lookupValues(op.getOperands());
    ArrayRef<llvm::Value *> operandsRef(operands);
    if (auto attr = op.getAttrOfType<FlatSymbolRefAttr>("callee")) {
      return builder.CreateCall(functionMapping.lookup(attr.getValue()),
                                operandsRef);
    } else {
      auto *calleePtrType =
          cast<llvm::PointerType>(operandsRef.front()->getType());
      auto *calleeType =
          cast<llvm::FunctionType>(calleePtrType->getElementType());
      return builder.CreateCall(calleeType, operandsRef.front(),
                                operandsRef.drop_front());
    }
  };

  // Emit calls.  If the called function has a result, remap the corresponding
  // value.  Note that LLVM IR dialect CallOp has either 0 or 1 result.
  if (isa<LLVM::CallOp>(opInst)) {
    llvm::Value *result = convertCall(opInst);
    if (opInst.getNumResults() != 0) {
      valueMapping[opInst.getResult(0)] = result;
      return success();
    }
    // Check that LLVM call returns void for 0-result functions.
    return success(result->getType()->isVoidTy());
  }

  if (auto invOp = dyn_cast<LLVM::InvokeOp>(opInst)) {
    auto operands = lookupValues(opInst.getOperands());
    ArrayRef<llvm::Value *> operandsRef(operands);
    if (auto attr = opInst.getAttrOfType<FlatSymbolRefAttr>("callee")) {
      builder.CreateInvoke(functionMapping.lookup(attr.getValue()),
                           blockMapping[invOp.getSuccessor(0)],
                           blockMapping[invOp.getSuccessor(1)], operandsRef);
    } else {
      auto *calleePtrType =
          cast<llvm::PointerType>(operandsRef.front()->getType());
      auto *calleeType =
          cast<llvm::FunctionType>(calleePtrType->getElementType());
      builder.CreateInvoke(
          calleeType, operandsRef.front(), blockMapping[invOp.getSuccessor(0)],
          blockMapping[invOp.getSuccessor(1)], operandsRef.drop_front());
    }
    return success();
  }

  if (auto lpOp = dyn_cast<LLVM::LandingpadOp>(opInst)) {
    llvm::Type *ty = lpOp.getType().dyn_cast<LLVMType>().getUnderlyingType();
    llvm::LandingPadInst *lpi =
        builder.CreateLandingPad(ty, lpOp.getNumOperands());

    // Add clauses
    for (auto operand : lookupValues(lpOp.getOperands())) {
      // All operands should be constant - checked by verifier
      if (auto constOperand = dyn_cast<llvm::Constant>(operand))
        lpi->addClause(constOperand);
    }
    valueMapping[lpOp.getResult()] = lpi;
    return success();
  }

  // Emit branches.  We need to look up the remapped blocks and ignore the block
  // arguments that were transformed into PHI nodes.
  if (auto brOp = dyn_cast<LLVM::BrOp>(opInst)) {
    builder.CreateBr(blockMapping[brOp.getSuccessor()]);
    return success();
  }
  if (auto condbrOp = dyn_cast<LLVM::CondBrOp>(opInst)) {
    builder.CreateCondBr(valueMapping.lookup(condbrOp.getOperand(0)),
                         blockMapping[condbrOp.getSuccessor(0)],
                         blockMapping[condbrOp.getSuccessor(1)]);
    return success();
  }

  // Emit addressof.  We need to look up the global value referenced by the
  // operation and store it in the MLIR-to-LLVM value mapping.  This does not
  // emit any LLVM instruction.
  if (auto addressOfOp = dyn_cast<LLVM::AddressOfOp>(opInst)) {
    LLVM::GlobalOp global = addressOfOp.getGlobal();
    LLVM::LLVMFuncOp function = addressOfOp.getFunction();

    // The verifier should not have allowed this.
    assert((global || function) &&
           "referencing an undefined global or function");

    valueMapping[addressOfOp.getResult()] =
        global ? globalsMapping.lookup(global)
               : functionMapping.lookup(function.getName());
    return success();
  }

  if (opInst.getDialect() == ompDialect) {
    return convertOmpOperation(opInst, builder);
  }

  return opInst.emitError("unsupported or non-LLVM operation: ")
         << opInst.getName();
}

/// Convert block to LLVM IR.  Unless `ignoreArguments` is set, emit PHI nodes
/// to define values corresponding to the MLIR block arguments.  These nodes
/// are not connected to the source basic blocks, which may not exist yet.
LogicalResult ModuleTranslation::convertBlock(Block &bb, bool ignoreArguments) {
  llvm::IRBuilder<> builder(blockMapping[&bb]);
  auto *subprogram = builder.GetInsertBlock()->getParent()->getSubprogram();

  // Before traversing operations, make block arguments available through
  // value remapping and PHI nodes, but do not add incoming edges for the PHI
  // nodes just yet: those values may be defined by this or following blocks.
  // This step is omitted if "ignoreArguments" is set.  The arguments of the
  // first block have been already made available through the remapping of
  // LLVM function arguments.
  if (!ignoreArguments) {
    auto predecessors = bb.getPredecessors();
    unsigned numPredecessors =
        std::distance(predecessors.begin(), predecessors.end());
    for (auto arg : bb.getArguments()) {
      auto wrappedType = arg.getType().dyn_cast<LLVM::LLVMType>();
      if (!wrappedType)
        return emitError(bb.front().getLoc(),
                         "block argument does not have an LLVM type");
      llvm::Type *type = wrappedType.getUnderlyingType();
      llvm::PHINode *phi = builder.CreatePHI(type, numPredecessors);
      valueMapping[arg] = phi;
    }
  }

  // Traverse operations.
  for (auto &op : bb) {
    // Set the current debug location within the builder.
    builder.SetCurrentDebugLocation(
        debugTranslation->translateLoc(op.getLoc(), subprogram));

    if (failed(convertOperation(op, builder)))
      return failure();
  }

  return success();
}

/// Create named global variables that correspond to llvm.mlir.global
/// definitions.
LogicalResult ModuleTranslation::convertGlobals() {
  // Lock access to the llvm context.
  llvm::sys::SmartScopedLock<true> scopedLock(
      llvmDialect->getLLVMContextMutex());
  for (auto op : getModuleBody(mlirModule).getOps<LLVM::GlobalOp>()) {
    llvm::Type *type = op.getType().getUnderlyingType();
    llvm::Constant *cst = llvm::UndefValue::get(type);
    if (op.getValueOrNull()) {
      // String attributes are treated separately because they cannot appear as
      // in-function constants and are thus not supported by getLLVMConstant.
      if (auto strAttr = op.getValueOrNull().dyn_cast_or_null<StringAttr>()) {
        cst = llvm::ConstantDataArray::getString(
            llvmModule->getContext(), strAttr.getValue(), /*AddNull=*/false);
        type = cst->getType();
      } else if (!(cst = getLLVMConstant(type, op.getValueOrNull(),
                                         op.getLoc()))) {
        return failure();
      }
    } else if (Block *initializer = op.getInitializerBlock()) {
      llvm::IRBuilder<> builder(llvmModule->getContext());
      for (auto &op : initializer->without_terminator()) {
        if (failed(convertOperation(op, builder)) ||
            !isa<llvm::Constant>(valueMapping.lookup(op.getResult(0))))
          return emitError(op.getLoc(), "unemittable constant value");
      }
      ReturnOp ret = cast<ReturnOp>(initializer->getTerminator());
      cst = cast<llvm::Constant>(valueMapping.lookup(ret.getOperand(0)));
    }

    auto linkage = convertLinkageToLLVM(op.linkage());
    bool anyExternalLinkage =
        ((linkage == llvm::GlobalVariable::ExternalLinkage &&
          isa<llvm::UndefValue>(cst)) ||
         linkage == llvm::GlobalVariable::ExternalWeakLinkage);
    auto addrSpace = op.addr_space().getLimitedValue();
    auto *var = new llvm::GlobalVariable(
        *llvmModule, type, op.constant(), linkage,
        anyExternalLinkage ? nullptr : cst, op.sym_name(),
        /*InsertBefore=*/nullptr, llvm::GlobalValue::NotThreadLocal, addrSpace);

    globalsMapping.try_emplace(op, var);
  }

  return success();
}

/// Get the SSA value passed to the current block from the terminator operation
/// of its predecessor.
static Value getPHISourceValue(Block *current, Block *pred,
                               unsigned numArguments, unsigned index) {
  auto &terminator = *pred->getTerminator();
  if (isa<LLVM::BrOp>(terminator)) {
    return terminator.getOperand(index);
  }

  // For conditional branches, we need to check if the current block is reached
  // through the "true" or the "false" branch and take the relevant operands.
  auto condBranchOp = dyn_cast<LLVM::CondBrOp>(terminator);
  assert(condBranchOp &&
         "only branch operations can be terminators of a block that "
         "has successors");
  assert((condBranchOp.getSuccessor(0) != condBranchOp.getSuccessor(1)) &&
         "successors with arguments in LLVM conditional branches must be "
         "different blocks");

  return condBranchOp.getSuccessor(0) == current
             ? condBranchOp.trueDestOperands()[index]
             : condBranchOp.falseDestOperands()[index];
}

void ModuleTranslation::connectPHINodes(LLVMFuncOp func) {
  // Skip the first block, it cannot be branched to and its arguments correspond
  // to the arguments of the LLVM function.
  for (auto it = std::next(func.begin()), eit = func.end(); it != eit; ++it) {
    Block *bb = &*it;
    llvm::BasicBlock *llvmBB = blockMapping.lookup(bb);
    auto phis = llvmBB->phis();
    auto numArguments = bb->getNumArguments();
    assert(numArguments == std::distance(phis.begin(), phis.end()));
    for (auto &numberedPhiNode : llvm::enumerate(phis)) {
      auto &phiNode = numberedPhiNode.value();
      unsigned index = numberedPhiNode.index();
      for (auto *pred : bb->getPredecessors()) {
        phiNode.addIncoming(valueMapping.lookup(getPHISourceValue(
                                bb, pred, numArguments, index)),
                            blockMapping.lookup(pred));
      }
    }
  }
}

// TODO(mlir-team): implement an iterative version
static void topologicalSortImpl(llvm::SetVector<Block *> &blocks, Block *b) {
  blocks.insert(b);
  for (Block *bb : b->getSuccessors()) {
    if (blocks.count(bb) == 0)
      topologicalSortImpl(blocks, bb);
  }
}

/// Sort function blocks topologically.
static llvm::SetVector<Block *> topologicalSort(LLVMFuncOp f) {
  // For each blocks that has not been visited yet (i.e. that has no
  // predecessors), add it to the list and traverse its successors in DFS
  // preorder.
  llvm::SetVector<Block *> blocks;
  for (Block &b : f) {
    if (blocks.count(&b) == 0)
      topologicalSortImpl(blocks, &b);
  }
  assert(blocks.size() == f.getBlocks().size() && "some blocks are not sorted");

  return blocks;
}

/// Attempts to add an attribute identified by `key`, optionally with the given
/// `value` to LLVM function `llvmFunc`. Reports errors at `loc` if any. If the
/// attribute has a kind known to LLVM IR, create the attribute of this kind,
/// otherwise keep it as a string attribute. Performs additional checks for
/// attributes known to have or not have a value in order to avoid assertions
/// inside LLVM upon construction.
static LogicalResult checkedAddLLVMFnAttribute(Location loc,
                                               llvm::Function *llvmFunc,
                                               StringRef key,
                                               StringRef value = StringRef()) {
  auto kind = llvm::Attribute::getAttrKindFromName(key);
  if (kind == llvm::Attribute::None) {
    llvmFunc->addFnAttr(key, value);
    return success();
  }

  if (llvm::Attribute::doesAttrKindHaveArgument(kind)) {
    if (value.empty())
      return emitError(loc) << "LLVM attribute '" << key << "' expects a value";

    int result;
    if (!value.getAsInteger(/*Radix=*/0, result))
      llvmFunc->addFnAttr(
          llvm::Attribute::get(llvmFunc->getContext(), kind, result));
    else
      llvmFunc->addFnAttr(key, value);
    return success();
  }

  if (!value.empty())
    return emitError(loc) << "LLVM attribute '" << key
                          << "' does not expect a value, found '" << value
                          << "'";

  llvmFunc->addFnAttr(kind);
  return success();
}

/// Attaches the attributes listed in the given array attribute to `llvmFunc`.
/// Reports error to `loc` if any and returns immediately. Expects `attributes`
/// to be an array attribute containing either string attributes, treated as
/// value-less LLVM attributes, or array attributes containing two string
/// attributes, with the first string being the name of the corresponding LLVM
/// attribute and the second string beings its value. Note that even integer
/// attributes are expected to have their values expressed as strings.
static LogicalResult
forwardPassthroughAttributes(Location loc, Optional<ArrayAttr> attributes,
                             llvm::Function *llvmFunc) {
  if (!attributes)
    return success();

  for (Attribute attr : *attributes) {
    if (auto stringAttr = attr.dyn_cast<StringAttr>()) {
      if (failed(
              checkedAddLLVMFnAttribute(loc, llvmFunc, stringAttr.getValue())))
        return failure();
      continue;
    }

    auto arrayAttr = attr.dyn_cast<ArrayAttr>();
    if (!arrayAttr || arrayAttr.size() != 2)
      return emitError(loc)
             << "expected 'passthrough' to contain string or array attributes";

    auto keyAttr = arrayAttr[0].dyn_cast<StringAttr>();
    auto valueAttr = arrayAttr[1].dyn_cast<StringAttr>();
    if (!keyAttr || !valueAttr)
      return emitError(loc)
             << "expected arrays within 'passthrough' to contain two strings";

    if (failed(checkedAddLLVMFnAttribute(loc, llvmFunc, keyAttr.getValue(),
                                         valueAttr.getValue())))
      return failure();
  }
  return success();
}

LogicalResult ModuleTranslation::convertOneFunction(LLVMFuncOp func) {
  // Clear the block and value mappings, they are only relevant within one
  // function.
  blockMapping.clear();
  valueMapping.clear();
  llvm::Function *llvmFunc = functionMapping.lookup(func.getName());

  // Translate the debug information for this function.
  debugTranslation->translate(func, *llvmFunc);

  // Add function arguments to the value remapping table.
  // If there was noalias info then we decorate each argument accordingly.
  unsigned int argIdx = 0;
  for (auto kvp : llvm::zip(func.getArguments(), llvmFunc->args())) {
    llvm::Argument &llvmArg = std::get<1>(kvp);
    BlockArgument mlirArg = std::get<0>(kvp);

    if (auto attr = func.getArgAttrOfType<BoolAttr>(argIdx, "llvm.noalias")) {
      // NB: Attribute already verified to be boolean, so check if we can indeed
      // attach the attribute to this argument, based on its type.
      auto argTy = mlirArg.getType().dyn_cast<LLVM::LLVMType>();
      if (!argTy.getUnderlyingType()->isPointerTy())
        return func.emitError(
            "llvm.noalias attribute attached to LLVM non-pointer argument");
      if (attr.getValue())
        llvmArg.addAttr(llvm::Attribute::AttrKind::NoAlias);
    }

    if (auto attr = func.getArgAttrOfType<IntegerAttr>(argIdx, "llvm.align")) {
      // NB: Attribute already verified to be int, so check if we can indeed
      // attach the attribute to this argument, based on its type.
      auto argTy = mlirArg.getType().dyn_cast<LLVM::LLVMType>();
      if (!argTy.getUnderlyingType()->isPointerTy())
        return func.emitError(
            "llvm.align attribute attached to LLVM non-pointer argument");
      llvmArg.addAttrs(
          llvm::AttrBuilder().addAlignmentAttr(llvm::Align(attr.getInt())));
    }

    valueMapping[mlirArg] = &llvmArg;
    argIdx++;
  }

  // Check the personality and set it.
  if (func.personality().hasValue()) {
    llvm::Type *ty = llvm::Type::getInt8PtrTy(llvmFunc->getContext());
    if (llvm::Constant *pfunc =
            getLLVMConstant(ty, func.personalityAttr(), func.getLoc()))
      llvmFunc->setPersonalityFn(pfunc);
  }

  // First, create all blocks so we can jump to them.
  llvm::LLVMContext &llvmContext = llvmFunc->getContext();
  for (auto &bb : func) {
    auto *llvmBB = llvm::BasicBlock::Create(llvmContext);
    llvmBB->insertInto(llvmFunc);
    blockMapping[&bb] = llvmBB;
  }

  // Then, convert blocks one by one in topological order to ensure defs are
  // converted before uses.
  auto blocks = topologicalSort(func);
  for (auto indexedBB : llvm::enumerate(blocks)) {
    auto *bb = indexedBB.value();
    if (failed(convertBlock(*bb, /*ignoreArguments=*/indexedBB.index() == 0)))
      return failure();
  }

  // Finally, after all blocks have been traversed and values mapped, connect
  // the PHI nodes to the results of preceding blocks.
  connectPHINodes(func);
  return success();
}

LogicalResult ModuleTranslation::checkSupportedModuleOps(Operation *m) {
  for (Operation &o : getModuleBody(m).getOperations())
    if (!isa<LLVM::LLVMFuncOp, LLVM::GlobalOp>(&o) && !o.isKnownTerminator())
      return o.emitOpError("unsupported module-level operation");
  return success();
}

LogicalResult ModuleTranslation::convertFunctions() {
  // Lock access to the llvm context.
  llvm::sys::SmartScopedLock<true> scopedLock(
      llvmDialect->getLLVMContextMutex());
  // Declare all functions first because there may be function calls that form a
  // call graph with cycles.
  for (auto function : getModuleBody(mlirModule).getOps<LLVMFuncOp>()) {
    llvm::FunctionCallee llvmFuncCst = llvmModule->getOrInsertFunction(
        function.getName(),
        cast<llvm::FunctionType>(function.getType().getUnderlyingType()));
    llvm::Function *llvmFunc = cast<llvm::Function>(llvmFuncCst.getCallee());
    functionMapping[function.getName()] = llvmFunc;

    // Forward the pass-through attributes to LLVM.
    if (failed(forwardPassthroughAttributes(function.getLoc(),
                                            function.passthrough(), llvmFunc)))
      return failure();
  }

  // Convert functions.
  for (auto function : getModuleBody(mlirModule).getOps<LLVMFuncOp>()) {
    // Ignore external functions.
    if (function.isExternal())
      continue;

    if (failed(convertOneFunction(function)))
      return failure();
  }

  return success();
}

/// A helper to look up remapped operands in the value remapping table.`
SmallVector<llvm::Value *, 8>
ModuleTranslation::lookupValues(ValueRange values) {
  SmallVector<llvm::Value *, 8> remapped;
  remapped.reserve(values.size());
  for (Value v : values) {
    assert(valueMapping.count(v) && "referencing undefined value");
    remapped.push_back(valueMapping.lookup(v));
  }
  return remapped;
}

std::unique_ptr<llvm::Module>
ModuleTranslation::prepareLLVMModule(Operation *m) {
  auto *dialect = m->getContext()->getRegisteredDialect<LLVM::LLVMDialect>();
  assert(dialect && "LLVM dialect must be registered");
  // Lock the LLVM context as we might create new types here.
  llvm::sys::SmartScopedLock<true> scopedLock(dialect->getLLVMContextMutex());

  auto llvmModule = llvm::CloneModule(dialect->getLLVMModule());
  if (!llvmModule)
    return nullptr;

  llvm::LLVMContext &llvmContext = llvmModule->getContext();
  llvm::IRBuilder<> builder(llvmContext);

  // Inject declarations for `malloc` and `free` functions that can be used in
  // memref allocation/deallocation coming from standard ops lowering.
  llvmModule->getOrInsertFunction("malloc", builder.getInt8PtrTy(),
                                  builder.getInt64Ty());
  llvmModule->getOrInsertFunction("free", builder.getVoidTy(),
                                  builder.getInt8PtrTy());

  return llvmModule;
}