carbon-lang/toolchain/check/subst.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/check/subst.h"

#include "toolchain/base/kind_switch.h"
#include "toolchain/check/eval.h"
#include "toolchain/check/generic.h"
#include "toolchain/check/inst.h"
#include "toolchain/sem_ir/copy_on_write_block.h"
#include "toolchain/sem_ir/ids.h"
#include "toolchain/sem_ir/inst.h"

namespace Carbon::Check {

namespace {

// Information about an instruction that we are substituting into.
struct WorklistItem {
  // The instruction that we are substituting into.
  SemIR::InstId inst_id;
  // Whether the operands of this instruction have been added to the worklist.
  bool is_expanded : 1;
  // The index of the worklist item to process after we finish updating this
  // one. For the final child of an instruction, this is the parent. For any
  // other child, this is the index of the next child of the parent. For the
  // root, this is -1.
  int next_index : 31;
};

// A list of instructions that we're currently in the process of substituting
// into. For details of the algorithm used here, see `SubstConstant`.
class Worklist {
 public:
  explicit Worklist(SemIR::InstId root_id) {
    worklist_.push_back(
        {.inst_id = root_id, .is_expanded = false, .next_index = -1});
  }

  auto operator[](int index) -> WorklistItem& { return worklist_[index]; }
  auto size() -> int { return worklist_.size(); }
  auto back() -> WorklistItem& { return worklist_.back(); }

  auto Push(SemIR::InstId inst_id) -> void {
    CARBON_CHECK(inst_id.has_value());
    worklist_.push_back({.inst_id = inst_id,
                         .is_expanded = false,
                         .next_index = static_cast<int>(worklist_.size() + 1)});
    CARBON_CHECK(worklist_.back().next_index > 0, "Constant too large.");
  }
  auto Pop() -> SemIR::InstId { return worklist_.pop_back_val().inst_id; }

 private:
  // Constants can get pretty large, so use a large worklist. This should be
  // about 4KiB, which should be small enough to comfortably fit on the stack,
  // but large enough that it's unlikely that we'll need a heap allocation.
  llvm::SmallVector<WorklistItem, 512> worklist_;
};

}  // namespace

// Pushes the specified operand onto the worklist.
static auto PushOperand(Context& context, Worklist& worklist,
                        SemIR::Inst::ArgAndKind arg) -> void {
  auto push_block = [&](SemIR::InstBlockId block_id) {
    for (auto inst_id :
         context.inst_blocks().Get(SemIR::InstBlockId(block_id))) {
      worklist.Push(inst_id);
    }
  };

  auto push_specific = [&](SemIR::SpecificId specific_id) {
    if (specific_id.has_value()) {
      push_block(context.specifics().Get(specific_id).args_id);
    }
  };

  CARBON_KIND_SWITCH(arg) {
    case CARBON_KIND(SemIR::InstId inst_id): {
      if (inst_id.has_value()) {
        worklist.Push(inst_id);
      }
      break;
    }
    case CARBON_KIND(SemIR::MetaInstId inst_id): {
      if (inst_id.has_value()) {
        worklist.Push(inst_id);
      }
      break;
    }
    case CARBON_KIND(SemIR::InstBlockId inst_block_id): {
      push_block(inst_block_id);
      break;
    }
    case CARBON_KIND(SemIR::StructTypeFieldsId fields_id): {
      for (auto field : context.struct_type_fields().Get(fields_id)) {
        worklist.Push(field.type_inst_id);
      }
      break;
    }
    case CARBON_KIND(SemIR::SpecificId specific_id): {
      push_specific(specific_id);
      break;
    }
    case CARBON_KIND(SemIR::SpecificInterfaceId interface_id): {
      auto interface = context.specific_interfaces().Get(interface_id);
      push_specific(interface.specific_id);
      break;
    }
    case CARBON_KIND(SemIR::FacetTypeId facet_type_id): {
      const auto& facet_type_info = context.facet_types().Get(facet_type_id);
      for (auto interface : facet_type_info.extend_constraints) {
        push_specific(interface.specific_id);
      }
      for (auto interface : facet_type_info.self_impls_constraints) {
        push_specific(interface.specific_id);
      }
      for (auto rewrite : facet_type_info.rewrite_constraints) {
        worklist.Push(rewrite.lhs_id);
        worklist.Push(rewrite.rhs_id);
      }
      // TODO: Process other requirements as well.
      break;
    }
    default:
      break;
  }
}

// Converts the operands of this instruction into `InstId`s and pushes them onto
// the worklist.
static auto ExpandOperands(Context& context, Worklist& worklist,
                           SemIR::InstId inst_id) -> void {
  auto inst = context.insts().Get(inst_id);
  if (inst.type_id().has_value()) {
    worklist.Push(context.types().GetInstId(inst.type_id()));
  }
  PushOperand(context, worklist, inst.arg0_and_kind());
  PushOperand(context, worklist, inst.arg1_and_kind());
}

// Pops the specified operand from the worklist and returns it.
static auto PopOperand(Context& context, Worklist& worklist,
                       SemIR::Inst::ArgAndKind arg) -> int32_t {
  auto pop_block_id = [&](SemIR::InstBlockId old_inst_block_id) {
    auto size = context.inst_blocks().Get(old_inst_block_id).size();
    SemIR::CopyOnWriteInstBlock new_inst_block(&context.sem_ir(),
                                               old_inst_block_id);
    for (auto i : llvm::reverse(llvm::seq(size))) {
      new_inst_block.Set(i, worklist.Pop());
    }
    return new_inst_block.GetCanonical();
  };

  auto pop_specific = [&](SemIR::SpecificId specific_id) {
    if (!specific_id.has_value()) {
      return specific_id;
    }
    auto& specific = context.specifics().Get(specific_id);
    auto args_id = pop_block_id(specific.args_id);
    return context.specifics().GetOrAdd(specific.generic_id, args_id);
  };

  CARBON_KIND_SWITCH(arg) {
    case CARBON_KIND(SemIR::InstId inst_id): {
      if (!inst_id.has_value()) {
        return arg.value();
      }
      return worklist.Pop().index;
    }
    case CARBON_KIND(SemIR::MetaInstId inst_id): {
      if (!inst_id.has_value()) {
        return arg.value();
      }
      return worklist.Pop().index;
    }
    case CARBON_KIND(SemIR::InstBlockId inst_block_id): {
      return pop_block_id(inst_block_id).index;
    }
    case CARBON_KIND(SemIR::StructTypeFieldsId old_fields_id): {
      auto old_fields = context.struct_type_fields().Get(old_fields_id);
      SemIR::CopyOnWriteStructTypeFieldsBlock new_fields(&context.sem_ir(),
                                                         old_fields_id);
      for (auto i : llvm::reverse(llvm::seq(old_fields.size()))) {
        new_fields.Set(i, {.name_id = old_fields[i].name_id,
                           .type_inst_id = worklist.Pop()});
      }
      return new_fields.GetCanonical().index;
    }
    case CARBON_KIND(SemIR::SpecificId specific_id): {
      return pop_specific(specific_id).index;
    }
    case CARBON_KIND(SemIR::SpecificInterfaceId interface_id): {
      auto interface = context.specific_interfaces().Get(interface_id);
      auto specific_id = pop_specific(interface.specific_id);
      return context.specific_interfaces()
          .Add({
              .interface_id = interface.interface_id,
              .specific_id = specific_id,
          })
          .index;
    }
    case CARBON_KIND(SemIR::FacetTypeId facet_type_id): {
      const auto& old_facet_type_info =
          context.facet_types().Get(facet_type_id);
      SemIR::FacetTypeInfo new_facet_type_info;
      // Since these were added to a stack, we get them back in reverse order.
      new_facet_type_info.rewrite_constraints.resize(
          old_facet_type_info.rewrite_constraints.size(),
          SemIR::FacetTypeInfo::RewriteConstraint::None);
      for (auto& new_constraint :
           llvm::reverse(new_facet_type_info.rewrite_constraints)) {
        auto rhs_id = worklist.Pop();
        auto lhs_id = worklist.Pop();
        new_constraint = {.lhs_id = lhs_id, .rhs_id = rhs_id};
      }
      new_facet_type_info.self_impls_constraints.resize(
          old_facet_type_info.self_impls_constraints.size(),
          SemIR::SpecificInterface::None);
      for (auto [old_constraint, new_constraint] : llvm::reverse(
               llvm::zip(old_facet_type_info.self_impls_constraints,
                         new_facet_type_info.self_impls_constraints))) {
        new_constraint = {
            .interface_id = old_constraint.interface_id,
            .specific_id = pop_specific(old_constraint.specific_id)};
      }
      new_facet_type_info.extend_constraints.resize(
          old_facet_type_info.extend_constraints.size(),
          SemIR::SpecificInterface::None);
      for (auto [old_constraint, new_constraint] :
           llvm::reverse(llvm::zip(old_facet_type_info.extend_constraints,
                                   new_facet_type_info.extend_constraints))) {
        new_constraint = {
            .interface_id = old_constraint.interface_id,
            .specific_id = pop_specific(old_constraint.specific_id)};
      }
      new_facet_type_info.other_requirements =
          old_facet_type_info.other_requirements;
      new_facet_type_info.Canonicalize();
      return context.facet_types().Add(new_facet_type_info).index;
    }
    default:
      return arg.value();
  }
}

// Pops the operands of the specified instruction off the worklist and rebuilds
// the instruction with the updated operands if it has changed.
static auto Rebuild(Context& context, Worklist& worklist, SemIR::InstId inst_id,
                    const SubstInstCallbacks& callbacks) -> SemIR::InstId {
  auto inst = context.insts().Get(inst_id);

  // Note that we pop in reverse order because we pushed them in forwards order.
  int32_t arg1 = PopOperand(context, worklist, inst.arg1_and_kind());
  int32_t arg0 = PopOperand(context, worklist, inst.arg0_and_kind());
  auto type_id = inst.type_id().has_value()
                     ? context.types().GetTypeIdForTypeInstId(worklist.Pop())
                     : SemIR::TypeId::None;
  if (type_id == inst.type_id() && arg0 == inst.arg0() && arg1 == inst.arg1()) {
    return callbacks.ReuseUnchanged(inst_id);
  }

  // TODO: Do we need to require this type to be complete?
  inst.SetType(type_id);
  inst.SetArgs(arg0, arg1);
  return callbacks.Rebuild(inst_id, inst);
}

auto SubstInst(Context& context, SemIR::InstId inst_id,
               const SubstInstCallbacks& callbacks) -> SemIR::InstId {
  Worklist worklist(inst_id);

  // For each instruction that forms part of the constant, we will visit it
  // twice:
  //
  // - First, we visit it with `is_expanded == false`, we add all of its
  //   operands onto the worklist, and process them by following this same
  //   process.
  // - Then, once all operands are processed, we visit the instruction with
  //   `is_expanded == true`, pop the operands back off the worklist, and if any
  //   of them changed, rebuild this instruction.
  //
  // The second step is skipped if we can detect in the first step that the
  // instruction will not need to be rebuilt.
  int index = 0;
  while (index != -1) {
    auto& item = worklist[index];

    if (item.is_expanded) {
      // Rebuild this item if necessary. Note that this might pop items from the
      // worklist but does not reallocate, so does not invalidate `item`.
      item.inst_id = Rebuild(context, worklist, item.inst_id, callbacks);
      index = item.next_index;
      continue;
    }

    if (callbacks.Subst(item.inst_id)) {
      index = item.next_index;
      continue;
    }

    // Extract the operands of this item into the worklist. Note that this
    // modifies the worklist, so it's not safe to use `item` after
    // `ExpandOperands` returns.
    item.is_expanded = true;
    int first_operand = worklist.size();
    int next_index = item.next_index;
    ExpandOperands(context, worklist, item.inst_id);

    // If there are any operands, go and update them before rebuilding this
    // item.
    if (worklist.size() > first_operand) {
      worklist.back().next_index = index;
      index = first_operand;
    } else {
      // No need to rebuild this instruction: its operands can't be changed by
      // substitution because it has none.
      index = next_index;
    }
  }

  CARBON_CHECK(worklist.size() == 1,
               "Unexpected data left behind in work list");
  return worklist.back().inst_id;
}

namespace {
// Callbacks for performing substitution of a set of Substitutions into a
// symbolic constant.
class SubstConstantCallbacks final : public SubstInstCallbacks {
 public:
  // `context` must not be null.
  SubstConstantCallbacks(Context* context, Substitutions substitutions)
      : context_(context), substitutions_(substitutions) {}

  // Applies the given Substitutions to an instruction, in order to replace
  // BindSymbolicName instructions with the value of the binding.
  auto Subst(SemIR::InstId& inst_id) const -> bool override {
    if (context_->constant_values().Get(inst_id).is_concrete()) {
      // This instruction is a concrete constant, so can't contain any
      // bindings that need to be substituted.
      return true;
    }

    auto entity_name_id = SemIR::EntityNameId::None;
    if (auto bind =
            context_->insts().TryGetAs<SemIR::BindSymbolicName>(inst_id)) {
      entity_name_id = bind->entity_name_id;
    } else if (auto bind =
                   context_->insts().TryGetAs<SemIR::SymbolicBindingPattern>(
                       inst_id)) {
      entity_name_id = bind->entity_name_id;
    } else {
      return false;
    }

    // This is a symbolic binding. Check if we're substituting it.
    // TODO: Consider building a hash map for substitutions. We might have a
    // lot of them.
    for (auto [bind_index, replacement_id] : substitutions_) {
      if (context_->entity_names().Get(entity_name_id).bind_index() ==
          bind_index) {
        // This is the binding we're replacing. Perform substitution.
        inst_id = context_->constant_values().GetInstId(replacement_id);
        return true;
      }
    }

    // If it's not being substituted, don't look through it. Its constant
    // value doesn't depend on its operand.
    return true;
  }

  // Rebuilds an instruction by building a new constant.
  auto Rebuild(SemIR::InstId old_inst_id, SemIR::Inst new_inst) const
      -> SemIR::InstId override {
    auto const_id = EvalOrAddInst(
        *context_,
        SemIR::LocIdAndInst::UncheckedLoc(
            context_->insts().GetLocId(old_inst_id).ToImplicit(), new_inst));
    CARBON_CHECK(const_id.has_value(),
                 "Substitution into constant produced non-constant");
    return context_->constant_values().GetInstId(const_id);
  }

 private:
  Context* context_;
  Substitutions substitutions_;
};
}  // namespace

auto SubstConstant(Context& context, SemIR::ConstantId const_id,
                   Substitutions substitutions) -> SemIR::ConstantId {
  CARBON_CHECK(const_id.is_constant(), "Substituting into non-constant");

  if (substitutions.empty()) {
    // Nothing to substitute.
    return const_id;
  }

  if (!const_id.is_symbolic()) {
    // A concrete constant can't contain a reference to a symbolic binding.
    return const_id;
  }

  auto subst_inst_id =
      SubstInst(context, context.constant_values().GetInstId(const_id),
                SubstConstantCallbacks(&context, substitutions));
  return context.constant_values().Get(subst_inst_id);
}

}  // namespace Carbon::Check