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// 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_INTERNAL_H_
#define ABSL_STRINGS_INTERNAL_CORD_INTERNAL_H_
#include <atomic>
#include <cassert>
#include <cstddef>
#include <cstdint>
#include <type_traits>
#include "absl/meta/type_traits.h"
#include "absl/strings/string_view.h"
namespace absl {
ABSL_NAMESPACE_BEGIN
namespace cord_internal {
// Wraps std::atomic for reference counting.
class Refcount {
public:
Refcount() : count_{1} {}
~Refcount() {}
// Increments the reference count by 1. Imposes no memory ordering.
inline void Increment() { count_.fetch_add(1, std::memory_order_relaxed); }
// Asserts that the current refcount is greater than 0. If the refcount is
// greater than 1, decrements the reference count by 1.
//
// Returns false if there are no references outstanding; true otherwise.
// Inserts barriers to ensure that state written before this method returns
// false will be visible to a thread that just observed this method returning
// false.
inline bool Decrement() {
int32_t refcount = count_.load(std::memory_order_acquire);
assert(refcount > 0);
return refcount != 1 && count_.fetch_sub(1, std::memory_order_acq_rel) != 1;
}
// Same as Decrement but expect that refcount is greater than 1.
inline bool DecrementExpectHighRefcount() {
int32_t refcount = count_.fetch_sub(1, std::memory_order_acq_rel);
assert(refcount > 0);
return refcount != 1;
}
// Returns the current reference count using acquire semantics.
inline int32_t Get() const { return count_.load(std::memory_order_acquire); }
// Returns whether the atomic integer is 1.
// If the reference count is used in the conventional way, a
// reference count of 1 implies that the current thread owns the
// reference and no other thread shares it.
// This call performs the test for a reference count of one, and
// performs the memory barrier needed for the owning thread
// to act on the object, knowing that it has exclusive access to the
// object.
inline bool IsOne() { return count_.load(std::memory_order_acquire) == 1; }
private:
std::atomic<int32_t> count_;
};
// The overhead of a vtable is too much for Cord, so we roll our own subclasses
// using only a single byte to differentiate classes from each other - the "tag"
// byte. Define the subclasses first so we can provide downcasting helper
// functions in the base class.
struct CordRepConcat;
struct CordRepSubstring;
struct CordRepExternal;
struct CordRep {
// The following three fields have to be less than 32 bytes since
// that is the smallest supported flat node size.
// We use uint64_t for the length even in 32-bit binaries.
uint64_t length;
Refcount refcount;
// If tag < FLAT, it represents CordRepKind and indicates the type of node.
// Otherwise, the node type is CordRepFlat and the tag is the encoded size.
uint8_t tag;
char data[1]; // Starting point for flat array: MUST BE LAST FIELD of CordRep
inline CordRepConcat* concat();
inline const CordRepConcat* concat() const;
inline CordRepSubstring* substring();
inline const CordRepSubstring* substring() const;
inline CordRepExternal* external();
inline const CordRepExternal* external() const;
};
struct CordRepConcat : public CordRep {
CordRep* left;
CordRep* right;
uint8_t depth() const { return static_cast<uint8_t>(data[0]); }
void set_depth(uint8_t depth) { data[0] = static_cast<char>(depth); }
};
struct CordRepSubstring : public CordRep {
size_t start; // Starting offset of substring in child
CordRep* child;
};
// TODO(strel): replace the following logic (and related functions in cord.cc)
// with container_internal::Layout.
// Alignment requirement for CordRepExternal so that the type erased releaser
// will be stored at a suitably aligned address.
constexpr size_t ExternalRepAlignment() {
#if defined(__STDCPP_DEFAULT_NEW_ALIGNMENT__)
return __STDCPP_DEFAULT_NEW_ALIGNMENT__;
#else
return alignof(max_align_t);
#endif
}
// Type for function pointer that will invoke and destroy the type-erased
// releaser function object. Accepts a pointer to the releaser and the
// `string_view` that were passed in to `NewExternalRep` below. The return value
// is the size of the `Releaser` type.
using ExternalReleaserInvoker = size_t (*)(void*, absl::string_view);
// External CordReps are allocated together with a type erased releaser. The
// releaser is stored in the memory directly following the CordRepExternal.
struct alignas(ExternalRepAlignment()) CordRepExternal : public CordRep {
const char* base;
// Pointer to function that knows how to call and destroy the releaser.
ExternalReleaserInvoker releaser_invoker;
};
// TODO(strel): look into removing, it doesn't seem like anything relies on this
static_assert(sizeof(CordRepConcat) == sizeof(CordRepSubstring), "");
} // namespace cord_internal
ABSL_NAMESPACE_END
} // namespace absl
#endif // ABSL_STRINGS_INTERNAL_CORD_INTERNAL_H_
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