carbon-lang/toolchain/lower/constant.cpp

// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
// Exceptions. See /LICENSE for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception

#include "toolchain/lower/constant.h"

#include "llvm/ADT/STLExtras.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/IRBuilder.h"
#include "llvm/IR/Value.h"
#include "toolchain/base/kind_switch.h"
#include "toolchain/lower/file_context.h"
#include "toolchain/sem_ir/expr_info.h"
#include "toolchain/sem_ir/inst.h"
#include "toolchain/sem_ir/typed_insts.h"

namespace Carbon::Lower {

// Context and shared functionality for lowering constant values.
class ConstantContext {
 public:
  explicit ConstantContext(FileContext& file_context,
                           const FileContext::LoweredConstantStore* constants)
      : file_context_(&file_context), constants_(constants) {}

  // Gets the lowered constant value for an instruction, which must have a
  // constant value that has already been lowered. Returns nullptr if it
  // hasn't been, in which case it should not be needed.
  auto GetConstant(SemIR::InstId inst_id) const -> llvm::Constant* {
    return GetConstant(file_context_->sem_ir().constant_values().Get(inst_id));
  }

  // Gets the lowered constant value for a constant that has already been
  // lowered. Returns nullptr if it hasn't been, in which case it should not be
  // needed.
  auto GetConstant(SemIR::ConstantId const_id) const -> llvm::Constant* {
    CARBON_CHECK(const_id.is_concrete(), "Unexpected constant ID {0}",
                 const_id);
    auto inst_id = sem_ir().constant_values().GetInstId(const_id);
    CARBON_CHECK(sem_ir().insts().GetRawIndex(inst_id) <
                     sem_ir().insts().GetRawIndex(current_constant_inst_id_),
                 "Accessed constants out of order: {0} {1} uses {2} {3}",
                 current_constant_inst_id_,
                 sem_ir().insts().Get(current_constant_inst_id_), inst_id,
                 sem_ir().insts().Get(inst_id));
    return constants_->Get(inst_id);
  }

  // Returns a constant for the case of a value that should never be used.
  auto GetUnusedConstant(SemIR::TypeId /*type_id*/) const -> llvm::Constant* {
    // TODO: Consider using a poison value of the appropriate type.
    return nullptr;
  }

  // Gets the value to use for a literal.
  auto GetLiteralAsValue() const -> llvm::Constant* {
    return file_context_->GetLiteralAsValue();
  }

  // Gets a callable's function. Returns nullptr for a builtin.
  auto GetFunction(SemIR::FunctionId function_id) -> llvm::Function* {
    return file_context_->GetFunction(function_id);
  }

  auto GetVtable(SemIR::VtableId vtable_id, SemIR::SpecificId specific_id)
      -> llvm::GlobalVariable* {
    return file_context_->GetVtable(vtable_id, specific_id);
  }

  // Returns a lowered type for the given type_id.
  auto GetType(SemIR::TypeId type_id) const -> llvm::Type* {
    return file_context_->GetType(type_id);
  }

  // Returns a lowered value to use for a value of type `type`.
  auto GetTypeAsValue() const -> llvm::Constant* {
    return file_context_->GetTypeAsValue();
  }

  // Returns a lowered global variable declaration.
  auto BuildGlobalVariableDecl(SemIR::VarStorage inst) -> llvm::Constant* {
    return file_context_->BuildGlobalVariableDecl(inst);
  }

  // Sets the InstId of the constant we're currently lowering. This is used to
  // check we don't look at constants that we've not lowered yet.
  auto SetCurrentConstantInstId(SemIR::InstId inst_id) -> void {
    CARBON_CHECK(
        !current_constant_inst_id_.has_value() ||
            sem_ir().insts().GetRawIndex(inst_id) >
                sem_ir().insts().GetRawIndex(current_constant_inst_id_),
        "Visited constants out of order");
    current_constant_inst_id_ = inst_id;
  }

  auto llvm_context() const -> llvm::LLVMContext& {
    return file_context_->llvm_context();
  }
  auto llvm_module() const -> llvm::Module& {
    return file_context_->llvm_module();
  }
  auto sem_ir() const -> const SemIR::File& { return file_context_->sem_ir(); }
  auto inst_namer() const -> const SemIR::InstNamer* {
    return file_context_->inst_namer();
  }

 private:
  FileContext* file_context_;
  const FileContext::LoweredConstantStore* constants_;
  SemIR::InstId current_constant_inst_id_ = SemIR::InstId::None;
};

// Get the element type at a given index from a struct.
static auto GetElementType(llvm::StructType* struct_type, int index)
    -> llvm::Type* {
  return struct_type->getElementType(index);
}

// Get the element type from an array.
static auto GetElementType(llvm::ArrayType* array_type, int /*index*/)
    -> llvm::Type* {
  return array_type->getElementType();
}

// Add padding if necessary to convert the given constant to the specified,
// possibly-padded LLVM type. The type of the constant will either already be
// `llvm_type`, or will be a tail-padded type `<{llvm_type, [i8 x N]}>`.
static auto PadToType(llvm::Constant* constant, llvm::Type* llvm_type)
    -> llvm::Constant* {
  if (constant->getType() == llvm_type) {
    return constant;
  }
  auto* padded = cast<llvm::StructType>(llvm_type);
  CARBON_CHECK(padded->getNumElements() == 2 &&
                   padded->getElementType(0) == constant->getType(),
               "Unexpected type {0} for constant {1}", *llvm_type, *constant);
  return llvm::ConstantStruct::get(
      padded, constant, llvm::PoisonValue::get(padded->getElementType(1)));
}

// Emits an aggregate constant of LLVM type `Type` whose elements are the
// contents of `refs_id`.
template <typename ConstantType, typename Type>
static auto EmitAggregateConstant(ConstantContext& context,
                                  SemIR::InstBlockId refs_id, Type* llvm_type)
    -> llvm::Constant* {
  auto refs = context.sem_ir().inst_blocks().Get(refs_id);
  llvm::SmallVector<llvm::Constant*> elements;
  elements.reserve(refs.size());
  for (auto [i, ref] : llvm::enumerate(refs)) {
    if (auto* constant = context.GetConstant(ref)) {
      auto* elem_type = GetElementType(llvm_type, i);
      elements.push_back(PadToType(constant, elem_type));
    } else {
      return nullptr;
    }
  }

  return ConstantType::get(llvm_type, elements);
}

// For each instruction InstT that can be emitted as a constant, there is a
// function below to convert it to an `llvm::Constant*`:
//
// auto EmitAsConstant(ConstantContext& context, SemIR::InstT inst)
//     -> llvm::Constant*;

// Represent facet values the same as types.
static auto EmitAsConstant(ConstantContext& context, SemIR::FacetValue /*inst*/)
    -> llvm::Constant* {
  return context.GetTypeAsValue();
}

static auto EmitAsConstant(ConstantContext& context, SemIR::StructValue inst)
    -> llvm::Constant* {
  return EmitAggregateConstant<llvm::ConstantStruct>(
      context, inst.elements_id,
      cast<llvm::StructType>(context.GetType(inst.type_id)));
}

static auto EmitAsConstant(ConstantContext& context, SemIR::TupleValue inst)
    -> llvm::Constant* {
  // TODO: Add an ArrayValue instruction and stop using TupleValues to represent
  // array constants.
  if (context.sem_ir().types().Is<SemIR::ArrayType>(inst.type_id)) {
    return EmitAggregateConstant<llvm::ConstantArray>(
        context, inst.elements_id,
        cast<llvm::ArrayType>(context.GetType(inst.type_id)));
  }

  return EmitAggregateConstant<llvm::ConstantStruct>(
      context, inst.elements_id,
      cast<llvm::StructType>(context.GetType(inst.type_id)));
}

static auto EmitAsConstant(ConstantContext& context, SemIR::AddrOf inst)
    -> llvm::Constant* {
  // A constant reference expression is lowered as a pointer, so `AddrOf` is a
  // no-op.
  return context.GetConstant(inst.lvalue_id);
}

static auto EmitAsConstant(ConstantContext& context, SemIR::VtablePtr inst)
    -> llvm::Constant* {
  return context.GetVtable(inst.vtable_id, inst.specific_id);
}

static auto EmitAsConstant(ConstantContext& context,
                           SemIR::AnyAggregateAccess inst) -> llvm::Constant* {
  auto* aggr_addr = context.GetConstant(inst.aggregate_id);
  if (!aggr_addr) {
    return nullptr;
  }
  auto* aggr_type = context.GetType(
      context.sem_ir().insts().Get(inst.aggregate_id).type_id());

  auto* i32_type = llvm::Type::getInt32Ty(context.llvm_context());
  // For now, we rely on the LLVM type's GEP indexes matching the SemIR
  // aggregate element indexes.
  llvm::Constant* indexes[2] = {
      llvm::ConstantInt::get(i32_type, 0),
      llvm::ConstantInt::get(i32_type, inst.index.index),
  };
  auto no_wrap_flags =
      llvm::GEPNoWrapFlags::inBounds() | llvm::GEPNoWrapFlags::noUnsignedWrap();
  return llvm::ConstantExpr::getGetElementPtr(aggr_type, aggr_addr, indexes,
                                              no_wrap_flags);
}

template <typename InstT>
  requires(SemIR::Internal::InstLikeTypeInfo<SemIR::AnyAggregateAccess>::IsKind(
      InstT::Kind))
static auto EmitAsConstant(ConstantContext& context, InstT inst)
    -> llvm::Constant* {
  return EmitAsConstant(context,
                        SemIR::Inst(inst).As<SemIR::AnyAggregateAccess>());
}

static auto EmitAsConstant(ConstantContext& context,
                           SemIR::AssociatedEntity inst) -> llvm::Constant* {
  return context.GetUnusedConstant(inst.type_id);
}

static auto EmitAsConstant(ConstantContext& context, SemIR::BaseDecl inst)
    -> llvm::Constant* {
  return context.GetUnusedConstant(inst.type_id);
}

static auto EmitAsConstant(ConstantContext& context, SemIR::BoolLiteral inst)
    -> llvm::Constant* {
  return llvm::ConstantInt::get(llvm::Type::getInt1Ty(context.llvm_context()),
                                inst.value.index);
}

static auto EmitAsConstant(ConstantContext& context, SemIR::BoundMethod inst)
    -> llvm::Constant* {
  // Propagate just the function; the object is separately provided to the
  // enclosing call as an implicit argument.
  return context.GetConstant(inst.function_decl_id);
}

static auto EmitAsConstant(ConstantContext& context,
                           SemIR::CharLiteralValue /*inst*/)
    -> llvm::Constant* {
  return context.GetLiteralAsValue();
}

static auto EmitAsConstant(ConstantContext& context,
                           SemIR::CompleteTypeWitness inst) -> llvm::Constant* {
  return context.GetUnusedConstant(inst.type_id);
}

static auto EmitAsConstant(ConstantContext& context,
                           SemIR::CppOverloadSetValue /*inst*/)
    -> llvm::Constant* {
  return context.GetLiteralAsValue();
}

static auto EmitAsConstant(ConstantContext& context, SemIR::FieldDecl inst)
    -> llvm::Constant* {
  return context.GetUnusedConstant(inst.type_id);
}

static auto EmitAsConstant(ConstantContext& context,
                           SemIR::FloatLiteralValue /*inst*/)
    -> llvm::Constant* {
  return context.GetLiteralAsValue();
}

static auto EmitAsConstant(ConstantContext& context, SemIR::FloatValue inst)
    -> llvm::Constant* {
  const llvm::APFloat& value = context.sem_ir().floats().Get(inst.float_id);
  return llvm::ConstantFP::get(context.GetType(inst.type_id), value);
}

static auto EmitAsConstant(ConstantContext& context, SemIR::IntValue inst)
    -> llvm::Constant* {
  auto* type = context.GetType(inst.type_id);

  // IntLiteral is represented as an empty struct. All other integer types are
  // represented as an LLVM integer type.
  auto* int_type = dyn_cast<llvm::IntegerType>(type);
  if (!int_type) {
    auto* literal_value = context.GetLiteralAsValue();
    CARBON_CHECK(literal_value->getType() == type);
    return literal_value;
  }

  int bit_width = int_type->getBitWidth();
  auto val = context.sem_ir().ints().GetAtWidth(inst.int_id, bit_width);
  return llvm::ConstantInt::get(type, val);
}

static auto EmitAsConstant(ConstantContext& context, SemIR::Namespace inst)
    -> llvm::Constant* {
  return context.GetUnusedConstant(inst.type_id);
}

static auto EmitAsConstant(ConstantContext& context,
                           SemIR::SpecificFunction inst) -> llvm::Constant* {
  return context.GetUnusedConstant(inst.type_id);
}

static auto EmitAsConstant(ConstantContext& context, SemIR::StringLiteral inst)
    -> llvm::Constant* {
  return llvm::IRBuilder<>(context.llvm_context())
      .CreateGlobalString(
          context.sem_ir().string_literal_values().Get(inst.string_literal_id),
          /*Name=*/"", /*AddressSpace=*/0, &context.llvm_module());
}

static auto EmitAsConstant(ConstantContext& context, SemIR::Temporary inst)
    -> llvm::Constant* {
  auto* const_value = context.GetConstant(inst.init_id);

  llvm::StringRef const_name;
  if (context.inst_namer()) {
    const_name = context.inst_namer()->GetUnscopedNameFor(inst.init_id);
  }
  if (const_name.empty()) {
    const_name = "const";
  }

  auto* global_variable = new llvm::GlobalVariable(
      context.llvm_module(), context.GetType(inst.type_id),
      /*isConstant=*/true, llvm::GlobalVariable::InternalLinkage, const_value,
      const_name);
  return global_variable;
}

static auto EmitAsConstant(ConstantContext& context,
                           SemIR::UninitializedValue inst) -> llvm::Constant* {
  return llvm::PoisonValue::get(context.GetType(inst.type_id));
}

static auto EmitAsConstant(ConstantContext& context, SemIR::VarStorage inst)
    -> llvm::Constant* {
  if (!inst.pattern_id.has_value()) {
    // This constant is a placeholder and should not be used by lowering.
    return nullptr;
  }
  // Create the corresponding global variable declaration.
  return context.BuildGlobalVariableDecl(inst);
}

// Tries to emit an LLVM constant value for this constant instruction. Centrally
// handles some common cases and then dispatches to the relevant EmitAsConstant
// overload based on the type of the instruction for the remaining cases.
template <typename InstT>
static auto MaybeEmitAsConstant(ConstantContext& context, InstT inst)
    -> llvm::Constant* {
  if constexpr (InstT::Kind.constant_kind() == SemIR::InstConstantKind::Never ||
                InstT::Kind.constant_kind() ==
                    SemIR::InstConstantKind::Indirect ||
                InstT::Kind.constant_kind() ==
                    SemIR::InstConstantKind::SymbolicOnly) {
    CARBON_FATAL("Unexpected constant instruction kind {0}", inst);
  } else if constexpr (!InstT::Kind.is_lowered()) {
    // This instruction has a constant value, but that constant value will never
    // be used by lowering.
    return nullptr;
  } else if constexpr (InstT::Kind.is_type() == SemIR::InstIsType::Always) {
    // All types are lowered to the same value.
    return context.GetTypeAsValue();
  } else {
    return EmitAsConstant(context, inst);
  }
}

auto LowerConstants(FileContext& file_context,
                    FileContext::LoweredConstantStore& constants) -> void {
  ConstantContext context(file_context, &constants);
  // Lower each constant in InstId order. This guarantees we lower the
  // dependencies of a constant before we lower the constant itself.
  for (auto [inst_id, const_id] :
       file_context.sem_ir().constant_values().enumerate()) {
    if (!const_id.has_value() || !const_id.is_concrete()) {
      // We are only interested in lowering concrete constants.
      continue;
    }
    auto defining_inst_id =
        file_context.sem_ir().constant_values().GetInstId(const_id);
    if (defining_inst_id != inst_id) {
      // This isn't the instruction that defines the constant.
      continue;
    }

    auto inst = file_context.sem_ir().insts().Get(inst_id);
    if (inst.type_id().has_value() &&
        !file_context.sem_ir().types().IsComplete(inst.type_id()) &&
        !IsRefCategory(SemIR::GetExprCategory(context.sem_ir(), inst_id))) {
      // If a non-reference constant doesn't have a complete type, that means we
      // imported it but didn't actually use it.
      continue;
    }

    context.SetCurrentConstantInstId(inst_id);

    llvm::Constant* value = nullptr;
    CARBON_KIND_SWITCH(inst) {
#define CARBON_SEM_IR_INST_KIND(Name)                 \
  case CARBON_KIND(SemIR::Name const_inst): {         \
    value = MaybeEmitAsConstant(context, const_inst); \
    break;                                            \
  }
#include "toolchain/sem_ir/inst_kind.def"
    }

    constants.Set(inst_id, value);
  }
}

}  // namespace Carbon::Lower