| // Copyright 2020 The Abseil Authors |
| // |
| // Licensed under the Apache License, Version 2.0 (the "License"); |
| // you may not use this file except in compliance with the License. |
| // You may obtain a copy of the License at |
| // |
| // https://www.apache.org/licenses/LICENSE-2.0 |
| // |
| // Unless required by applicable law or agreed to in writing, software |
| // distributed under the License is distributed on an "AS IS" BASIS, |
| // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. |
| // See the License for the specific language governing permissions and |
| // limitations under the License. |
| |
| #ifndef ABSL_STRINGS_INTERNAL_CORD_REP_FLAT_H_ |
| #define ABSL_STRINGS_INTERNAL_CORD_REP_FLAT_H_ |
| |
| #include <cassert> |
| #include <cstddef> |
| #include <cstdint> |
| #include <memory> |
| |
| #include "absl/strings/internal/cord_internal.h" |
| |
| namespace absl { |
| ABSL_NAMESPACE_BEGIN |
| namespace cord_internal { |
| |
| // Note: all constants below are never ODR used and internal to cord, we define |
| // these as static constexpr to avoid 'in struct' definition and usage clutter. |
| |
| // Largest and smallest flat node lengths we are willing to allocate |
| // Flat allocation size is stored in tag, which currently can encode sizes up |
| // to 4K, encoded as multiple of either 8 or 32 bytes. |
| // If we allow for larger sizes, we need to change this to 8/64, 16/128, etc. |
| // kMinFlatSize is bounded by tag needing to be at least FLAT * 8 bytes, and |
| // ideally a 'nice' size aligning with allocation and cacheline sizes like 32. |
| // kMaxFlatSize is bounded by the size resulting in a computed tag no greater |
| // than MAX_FLAT_TAG. MAX_FLAT_TAG provides for additional 'high' tag values. |
| static constexpr size_t kFlatOverhead = offsetof(CordRep, storage); |
| static constexpr size_t kMinFlatSize = 32; |
| static constexpr size_t kMaxFlatSize = 4096; |
| static constexpr size_t kMaxFlatLength = kMaxFlatSize - kFlatOverhead; |
| static constexpr size_t kMinFlatLength = kMinFlatSize - kFlatOverhead; |
| |
| constexpr uint8_t AllocatedSizeToTagUnchecked(size_t size) { |
| return static_cast<uint8_t>((size <= 1024) ? size / 8 |
| : 128 + size / 32 - 1024 / 32); |
| } |
| |
| static_assert(kMinFlatSize / 8 >= FLAT, ""); |
| static_assert(AllocatedSizeToTagUnchecked(kMaxFlatSize) <= MAX_FLAT_TAG, ""); |
| |
| // Helper functions for rounded div, and rounding to exact sizes. |
| constexpr size_t DivUp(size_t n, size_t m) { return (n + m - 1) / m; } |
| constexpr size_t RoundUp(size_t n, size_t m) { return DivUp(n, m) * m; } |
| |
| // Returns the size to the nearest equal or larger value that can be |
| // expressed exactly as a tag value. |
| inline size_t RoundUpForTag(size_t size) { |
| return RoundUp(size, (size <= 1024) ? 8 : 32); |
| } |
| |
| // Converts the allocated size to a tag, rounding down if the size |
| // does not exactly match a 'tag expressible' size value. The result is |
| // undefined if the size exceeds the maximum size that can be encoded in |
| // a tag, i.e., if size is larger than TagToAllocatedSize(<max tag>). |
| inline uint8_t AllocatedSizeToTag(size_t size) { |
| const uint8_t tag = AllocatedSizeToTagUnchecked(size); |
| assert(tag <= MAX_FLAT_TAG); |
| return tag; |
| } |
| |
| // Converts the provided tag to the corresponding allocated size |
| constexpr size_t TagToAllocatedSize(uint8_t tag) { |
| return (tag <= 128) ? (tag * 8) : (1024 + (tag - 128) * 32); |
| } |
| |
| // Converts the provided tag to the corresponding available data length |
| constexpr size_t TagToLength(uint8_t tag) { |
| return TagToAllocatedSize(tag) - kFlatOverhead; |
| } |
| |
| // Enforce that kMaxFlatSize maps to a well-known exact tag value. |
| static_assert(TagToAllocatedSize(224) == kMaxFlatSize, "Bad tag logic"); |
| |
| struct CordRepFlat : public CordRep { |
| // Creates a new flat node. |
| static CordRepFlat* New(size_t len) { |
| if (len <= kMinFlatLength) { |
| len = kMinFlatLength; |
| } else if (len > kMaxFlatLength) { |
| len = kMaxFlatLength; |
| } |
| |
| // Round size up so it matches a size we can exactly express in a tag. |
| const size_t size = RoundUpForTag(len + kFlatOverhead); |
| void* const raw_rep = ::operator new(size); |
| CordRepFlat* rep = new (raw_rep) CordRepFlat(); |
| rep->tag = AllocatedSizeToTag(size); |
| return rep; |
| } |
| |
| // Deletes a CordRepFlat instance created previously through a call to New(). |
| // Flat CordReps are allocated and constructed with raw ::operator new and |
| // placement new, and must be destructed and deallocated accordingly. |
| static void Delete(CordRep*rep) { |
| assert(rep->tag >= FLAT && rep->tag <= MAX_FLAT_TAG); |
| |
| #if defined(__cpp_sized_deallocation) |
| size_t size = TagToAllocatedSize(rep->tag); |
| rep->~CordRep(); |
| ::operator delete(rep, size); |
| #else |
| rep->~CordRep(); |
| ::operator delete(rep); |
| #endif |
| } |
| |
| // Returns a pointer to the data inside this flat rep. |
| char* Data() { return storage; } |
| const char* Data() const { return storage; } |
| |
| // Returns the maximum capacity (payload size) of this instance. |
| size_t Capacity() const { return TagToLength(tag); } |
| |
| // Returns the allocated size (payload + overhead) of this instance. |
| size_t AllocatedSize() const { return TagToAllocatedSize(tag); } |
| }; |
| |
| // Now that CordRepFlat is defined, we can define CordRep's helper casts: |
| inline CordRepFlat* CordRep::flat() { |
| assert(tag >= FLAT && tag <= MAX_FLAT_TAG); |
| return reinterpret_cast<CordRepFlat*>(this); |
| } |
| |
| inline const CordRepFlat* CordRep::flat() const { |
| assert(tag >= FLAT && tag <= MAX_FLAT_TAG); |
| return reinterpret_cast<const CordRepFlat*>(this); |
| } |
| |
| } // namespace cord_internal |
| ABSL_NAMESPACE_END |
| } // namespace absl |
| |
| #endif // ABSL_STRINGS_INTERNAL_CORD_REP_FLAT_H_ |