carbon-lang/toolchain/base/value_store.h

// 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

#ifndef CARBON_TOOLCHAIN_BASE_VALUE_STORE_H_
#define CARBON_TOOLCHAIN_BASE_VALUE_STORE_H_

#include <bit>
#include <cstddef>
#include <limits>
#include <type_traits>
#include <utility>

#include "common/check.h"
#include "common/ostream.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/Sequence.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/Support/MemAlloc.h"
#include "toolchain/base/mem_usage.h"
#include "toolchain/base/value_store_types.h"
#include "toolchain/base/yaml.h"

namespace Carbon {

namespace Internal {

// Used as a parent class for non-printable types. This is just for
// std::conditional, not as an API.
class ValueStoreNotPrintable {};

}  // namespace Internal

// A simple wrapper for accumulating values, providing IDs to later retrieve the
// value. This does not do deduplication.
template <typename IdT, typename ValueT>
class ValueStore
    : public std::conditional<std::is_base_of_v<Printable<ValueT>, ValueT>,
                              Yaml::Printable<ValueStore<IdT, ValueT>>,
                              Internal::ValueStoreNotPrintable> {
 public:
  using IdType = IdT;
  using ValueType = ValueStoreTypes<IdT, ValueT>::ValueType;
  using RefType = ValueStoreTypes<IdT, ValueT>::RefType;
  using ConstRefType = ValueStoreTypes<IdT, ValueT>::ConstRefType;

  // A range over references to the values in a ValueStore, returned from
  // `ValueStore::values()`. Hides the complex type name of the iterator
  // internally to provide a type name (`Range`) that can be
  // referred to without auto and templates.
  class Range {
   public:
    explicit Range(const ValueStore& store [[clang::lifetimebound]])
        : flattened_range_(MakeFlattenedRange(store)) {}

    auto begin() const -> auto { return flattened_range_.begin(); }
    auto end() const -> auto { return flattened_range_.end(); }

   private:
    // Flattens the range of `Chunk`s of `ValueType`s into a single
    // range of `ValueType`s.
    static auto MakeFlattenedRange(const ValueStore& store) -> auto {
      // Because indices into `ValueStore` are all sequential values from 0, we
      // can use llvm::seq to walk all indices in the store.
      return llvm::map_range(
          llvm::seq(store.size_),
          [&](int32_t i) -> ConstRefType { return store.Get(IdType(i)); });
    }

    using FlattenedRangeType =
        decltype(MakeFlattenedRange(std::declval<const ValueStore&>()));
    FlattenedRangeType flattened_range_;
  };

  ValueStore() = default;

  // Stores the value and returns an ID to reference it.
  auto Add(ValueType value) -> IdType {
    // This routine is especially hot and the check here relatively expensive
    // for the value provided, so only do this in non-optimized builds to make
    // tracking down issues easier.
    CARBON_DCHECK(size_ < std::numeric_limits<int32_t>::max(), "Id overflow");

    IdType id(size_);
    auto [chunk_index, pos] = IdToChunkIndices(id);
    ++size_;

    CARBON_DCHECK(static_cast<size_t>(chunk_index) <= chunks_.size(),
                  "{0} <= {1}", chunk_index, chunks_.size());
    if (static_cast<size_t>(chunk_index) == chunks_.size()) {
      chunks_.emplace_back();
    }

    CARBON_DCHECK(pos == chunks_[chunk_index].size());
    chunks_[chunk_index].push(std::move(value));
    return id;
  }

  // Returns a mutable value for an ID.
  auto Get(IdType id) -> RefType {
    CARBON_DCHECK(id.index >= 0, "{0}", id);
    CARBON_DCHECK(id.index < size_, "{0}", id);
    auto [chunk_index, pos] = IdToChunkIndices(id);
    return chunks_[chunk_index].at(pos);
  }

  // Returns the value for an ID.
  auto Get(IdType id) const -> ConstRefType {
    CARBON_DCHECK(id.index >= 0, "{0}", id);
    CARBON_DCHECK(id.index < size_, "{0}", id);
    auto [chunk_index, pos] = IdToChunkIndices(id);
    return chunks_[chunk_index].at(pos);
  }

  // Reserves space.
  auto Reserve(size_t size) -> void {
    // We get the number of chunks needed to satisfy `size` by rounding any
    // partial result up.
    size_t num_more_chunks = (size + Chunk::Capacity() - 1) / Chunk::Capacity();
    if (chunks_.size() < num_more_chunks) {
      // We resize() rather than reserve() here to create the new `ChunkType`
      // objects, which will in turn allocate space for values in those chunks
      // (but not initialize them).
      chunks_.resize(num_more_chunks);
    }
  }

  // These are to support printable structures, and are not guaranteed.
  auto OutputYaml() const -> Yaml::OutputMapping {
    return Yaml::OutputMapping([&](Yaml::OutputMapping::Map map) {
      for (auto [id, value] : enumerate()) {
        map.Add(PrintToString(id), Yaml::OutputScalar(value));
      }
    });
  }

  // Collects memory usage of the values.
  auto CollectMemUsage(MemUsage& mem_usage, llvm::StringRef label) const
      -> void {
    mem_usage.Add(label.str(), size_ * sizeof(ValueType),
                  Chunk::CapacityBytes * chunks_.size());
  }

  auto size() const -> size_t { return size_; }

  // Makes an iterable range over references to all values in the ValueStore.
  auto values() const [[clang::lifetimebound]] -> Range { return Range(*this); }

  // Makes an iterable range over pairs of the index and a reference to the
  // value for each value in the store.
  //
  // The range is over references to the values in the store, even if used with
  // `auto` to destructure the pair. In this example, the `value` is a
  // `ConstRefType`:
  // ```
  // for (auto [id, value] : store.enumerate()) { ... }
  // ```
  auto enumerate() const [[clang::lifetimebound]] -> auto {
    // For `it->val`, writing `const std::pair` is required; otherwise
    // `mapped_iterator` incorrectly infers the pointer type for `PointerProxy`.
    // NOLINTNEXTLINE(readability-const-return-type)
    auto index_to_id = [&](int32_t i) -> const std::pair<IdType, ConstRefType> {
      return std::pair<IdType, ConstRefType>(IdType(i), Get(IdType(i)));
    };
    // Because indices into `ValueStore` are all sequential values from 0, we
    // can use llvm::seq to walk all indices in the store.
    return llvm::map_range(llvm::seq(size_), index_to_id);
  }

 private:
  // A chunk of `ValueType`s which has a fixed capacity, but variable size.
  // Tracks the size internally for verifying bounds.
  struct Chunk {
   public:
    // The max size of each chunk allocation for `ValueStore`. This is based on
    // TLB page sizes for the target platform.
    //
    // See https://docs.kernel.org/admin-guide/mm/hugetlbpage.html
    //
    // A 4K chunk size outperforms a 1M chunk size on Linux-x64 and MacOS-arm64
    // in benchmarks and when running file_test.
    //
    // Linux-x64: x64 CPUs support 4K and 2M page sizes, but we see 1M is slower
    // than 4K with tcmalloc in opt builds for our tests.
    //
    // Mac-arm64: arm64 CPUs support 4K, 8K, 64K, 256K, 1M, 4M and up. Like for
    // Linux-x64, 4K outperformed 1M. We didn't try other sizes yet.
    //
    // TODO: Is there a more optimize size for Mac-arm64? What should
    // Linux-arm64 and Mac-x64 use? What should Windows use?
    //
    // TODO: The previous SmallVector<ValueType> seems to outperform 4K chunks
    // (they may be slower by up to 5%) in benchmarks. Find ways to make
    // chunking faster. Should successive chunks get larger in size? That will
    // greatly complicate math for choosing a chunk though.
    static constexpr auto MaxAllocationBytes() -> int32_t {
#if !defined(NDEBUG) || LLVM_ADDRESS_SANITIZER_BUILD
      // Use a small size in unoptimized builds to ensure multiple chunks get
      // used. And do the same in ASAN builds to reduce bookkeeping overheads.
      // Using large allocations (e.g. 1M+) incurs a 10x runtime cost for our
      // tests under ASAN.
      return sizeof(ValueType) * 5;
#else
      return 4 * 1024;
#endif
    }

    // The number of elements stored in each chunk allocation.
    //
    // The number must be a power of two so that that there are no unused values
    // in bits indexing into the allocation.
    static constexpr auto Capacity() -> int32_t {
      constexpr auto MaxElements = MaxAllocationBytes() / sizeof(ValueType);
      return std::bit_floor(MaxElements);
    }

    // The number of bits needed to index each element in a chunk allocation.
    static constexpr auto IndexBits() -> int32_t {
      static_assert(Capacity() > 0);
      return std::bit_width(uint32_t{Capacity() - 1});
    }

    static constexpr auto CapacityBytes = Capacity() * sizeof(ValueType);

    explicit Chunk()
        : buf_(reinterpret_cast<ValueType*>(
              llvm::allocate_buffer(CapacityBytes, alignof(ValueType)))) {}

    // Moving leaves nullptr behind in the moved-from object so that the
    // destructor is a no-op (preventing double free).
    Chunk(Chunk&& rhs) noexcept
        : buf_(std::exchange(rhs.buf_, nullptr)), num_(rhs.num_) {}

    auto operator=(Chunk&& rhs) noexcept -> Chunk& {
      buf_ = std::exchange(rhs.buf_, nullptr);
      num_ = rhs.num_;
      return *this;
    }

    ~Chunk() {
      if (buf_) {
        if constexpr (!std::is_trivially_destructible_v<ValueType>) {
          std::destroy_n(buf_, num_);
        }
        llvm::deallocate_buffer(buf_, CapacityBytes, alignof(ValueType));
      }
    }

    auto at(int32_t i) -> ValueType& {
      CARBON_CHECK(i < num_, "{0}", i);
      return buf_[i];
    }
    auto at(int32_t i) const -> const ValueType& {
      CARBON_CHECK(i < num_, "{0}", i);
      return buf_[i];
    }

    auto push(ValueType&& value) -> void {
      CARBON_CHECK(num_ < Capacity());
      std::construct_at(buf_ + num_, std::move(value));
      ++num_;
    }

    auto size() const -> int32_t { return num_; }

   private:
    // Verify using an `int32_t` for `num_` is sound.
    static_assert(Capacity() <= std::numeric_limits<int32_t>::max());

    ValueType* buf_;
    int32_t num_ = 0;
  };

  // Converts an id into an index into the set of chunks, and an offset into
  // that specific chunk. Looks for index overflow in non-optimized builds.
  static auto IdToChunkIndices(IdType id) -> std::pair<int32_t, int32_t> {
    constexpr auto LowBits = Chunk::IndexBits();

    // Verify there are no unused bits when indexing up to the `Capacity`. This
    // ensures that ids are contiguous values from 0, as if the values were all
    // stored in a single array, and allows using the ids to index into other
    // arrays.
    static_assert((1 << LowBits) == Chunk::Capacity());
    // Simple check to make sure nothing went wildly wrong with the `Capacity`,
    // and we have some room for a chunk index, and that shifting by the number
    // of bits won't be UB in an int32_t.
    static_assert(LowBits < 30);

    // The index of the chunk is the high bits.
    auto chunk = id.index >> LowBits;
    // The index into the chunk is the low bits.
    auto pos = id.index & ((1 << LowBits) - 1);
    return {chunk, pos};
  }

  // Number of elements added to the store. The number should never exceed what
  // fits in an `int32_t`, which is checked in non-optimized builds in Add().
  int32_t size_ = 0;

  // Storage for the `ValueType` objects, indexed by the id. We use a vector of
  // chunks of `ValueType` instead of just a vector of `ValueType` so that
  // addresses of `ValueType` objects are stable. This allows the rest of the
  // toolchain to hold references into `ValueStore` without having to worry
  // about invalidation and use-after-free. We ensure at least one Chunk is held
  // inline so that in the case where there is only a single Chunk (i.e. small
  // files) we can avoid one indirection.
  llvm::SmallVector<Chunk, 1> chunks_;
};

}  // namespace Carbon

#endif  // CARBON_TOOLCHAIN_BASE_VALUE_STORE_H_