1e39f8626a
-- b842b7fd9b1289be31f0b7ee8e62e48e550747cf by Greg Falcon <gfalcon@google.com>: Change the Cord str_format formatter to use iteration instead of CordReader. When Cord is publicly released, CordReader is not going with it. PiperOrigin-RevId: 284780736 -- 28e76c08ea7185a7ff9f4e0e02ae565fbbf7980f by Greg Falcon <gfalcon@google.com>: Implementation detail change. Introduce ABSL_NAMESPACE_BEGIN and _END annotation macros which indicate the beginning and end of a `namespace absl` scope. Currently these do nothing, but they will be used to inject an inline namespace for LTS builds (to avoid symbol collisions against other Abseil versions). These macros should not be used by end users, because end users should never write `namespace absl {` in their own code. This CL applies these annotations to all code under //absl/base/. The rest of Abseil will be annotated in this way in follow-up CLs. PiperOrigin-RevId: 284776410 -- e1711dc6d696dcca50d4e7d4b4d8f3076575b7ec by Abseil Team <absl-team@google.com>: --help changed to report long flags. PiperOrigin-RevId: 284757720 -- 78f66a68f428bbbd19d8d60e1125f43ba765fd35 by Tom Manshreck <shreck@google.com>: Update comment on + or - in SimpleAToi() PiperOrigin-RevId: 284231843 GitOrigin-RevId: b842b7fd9b1289be31f0b7ee8e62e48e550747cf Change-Id: I3046b31391bd11c8bc4abab7785a863c377cd757
179 lines
6.6 KiB
C++
179 lines
6.6 KiB
C++
// Copyright 2017 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_BASE_INTERNAL_ATOMIC_HOOK_H_
|
|
#define ABSL_BASE_INTERNAL_ATOMIC_HOOK_H_
|
|
|
|
#include <atomic>
|
|
#include <cassert>
|
|
#include <cstdint>
|
|
#include <utility>
|
|
|
|
#include "absl/base/config.h"
|
|
|
|
#ifdef _MSC_FULL_VER
|
|
#define ABSL_HAVE_WORKING_ATOMIC_POINTER 0
|
|
#define ABSL_HAVE_WORKING_CONSTEXPR_STATIC_INIT 0
|
|
#else
|
|
#define ABSL_HAVE_WORKING_ATOMIC_POINTER 1
|
|
#define ABSL_HAVE_WORKING_CONSTEXPR_STATIC_INIT 1
|
|
#endif
|
|
|
|
namespace absl {
|
|
ABSL_NAMESPACE_BEGIN
|
|
namespace base_internal {
|
|
|
|
template <typename T>
|
|
class AtomicHook;
|
|
|
|
// `AtomicHook` is a helper class, templatized on a raw function pointer type,
|
|
// for implementing Abseil customization hooks. It is a callable object that
|
|
// dispatches to the registered hook. Objects of type `AtomicHook` must have
|
|
// static or thread storage duration.
|
|
//
|
|
// A default constructed object performs a no-op (and returns a default
|
|
// constructed object) if no hook has been registered.
|
|
//
|
|
// Hooks can be pre-registered via constant initialization, for example,
|
|
// `ABSL_CONST_INIT static AtomicHook<void(*)()> my_hook(DefaultAction);`
|
|
// and then changed at runtime via a call to `Store()`.
|
|
//
|
|
// Reads and writes guarantee memory_order_acquire/memory_order_release
|
|
// semantics.
|
|
template <typename ReturnType, typename... Args>
|
|
class AtomicHook<ReturnType (*)(Args...)> {
|
|
public:
|
|
using FnPtr = ReturnType (*)(Args...);
|
|
|
|
// Constructs an object that by default performs a no-op (and
|
|
// returns a default constructed object) when no hook as been registered.
|
|
constexpr AtomicHook() : AtomicHook(DummyFunction) {}
|
|
|
|
// Constructs an object that by default dispatches to/returns the
|
|
// pre-registered default_fn when no hook has been registered at runtime.
|
|
#if ABSL_HAVE_WORKING_ATOMIC_POINTER && ABSL_HAVE_WORKING_CONSTEXPR_STATIC_INIT
|
|
explicit constexpr AtomicHook(FnPtr default_fn)
|
|
: hook_(default_fn), default_fn_(default_fn) {}
|
|
#else
|
|
// On MSVC, this function sometimes executes after dynamic initialization =(.
|
|
// If a non-zero `hook_` has been installed by a dynamic initializer, we want
|
|
// to preserve it. If not, `hook_` will be zero initialized and we have no
|
|
// need to set it to `kUninitialized`.
|
|
// https://developercommunity.visualstudio.com/content/problem/336946/class-with-constexpr-constructor-not-using-static.html
|
|
explicit constexpr AtomicHook(FnPtr default_fn)
|
|
: /* hook_(deliberately omitted), */ default_fn_(default_fn) {
|
|
static_assert(kUninitialized == 0, "here we rely on zero-initialization");
|
|
}
|
|
#endif
|
|
|
|
// Stores the provided function pointer as the value for this hook.
|
|
//
|
|
// This is intended to be called once. Multiple calls are legal only if the
|
|
// same function pointer is provided for each call. The store is implemented
|
|
// as a memory_order_release operation, and read accesses are implemented as
|
|
// memory_order_acquire.
|
|
void Store(FnPtr fn) {
|
|
bool success = DoStore(fn);
|
|
static_cast<void>(success);
|
|
assert(success);
|
|
}
|
|
|
|
// Invokes the registered callback. If no callback has yet been registered, a
|
|
// default-constructed object of the appropriate type is returned instead.
|
|
template <typename... CallArgs>
|
|
ReturnType operator()(CallArgs&&... args) const {
|
|
return DoLoad()(std::forward<CallArgs>(args)...);
|
|
}
|
|
|
|
// Returns the registered callback, or nullptr if none has been registered.
|
|
// Useful if client code needs to conditionalize behavior based on whether a
|
|
// callback was registered.
|
|
//
|
|
// Note that atomic_hook.Load()() and atomic_hook() have different semantics:
|
|
// operator()() will perform a no-op if no callback was registered, while
|
|
// Load()() will dereference a null function pointer. Prefer operator()() to
|
|
// Load()() unless you must conditionalize behavior on whether a hook was
|
|
// registered.
|
|
FnPtr Load() const {
|
|
FnPtr ptr = DoLoad();
|
|
return (ptr == DummyFunction) ? nullptr : ptr;
|
|
}
|
|
|
|
private:
|
|
static ReturnType DummyFunction(Args...) {
|
|
return ReturnType();
|
|
}
|
|
|
|
// Current versions of MSVC (as of September 2017) have a broken
|
|
// implementation of std::atomic<T*>: Its constructor attempts to do the
|
|
// equivalent of a reinterpret_cast in a constexpr context, which is not
|
|
// allowed.
|
|
//
|
|
// This causes an issue when building with LLVM under Windows. To avoid this,
|
|
// we use a less-efficient, intptr_t-based implementation on Windows.
|
|
#if ABSL_HAVE_WORKING_ATOMIC_POINTER
|
|
// Return the stored value, or DummyFunction if no value has been stored.
|
|
FnPtr DoLoad() const { return hook_.load(std::memory_order_acquire); }
|
|
|
|
// Store the given value. Returns false if a different value was already
|
|
// stored to this object.
|
|
bool DoStore(FnPtr fn) {
|
|
assert(fn);
|
|
FnPtr expected = default_fn_;
|
|
const bool store_succeeded = hook_.compare_exchange_strong(
|
|
expected, fn, std::memory_order_acq_rel, std::memory_order_acquire);
|
|
const bool same_value_already_stored = (expected == fn);
|
|
return store_succeeded || same_value_already_stored;
|
|
}
|
|
|
|
std::atomic<FnPtr> hook_;
|
|
#else // !ABSL_HAVE_WORKING_ATOMIC_POINTER
|
|
// Use a sentinel value unlikely to be the address of an actual function.
|
|
static constexpr intptr_t kUninitialized = 0;
|
|
|
|
static_assert(sizeof(intptr_t) >= sizeof(FnPtr),
|
|
"intptr_t can't contain a function pointer");
|
|
|
|
FnPtr DoLoad() const {
|
|
const intptr_t value = hook_.load(std::memory_order_acquire);
|
|
if (value == kUninitialized) {
|
|
return default_fn_;
|
|
}
|
|
return reinterpret_cast<FnPtr>(value);
|
|
}
|
|
|
|
bool DoStore(FnPtr fn) {
|
|
assert(fn);
|
|
const auto value = reinterpret_cast<intptr_t>(fn);
|
|
intptr_t expected = kUninitialized;
|
|
const bool store_succeeded = hook_.compare_exchange_strong(
|
|
expected, value, std::memory_order_acq_rel, std::memory_order_acquire);
|
|
const bool same_value_already_stored = (expected == value);
|
|
return store_succeeded || same_value_already_stored;
|
|
}
|
|
|
|
std::atomic<intptr_t> hook_;
|
|
#endif
|
|
|
|
const FnPtr default_fn_;
|
|
};
|
|
|
|
#undef ABSL_HAVE_WORKING_ATOMIC_POINTER
|
|
#undef ABSL_HAVE_WORKING_CONSTEXPR_STATIC_INIT
|
|
|
|
} // namespace base_internal
|
|
ABSL_NAMESPACE_END
|
|
} // namespace absl
|
|
|
|
#endif // ABSL_BASE_INTERNAL_ATOMIC_HOOK_H_
|