-- d85f04af95a6fdafb102f7dc393d78d4431b47e5 by Abseil Team <absl-team@google.com>: Internal change for cord ring PiperOrigin-RevId: 344541196 -- 1ff57908e31a09ec0c98d8316da1263092cc3a1c by Abseil Team <absl-team@google.com>: Fix typo in comment. PiperOrigin-RevId: 344214280 GitOrigin-RevId: d85f04af95a6fdafb102f7dc393d78d4431b47e5 Change-Id: I58b3c28f62a5d10dd665b17d58a121f371e1260apull/861/head
Abseil Team
4 years ago
committed by
vslashg
parent
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commit
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7 changed files with 180 additions and 109 deletions
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// Copyright 2020 The Abseil Authors
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//
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// Licensed under the Apache License, Version 2.0 (the "License");
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// you may not use this file except in compliance with the License.
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// You may obtain a copy of the License at
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//
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// https://www.apache.org/licenses/LICENSE-2.0
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//
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// Unless required by applicable law or agreed to in writing, software
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// distributed under the License is distributed on an "AS IS" BASIS,
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// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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// See the License for the specific language governing permissions and
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// limitations under the License.
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#ifndef ABSL_STRINGS_INTERNAL_CORD_REP_FLAT_H_ |
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#define ABSL_STRINGS_INTERNAL_CORD_REP_FLAT_H_ |
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#include <cassert> |
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#include <cstddef> |
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#include <cstdint> |
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#include <memory> |
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#include "absl/strings/internal/cord_internal.h" |
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namespace absl { |
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ABSL_NAMESPACE_BEGIN |
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namespace cord_internal { |
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// Note: all constants below are never ODR used and internal to cord, we define
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// these as static constexpr to avoid 'in struct' definition and usage clutter.
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// Largest and smallest flat node lengths we are willing to allocate
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// Flat allocation size is stored in tag, which currently can encode sizes up
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// to 4K, encoded as multiple of either 8 or 32 bytes.
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// If we allow for larger sizes, we need to change this to 8/64, 16/128, etc.
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// kMinFlatSize is bounded by tag needing to be at least FLAT * 8 bytes, and
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// ideally a 'nice' size aligning with allocation and cacheline sizes like 32.
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// kMaxFlatSize is bounded by the size resulting in a computed tag no greater
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// than MAX_FLAT_TAG. MAX_FLAT_TAG provides for additional 'high' tag values.
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static constexpr size_t kFlatOverhead = offsetof(CordRep, data); |
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static constexpr size_t kMinFlatSize = 32; |
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static constexpr size_t kMaxFlatSize = 4096; |
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static constexpr size_t kMaxFlatLength = kMaxFlatSize - kFlatOverhead; |
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static constexpr size_t kMinFlatLength = kMinFlatSize - kFlatOverhead; |
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static constexpr size_t AllocatedSizeToTagUnchecked(size_t size) { |
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return (size <= 1024) ? size / 8 : 128 + size / 32 - 1024 / 32; |
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} |
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static_assert(kMinFlatSize / 8 >= FLAT, ""); |
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static_assert(AllocatedSizeToTagUnchecked(kMaxFlatSize) <= MAX_FLAT_TAG, ""); |
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// Helper functions for rounded div, and rounding to exact sizes.
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static size_t DivUp(size_t n, size_t m) { return (n + m - 1) / m; } |
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static size_t RoundUp(size_t n, size_t m) { return DivUp(n, m) * m; } |
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// Returns the size to the nearest equal or larger value that can be
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// expressed exactly as a tag value.
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static size_t RoundUpForTag(size_t size) { |
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return RoundUp(size, (size <= 1024) ? 8 : 32); |
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} |
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// Converts the allocated size to a tag, rounding down if the size
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// does not exactly match a 'tag expressible' size value. The result is
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// undefined if the size exceeds the maximum size that can be encoded in
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// a tag, i.e., if size is larger than TagToAllocatedSize(<max tag>).
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static uint8_t AllocatedSizeToTag(size_t size) { |
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const size_t tag = AllocatedSizeToTagUnchecked(size); |
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assert(tag <= MAX_FLAT_TAG); |
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return tag; |
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} |
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// Converts the provided tag to the corresponding allocated size
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static constexpr size_t TagToAllocatedSize(uint8_t tag) { |
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return (tag <= 128) ? (tag * 8) : (1024 + (tag - 128) * 32); |
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} |
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// Converts the provided tag to the corresponding available data length
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static constexpr size_t TagToLength(uint8_t tag) { |
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return TagToAllocatedSize(tag) - kFlatOverhead; |
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} |
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// Enforce that kMaxFlatSize maps to a well-known exact tag value.
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static_assert(TagToAllocatedSize(224) == kMaxFlatSize, "Bad tag logic"); |
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struct CordRepFlat : public CordRep { |
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// Creates a new flat node.
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static CordRepFlat* New(size_t len) { |
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if (len <= kMinFlatLength) { |
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len = kMinFlatLength; |
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} else if (len > kMaxFlatLength) { |
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len = kMaxFlatLength; |
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} |
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// Round size up so it matches a size we can exactly express in a tag.
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const size_t size = RoundUpForTag(len + kFlatOverhead); |
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void* const raw_rep = ::operator new(size); |
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CordRepFlat* rep = new (raw_rep) CordRepFlat(); |
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rep->tag = AllocatedSizeToTag(size); |
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return rep; |
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} |
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// Deletes a CordRepFlat instance created previously through a call to New().
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// Flat CordReps are allocated and constructed with raw ::operator new and
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// placement new, and must be destructed and deallocated accordingly.
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static void Delete(CordRep*rep) { |
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assert(rep->tag >= FLAT); |
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#if defined(__cpp_sized_deallocation) |
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size_t size = TagToAllocatedSize(rep->tag); |
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rep->~CordRep(); |
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::operator delete(rep, size); |
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#else |
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rep->~CordRep(); |
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::operator delete(rep); |
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#endif |
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} |
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// Returns the maximum capacity (payload size) of this instance.
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size_t Capacity() const { return TagToLength(tag); } |
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// Returns the allocated size (payload + overhead) of this instance.
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size_t AllocatedSize() const { return TagToAllocatedSize(tag); } |
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}; |
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} // namespace cord_internal
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ABSL_NAMESPACE_END |
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} // namespace absl
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#endif // ABSL_STRINGS_INTERNAL_CORD_REP_FLAT_H_
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