diff options
Diffstat (limited to 'absl/flags/internal/flag.h')
| -rw-r--r-- | absl/flags/internal/flag.h | 259 |
1 files changed, 162 insertions, 97 deletions
diff --git a/absl/flags/internal/flag.h b/absl/flags/internal/flag.h index f27e558b25a4..19119bbb6193 100644 --- a/absl/flags/internal/flag.h +++ b/absl/flags/internal/flag.h @@ -53,56 +53,13 @@ enum class FlagOp { kStaticTypeId, kParse, kUnparse, + kValueOffset, }; using FlagOpFn = void* (*)(FlagOp, const void*, void*, void*); -// Flag value specific operations routine. +// Forward declaration for Flag value specific operations. template <typename T> -void* FlagOps(FlagOp op, const void* v1, void* v2, void* v3) { - switch (op) { - case FlagOp::kDelete: - delete static_cast<const T*>(v1); - return nullptr; - case FlagOp::kClone: - return new T(*static_cast<const T*>(v1)); - case FlagOp::kCopy: - *static_cast<T*>(v2) = *static_cast<const T*>(v1); - return nullptr; - case FlagOp::kCopyConstruct: - new (v2) T(*static_cast<const T*>(v1)); - return nullptr; - case FlagOp::kSizeof: - return reinterpret_cast<void*>(sizeof(T)); - case FlagOp::kStaticTypeId: { - auto* static_id = &FlagStaticTypeIdGen<T>; - - // Cast from function pointer to void* is not portable. - // We don't have an easy way to work around this, but it works fine - // on all the platforms we test and as long as size of pointers match - // we should be fine to do reinterpret cast. - static_assert(sizeof(void*) == sizeof(static_id), - "Flag's static type id does not work on this platform"); - return reinterpret_cast<void*>(static_id); - } - case FlagOp::kParse: { - // Initialize the temporary instance of type T based on current value in - // destination (which is going to be flag's default value). - T temp(*static_cast<T*>(v2)); - if (!absl::ParseFlag<T>(*static_cast<const absl::string_view*>(v1), &temp, - static_cast<std::string*>(v3))) { - return nullptr; - } - *static_cast<T*>(v2) = std::move(temp); - return v2; - } - case FlagOp::kUnparse: - *static_cast<std::string*>(v2) = - absl::UnparseFlag<T>(*static_cast<const T*>(v1)); - return nullptr; - default: - return nullptr; - } -} +void* FlagOps(FlagOp op, const void* v1, void* v2, void* v3); // Deletes memory interpreting obj as flag value type pointer. inline void Delete(FlagOpFn op, const void* obj) { @@ -144,6 +101,16 @@ inline FlagStaticTypeId StaticTypeId(FlagOpFn op) { return reinterpret_cast<FlagStaticTypeId>( op(FlagOp::kStaticTypeId, nullptr, nullptr, nullptr)); } +// Returns offset of the field value_ from the field impl_ inside of +// absl::Flag<T> data. Given FlagImpl pointer p you can get the +// location of the corresponding value as: +// reinterpret_cast<char*>(p) + ValueOffset(). +inline ptrdiff_t ValueOffset(FlagOpFn op) { + // This sequence of casts reverses the sequence from + // `flags_internal::FlagOps()` + return static_cast<ptrdiff_t>(reinterpret_cast<intptr_t>( + op(FlagOp::kValueOffset, nullptr, nullptr, nullptr))); +} /////////////////////////////////////////////////////////////////////////////// // Flag help auxiliary structs. @@ -239,6 +206,10 @@ using FlagUseOneWordStorage = std::integral_constant< struct alignas(16) AlignedTwoWords { int64_t first; int64_t second; + + bool IsInitialized() const { + return first != flags_internal::UninitializedFlagValue(); + } }; template <typename T> @@ -248,8 +219,14 @@ using FlagUseTwoWordsStorage = std::integral_constant< #else // This is actually unused and only here to avoid ifdefs in other palces. struct AlignedTwoWords { - constexpr AlignedTwoWords() = default; - constexpr AlignedTwoWords(int64_t, int64_t) {} + constexpr AlignedTwoWords() noexcept : dummy() {} + constexpr AlignedTwoWords(int64_t, int64_t) noexcept : dummy() {} + char dummy; + + bool IsInitialized() const { + std::abort(); + return true; + } }; // This trait should be type dependent, otherwise SFINAE below will fail @@ -269,23 +246,70 @@ enum class FlagValueStorageKind : uint8_t { kTwoWordsAtomic = 2 }; -union FlagValue { - constexpr explicit FlagValue(int64_t v) : one_word_atomic(v) {} +template <typename T> +static constexpr FlagValueStorageKind StorageKind() { + return FlagUseHeapStorage<T>::value + ? FlagValueStorageKind::kHeapAllocated + : FlagUseOneWordStorage<T>::value + ? FlagValueStorageKind::kOneWordAtomic + : FlagUseTwoWordsStorage<T>::value + ? FlagValueStorageKind::kTwoWordsAtomic + : FlagValueStorageKind::kHeapAllocated; +} + +struct FlagHeapAllocatedValue { + using value_type = void*; + + value_type value; +}; + +struct FlagOneWordValue { + using value_type = std::atomic<int64_t>; + constexpr FlagOneWordValue() : value(UninitializedFlagValue()) {} - template <typename T> - static constexpr FlagValueStorageKind Kind() { - return FlagUseHeapStorage<T>::value - ? FlagValueStorageKind::kHeapAllocated - : FlagUseOneWordStorage<T>::value - ? FlagValueStorageKind::kOneWordAtomic - : FlagUseTwoWordsStorage<T>::value - ? FlagValueStorageKind::kTwoWordsAtomic - : FlagValueStorageKind::kHeapAllocated; + value_type value; +}; + +struct FlagTwoWordsValue { + using value_type = std::atomic<AlignedTwoWords>; + constexpr FlagTwoWordsValue() + : value(AlignedTwoWords{UninitializedFlagValue(), 0}) {} + + value_type value; +}; + +template <typename T, + FlagValueStorageKind Kind = flags_internal::StorageKind<T>()> +struct FlagValue; + +template <typename T> +struct FlagValue<T, FlagValueStorageKind::kHeapAllocated> + : FlagHeapAllocatedValue { + bool Get(T*) const { return false; } +}; + +template <typename T> +struct FlagValue<T, FlagValueStorageKind::kOneWordAtomic> : FlagOneWordValue { + bool Get(T* dst) const { + int64_t one_word_val = value.load(std::memory_order_acquire); + if (ABSL_PREDICT_FALSE(one_word_val == UninitializedFlagValue())) { + return false; + } + std::memcpy(dst, static_cast<const void*>(&one_word_val), sizeof(T)); + return true; } +}; - void* dynamic; - std::atomic<int64_t> one_word_atomic; - std::atomic<flags_internal::AlignedTwoWords> two_words_atomic; +template <typename T> +struct FlagValue<T, FlagValueStorageKind::kTwoWordsAtomic> : FlagTwoWordsValue { + bool Get(T* dst) const { + AlignedTwoWords two_words_val = value.load(std::memory_order_acquire); + if (ABSL_PREDICT_FALSE(!two_words_val.IsInitialized())) { + return false; + } + std::memcpy(dst, static_cast<const void*>(&two_words_val), sizeof(T)); + return true; + } }; /////////////////////////////////////////////////////////////////////////////// @@ -333,35 +357,10 @@ class FlagImpl final : public flags_internal::CommandLineFlag { counter_(0), callback_(nullptr), default_value_(default_value_gen), - value_(flags_internal::UninitializedFlagValue()), data_guard_{} {} // Constant access methods void Read(void* dst) const override ABSL_LOCKS_EXCLUDED(*DataGuard()); - template <typename T, typename std::enable_if<FlagUseHeapStorage<T>::value, - int>::type = 0> - void Get(T* dst) const { - Read(dst); - } - template <typename T, typename std::enable_if<FlagUseOneWordStorage<T>::value, - int>::type = 0> - void Get(T* dst) const { - int64_t one_word_val = - value_.one_word_atomic.load(std::memory_order_acquire); - if (ABSL_PREDICT_FALSE(one_word_val == UninitializedFlagValue())) { - DataGuard(); // Make sure flag initialized - one_word_val = value_.one_word_atomic.load(std::memory_order_acquire); - } - std::memcpy(dst, static_cast<const void*>(&one_word_val), sizeof(T)); - } - template <typename T, typename std::enable_if< - FlagUseTwoWordsStorage<T>::value, int>::type = 0> - void Get(T* dst) const { - DataGuard(); // Make sure flag initialized - const auto two_words_val = - value_.two_words_atomic.load(std::memory_order_acquire); - std::memcpy(dst, &two_words_val, sizeof(T)); - } // Mutating access methods void Write(const void* src) ABSL_LOCKS_EXCLUDED(*DataGuard()); @@ -391,6 +390,25 @@ class FlagImpl final : public flags_internal::CommandLineFlag { ABSL_EXCLUSIVE_LOCKS_REQUIRED(*DataGuard()); // Flag initialization called via absl::call_once. void Init(); + + // Offset value access methods. One per storage kind. These methods to not + // respect const correctness, so be very carefull using them. + + // This is a shared helper routine which encapsulates most of the magic. Since + // it is only used inside the three routines below, which are defined in + // flag.cc, we can define it in that file as well. + template <typename StorageT> + typename StorageT::value_type& OffsetValue() const; + // This is an accessor for a value stored in heap allocated storage. + // Returns a mutable reference to a pointer to allow vlaue mutation. + void*& HeapAllocatedValue() const; + // This is an accessor for a value stored as one word atomic. Returns a + // mutable reference to an atomic value. + std::atomic<int64_t>& OneWordValue() const; + // This is an accessor for a value stored as two words atomic. Returns a + // mutable reference to an atomic value. + std::atomic<AlignedTwoWords>& TwoWordsValue() const; + // Attempts to parse supplied `value` string. If parsing is successful, // returns new value. Otherwise returns nullptr. std::unique_ptr<void, DynValueDeleter> TryParse(absl::string_view value, @@ -488,13 +506,6 @@ class FlagImpl final : public flags_internal::CommandLineFlag { // these two cases. FlagDefaultSrc default_value_; - // Atomically mutable flag's state - - // Flag's value. This can be either the atomically stored small value or - // pointer to the heap allocated dynamic value. value_storage_kind_ is used - // to distinguish these cases. - FlagValue value_; - // This is reserved space for an absl::Mutex to guard flag data. It will be // initialized in FlagImpl::Init via placement new. // We can't use "absl::Mutex data_guard_", since this class is not literal. @@ -514,8 +525,9 @@ class Flag { public: constexpr Flag(const char* name, const char* filename, const FlagHelpArg help, const FlagDfltGenFunc default_value_gen) - : impl_(name, filename, &FlagOps<T>, help, FlagValue::Kind<T>(), - default_value_gen) {} + : impl_(name, filename, &FlagOps<T>, help, + flags_internal::StorageKind<T>(), default_value_gen), + value_() {} T Get() const { // See implementation notes in CommandLineFlag::Get(). @@ -530,7 +542,7 @@ class Flag { impl_.AssertValidType(&flags_internal::FlagStaticTypeIdGen<T>); #endif - impl_.Get(&u.value); + if (!value_.Get(&u.value)) impl_.Read(&u.value); return std::move(u.value); } void Set(const T& v) { @@ -556,10 +568,63 @@ class Flag { private: template <typename U, bool do_register> friend class FlagRegistrar; + // Flag's data + // The implementation depends on value_ field to be placed exactly after the + // impl_ field, so that impl_ can figure out the offset to the value and + // access it. FlagImpl impl_; + FlagValue<T> value_; }; +/////////////////////////////////////////////////////////////////////////////// +// Implementation of Flag value specific operations routine. +template <typename T> +void* FlagOps(FlagOp op, const void* v1, void* v2, void* v3) { + switch (op) { + case FlagOp::kDelete: + delete static_cast<const T*>(v1); + return nullptr; + case FlagOp::kClone: + return new T(*static_cast<const T*>(v1)); + case FlagOp::kCopy: + *static_cast<T*>(v2) = *static_cast<const T*>(v1); + return nullptr; + case FlagOp::kCopyConstruct: + new (v2) T(*static_cast<const T*>(v1)); + return nullptr; + case FlagOp::kSizeof: + return reinterpret_cast<void*>(static_cast<uintptr_t>(sizeof(T))); + case FlagOp::kStaticTypeId: + return reinterpret_cast<void*>(&FlagStaticTypeIdGen<T>); + case FlagOp::kParse: { + // Initialize the temporary instance of type T based on current value in + // destination (which is going to be flag's default value). + T temp(*static_cast<T*>(v2)); + if (!absl::ParseFlag<T>(*static_cast<const absl::string_view*>(v1), &temp, + static_cast<std::string*>(v3))) { + return nullptr; + } + *static_cast<T*>(v2) = std::move(temp); + return v2; + } + case FlagOp::kUnparse: + *static_cast<std::string*>(v2) = + absl::UnparseFlag<T>(*static_cast<const T*>(v1)); + return nullptr; + case FlagOp::kValueOffset: { + // Round sizeof(FlagImp) to a multiple of alignof(FlagValue<T>) to get the + // offset of the data. + ptrdiff_t round_to = alignof(FlagValue<T>); + ptrdiff_t offset = + (sizeof(FlagImpl) + round_to - 1) / round_to * round_to; + return reinterpret_cast<void*>(offset); + } + } + return nullptr; +} + +/////////////////////////////////////////////////////////////////////////////// // This class facilitates Flag object registration and tail expression-based // flag definition, for example: // ABSL_FLAG(int, foo, 42, "Foo help").OnUpdate(NotifyFooWatcher); |