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 // 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.
 //
 // This header file defines macros for declaring attributes for functions,
 // types, and variables.
 //
 // These macros are used within Abseil and allow the compiler to optimize, where
 // applicable, certain function calls.
 //
 // Most macros here are exposing GCC or Clang features, and are stubbed out for
 // other compilers.
 //
 // GCC attributes documentation:
 //   https://gcc.gnu.org/onlinedocs/gcc-4.7.0/gcc/Function-Attributes.html
 //   https://gcc.gnu.org/onlinedocs/gcc-4.7.0/gcc/Variable-Attributes.html
 //   https://gcc.gnu.org/onlinedocs/gcc-4.7.0/gcc/Type-Attributes.html
 //
 // Most attributes in this file are already supported by GCC 4.7. However, some
 // of them are not supported in older version of Clang. Thus, we check
 // `__has_attribute()` first. If the check fails, we check if we are on GCC and
 // assume the attribute exists on GCC (which is verified on GCC 4.7).
 
 #ifndef ABSL_BASE_ATTRIBUTES_H_
 #define ABSL_BASE_ATTRIBUTES_H_
 
 #include "absl/base/config.h"
 
 // ABSL_HAVE_ATTRIBUTE
 //
 // A function-like feature checking macro that is a wrapper around
 // `__has_attribute`, which is defined by GCC 5+ and Clang and evaluates to a
 // nonzero constant integer if the attribute is supported or 0 if not.
 //
 // It evaluates to zero if `__has_attribute` is not defined by the compiler.
 //
 // GCC: https://gcc.gnu.org/gcc-5/changes.html
 // Clang: https://clang.llvm.org/docs/LanguageExtensions.html
 #ifdef __has_attribute
 #define ABSL_HAVE_ATTRIBUTE(x) __has_attribute(x)
 #else
 #define ABSL_HAVE_ATTRIBUTE(x) 0
 #endif
 
 // ABSL_HAVE_CPP_ATTRIBUTE
 //
 // A function-like feature checking macro that accepts C++11 style attributes.
 // It's a wrapper around `__has_cpp_attribute`, defined by ISO C++ SD-6
 // (https://en.cppreference.com/w/cpp/experimental/feature_test). If we don't
 // find `__has_cpp_attribute`, will evaluate to 0.
 #if defined(__cplusplus) && defined(__has_cpp_attribute)
 // NOTE: requiring __cplusplus above should not be necessary, but
 // works around https://bugs.llvm.org/show_bug.cgi?id=23435.
 #define ABSL_HAVE_CPP_ATTRIBUTE(x) __has_cpp_attribute(x)
 #else
 #define ABSL_HAVE_CPP_ATTRIBUTE(x) 0
 #endif
 
 // -----------------------------------------------------------------------------
 // Function Attributes
 // -----------------------------------------------------------------------------
 //
 // GCC: https://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html
 // Clang: https://clang.llvm.org/docs/AttributeReference.html
 
 // ABSL_PRINTF_ATTRIBUTE
 // ABSL_SCANF_ATTRIBUTE
 //
 // Tells the compiler to perform `printf` format string checking if the
 // compiler supports it; see the 'format' attribute in
 // <https://gcc.gnu.org/onlinedocs/gcc-4.7.0/gcc/Function-Attributes.html>.
 //
 // Note: As the GCC manual states, "[s]ince non-static C++ methods
 // have an implicit 'this' argument, the arguments of such methods
 // should be counted from two, not one."
 #if ABSL_HAVE_ATTRIBUTE(format) || (defined(__GNUC__) && !defined(__clang__))
 #define ABSL_PRINTF_ATTRIBUTE(string_index, first_to_check) \
   __attribute__((__format__(__printf__, string_index, first_to_check)))
 #define ABSL_SCANF_ATTRIBUTE(string_index, first_to_check) \
   __attribute__((__format__(__scanf__, string_index, first_to_check)))
 #else
 #define ABSL_PRINTF_ATTRIBUTE(string_index, first_to_check)
 #define ABSL_SCANF_ATTRIBUTE(string_index, first_to_check)
 #endif
 
 // ABSL_ATTRIBUTE_ALWAYS_INLINE
 // ABSL_ATTRIBUTE_NOINLINE
 //
 // Forces functions to either inline or not inline. Introduced in gcc 3.1.
 #if ABSL_HAVE_ATTRIBUTE(always_inline) || \
     (defined(__GNUC__) && !defined(__clang__))
 #define ABSL_ATTRIBUTE_ALWAYS_INLINE __attribute__((always_inline))
 #define ABSL_HAVE_ATTRIBUTE_ALWAYS_INLINE 1
 #else
 #define ABSL_ATTRIBUTE_ALWAYS_INLINE
 #endif
 
 #if ABSL_HAVE_ATTRIBUTE(noinline) || (defined(__GNUC__) && !defined(__clang__))
 #define ABSL_ATTRIBUTE_NOINLINE __attribute__((noinline))
 #define ABSL_HAVE_ATTRIBUTE_NOINLINE 1
 #else
 #define ABSL_ATTRIBUTE_NOINLINE
 #endif
 
 // ABSL_ATTRIBUTE_NO_TAIL_CALL
 //
 // Prevents the compiler from optimizing away stack frames for functions which
 // end in a call to another function.
 #if ABSL_HAVE_ATTRIBUTE(disable_tail_calls)
 #define ABSL_HAVE_ATTRIBUTE_NO_TAIL_CALL 1
 #define ABSL_ATTRIBUTE_NO_TAIL_CALL __attribute__((disable_tail_calls))
 #elif defined(__GNUC__) && !defined(__clang__) && !defined(__e2k__)
 #define ABSL_HAVE_ATTRIBUTE_NO_TAIL_CALL 1
 #define ABSL_ATTRIBUTE_NO_TAIL_CALL \
   __attribute__((optimize("no-optimize-sibling-calls")))
 #else
 #define ABSL_ATTRIBUTE_NO_TAIL_CALL
 #define ABSL_HAVE_ATTRIBUTE_NO_TAIL_CALL 0
 #endif
 
 // ABSL_ATTRIBUTE_WEAK
 //
 // Tags a function as weak for the purposes of compilation and linking.
 // Weak attributes did not work properly in LLVM's Windows backend before
 // 9.0.0, so disable them there. See https://bugs.llvm.org/show_bug.cgi?id=37598
 // for further information.
 // The MinGW compiler doesn't complain about the weak attribute until the link
 // step, presumably because Windows doesn't use ELF binaries.
 #if (ABSL_HAVE_ATTRIBUTE(weak) ||                                         \
      (defined(__GNUC__) && !defined(__clang__))) &&                       \
     (!defined(_WIN32) || (defined(__clang__) && __clang_major__ >= 9)) && \
     !defined(__MINGW32__)
 #undef ABSL_ATTRIBUTE_WEAK
 #define ABSL_ATTRIBUTE_WEAK __attribute__((weak))
 #define ABSL_HAVE_ATTRIBUTE_WEAK 1
 #else
 #define ABSL_ATTRIBUTE_WEAK
 #define ABSL_HAVE_ATTRIBUTE_WEAK 0
 #endif
 
 // ABSL_ATTRIBUTE_NONNULL
 //
 // Tells the compiler either (a) that a particular function parameter
 // should be a non-null pointer, or (b) that all pointer arguments should
 // be non-null.
 //
 // Note: As the GCC manual states, "[s]ince non-static C++ methods
 // have an implicit 'this' argument, the arguments of such methods
 // should be counted from two, not one."
 //
 // Args are indexed starting at 1.
 //
 // For non-static class member functions, the implicit `this` argument
 // is arg 1, and the first explicit argument is arg 2. For static class member
 // functions, there is no implicit `this`, and the first explicit argument is
 // arg 1.
 //
 // Example:
 //
 //   /* arg_a cannot be null, but arg_b can */
 //   void Function(void* arg_a, void* arg_b) ABSL_ATTRIBUTE_NONNULL(1);
 //
 //   class C {
 //     /* arg_a cannot be null, but arg_b can */
 //     void Method(void* arg_a, void* arg_b) ABSL_ATTRIBUTE_NONNULL(2);
 //
 //     /* arg_a cannot be null, but arg_b can */
 //     static void StaticMethod(void* arg_a, void* arg_b)
 //     ABSL_ATTRIBUTE_NONNULL(1);
 //   };
 //
 // If no arguments are provided, then all pointer arguments should be non-null.
 //
 //  /* No pointer arguments may be null. */
 //  void Function(void* arg_a, void* arg_b, int arg_c) ABSL_ATTRIBUTE_NONNULL();
 //
 // NOTE: The GCC nonnull attribute actually accepts a list of arguments, but
 // ABSL_ATTRIBUTE_NONNULL does not.
 #if ABSL_HAVE_ATTRIBUTE(nonnull) || (defined(__GNUC__) && !defined(__clang__))
 #define ABSL_ATTRIBUTE_NONNULL(arg_index) __attribute__((nonnull(arg_index)))
 #else
 #define ABSL_ATTRIBUTE_NONNULL(...)
 #endif
 
 // ABSL_ATTRIBUTE_NORETURN
 //
 // Tells the compiler that a given function never returns.
 //
 // Deprecated: Prefer the `[[noreturn]]` attribute standardized by C++11 over
 // this macro.
 #if ABSL_HAVE_ATTRIBUTE(noreturn) || (defined(__GNUC__) && !defined(__clang__))
 #define ABSL_ATTRIBUTE_NORETURN __attribute__((noreturn))
 #elif defined(_MSC_VER)
 #define ABSL_ATTRIBUTE_NORETURN __declspec(noreturn)
 #else
 #define ABSL_ATTRIBUTE_NORETURN
 #endif
 
 // ABSL_ATTRIBUTE_NO_SANITIZE_ADDRESS
 //
 // Tells the AddressSanitizer (or other memory testing tools) to ignore a given
 // function. Useful for cases when a function reads random locations on stack,
 // calls _exit from a cloned subprocess, deliberately accesses buffer
 // out of bounds or does other scary things with memory.
 // NOTE: GCC supports AddressSanitizer(asan) since 4.8.
 // https://gcc.gnu.org/gcc-4.8/changes.html
 #if defined(ABSL_HAVE_ADDRESS_SANITIZER) && \
     ABSL_HAVE_ATTRIBUTE(no_sanitize_address)
 #define ABSL_ATTRIBUTE_NO_SANITIZE_ADDRESS __attribute__((no_sanitize_address))
 #elif defined(ABSL_HAVE_ADDRESS_SANITIZER) && defined(_MSC_VER) && \
     _MSC_VER >= 1928
 // https://docs.microsoft.com/en-us/cpp/cpp/no-sanitize-address
 #define ABSL_ATTRIBUTE_NO_SANITIZE_ADDRESS __declspec(no_sanitize_address)
 #elif defined(ABSL_HAVE_HWADDRESS_SANITIZER) && ABSL_HAVE_ATTRIBUTE(no_sanitize)
 // HWAddressSanitizer is a sanitizer similar to AddressSanitizer, which uses CPU
 // features to detect similar bugs with less CPU and memory overhead.
 // NOTE: GCC supports HWAddressSanitizer(hwasan) since 11.
 // https://gcc.gnu.org/gcc-11/changes.html
 #define ABSL_ATTRIBUTE_NO_SANITIZE_ADDRESS \
   __attribute__((no_sanitize("hwaddress")))
 #else
 #define ABSL_ATTRIBUTE_NO_SANITIZE_ADDRESS
 #endif
 
 // ABSL_ATTRIBUTE_NO_SANITIZE_MEMORY
 //
 // Tells the MemorySanitizer to relax the handling of a given function. All "Use
 // of uninitialized value" warnings from such functions will be suppressed, and
 // all values loaded from memory will be considered fully initialized.  This
 // attribute is similar to the ABSL_ATTRIBUTE_NO_SANITIZE_ADDRESS attribute
 // above, but deals with initialized-ness rather than addressability issues.
 // NOTE: MemorySanitizer(msan) is supported by Clang but not GCC.
 #if ABSL_HAVE_ATTRIBUTE(no_sanitize_memory)
 #define ABSL_ATTRIBUTE_NO_SANITIZE_MEMORY __attribute__((no_sanitize_memory))
 #else
 #define ABSL_ATTRIBUTE_NO_SANITIZE_MEMORY
 #endif
 
 // ABSL_ATTRIBUTE_NO_SANITIZE_THREAD
 //
 // Tells the ThreadSanitizer to not instrument a given function.
 // NOTE: GCC supports ThreadSanitizer(tsan) since 4.8.
 // https://gcc.gnu.org/gcc-4.8/changes.html
 #if ABSL_HAVE_ATTRIBUTE(no_sanitize_thread)
 #define ABSL_ATTRIBUTE_NO_SANITIZE_THREAD __attribute__((no_sanitize_thread))
 #else
 #define ABSL_ATTRIBUTE_NO_SANITIZE_THREAD
 #endif
 
 // ABSL_ATTRIBUTE_NO_SANITIZE_UNDEFINED
 //
 // Tells the UndefinedSanitizer to ignore a given function. Useful for cases
 // where certain behavior (eg. division by zero) is being used intentionally.
 // NOTE: GCC supports UndefinedBehaviorSanitizer(ubsan) since 4.9.
 // https://gcc.gnu.org/gcc-4.9/changes.html
 #if ABSL_HAVE_ATTRIBUTE(no_sanitize_undefined)
 #define ABSL_ATTRIBUTE_NO_SANITIZE_UNDEFINED \
   __attribute__((no_sanitize_undefined))
 #elif ABSL_HAVE_ATTRIBUTE(no_sanitize)
 #define ABSL_ATTRIBUTE_NO_SANITIZE_UNDEFINED \
   __attribute__((no_sanitize("undefined")))
 #else
 #define ABSL_ATTRIBUTE_NO_SANITIZE_UNDEFINED
 #endif
 
 // ABSL_ATTRIBUTE_NO_SANITIZE_CFI
 //
 // Tells the ControlFlowIntegrity sanitizer to not instrument a given function.
 // See https://clang.llvm.org/docs/ControlFlowIntegrity.html for details.
 #if ABSL_HAVE_ATTRIBUTE(no_sanitize) && defined(__llvm__)
 #define ABSL_ATTRIBUTE_NO_SANITIZE_CFI __attribute__((no_sanitize("cfi")))
 #else
 #define ABSL_ATTRIBUTE_NO_SANITIZE_CFI
 #endif
 
 // ABSL_ATTRIBUTE_NO_SANITIZE_SAFESTACK
 //
 // Tells the SafeStack to not instrument a given function.
 // See https://clang.llvm.org/docs/SafeStack.html for details.
 #if ABSL_HAVE_ATTRIBUTE(no_sanitize)
 #define ABSL_ATTRIBUTE_NO_SANITIZE_SAFESTACK \
   __attribute__((no_sanitize("safe-stack")))
 #else
 #define ABSL_ATTRIBUTE_NO_SANITIZE_SAFESTACK
 #endif
 
 // ABSL_ATTRIBUTE_RETURNS_NONNULL
 //
 // Tells the compiler that a particular function never returns a null pointer.
 #if ABSL_HAVE_ATTRIBUTE(returns_nonnull)
 #define ABSL_ATTRIBUTE_RETURNS_NONNULL __attribute__((returns_nonnull))
 #else
 #define ABSL_ATTRIBUTE_RETURNS_NONNULL
 #endif
 
 // ABSL_HAVE_ATTRIBUTE_SECTION
 //
 // Indicates whether labeled sections are supported. Weak symbol support is
 // a prerequisite. Labeled sections are not supported on Darwin/iOS.
 #ifdef ABSL_HAVE_ATTRIBUTE_SECTION
 #error ABSL_HAVE_ATTRIBUTE_SECTION cannot be directly set
 #elif (ABSL_HAVE_ATTRIBUTE(section) ||                \
        (defined(__GNUC__) && !defined(__clang__))) && \
     !defined(__APPLE__) && ABSL_HAVE_ATTRIBUTE_WEAK
 #define ABSL_HAVE_ATTRIBUTE_SECTION 1
 
 // ABSL_ATTRIBUTE_SECTION
 //
 // Tells the compiler/linker to put a given function into a section and define
 // `__start_ ## name` and `__stop_ ## name` symbols to bracket the section.
 // This functionality is supported by GNU linker.  Any function annotated with
 // `ABSL_ATTRIBUTE_SECTION` must not be inlined, or it will be placed into
 // whatever section its caller is placed into.
 //
 #ifndef ABSL_ATTRIBUTE_SECTION
 #define ABSL_ATTRIBUTE_SECTION(name) \
   __attribute__((section(#name))) __attribute__((noinline))
 #endif
 
 // ABSL_ATTRIBUTE_SECTION_VARIABLE
 //
 // Tells the compiler/linker to put a given variable into a section and define
 // `__start_ ## name` and `__stop_ ## name` symbols to bracket the section.
 // This functionality is supported by GNU linker.
 #ifndef ABSL_ATTRIBUTE_SECTION_VARIABLE
 #ifdef _AIX
 // __attribute__((section(#name))) on AIX is achieved by using the `.csect`
 // psudo op which includes an additional integer as part of its syntax indcating
 // alignment. If data fall under different alignments then you might get a
 // compilation error indicating a `Section type conflict`.
 #define ABSL_ATTRIBUTE_SECTION_VARIABLE(name)
 #else
 #define ABSL_ATTRIBUTE_SECTION_VARIABLE(name) __attribute__((section(#name)))
 #endif
 #endif
 
 // ABSL_DECLARE_ATTRIBUTE_SECTION_VARS
 //
 // A weak section declaration to be used as a global declaration
 // for ABSL_ATTRIBUTE_SECTION_START|STOP(name) to compile and link
 // even without functions with ABSL_ATTRIBUTE_SECTION(name).
 // ABSL_DEFINE_ATTRIBUTE_SECTION should be in the exactly one file; it's
 // a no-op on ELF but not on Mach-O.
 //
 #ifndef ABSL_DECLARE_ATTRIBUTE_SECTION_VARS
 #define ABSL_DECLARE_ATTRIBUTE_SECTION_VARS(name)   \
   extern char __start_##name[] ABSL_ATTRIBUTE_WEAK; \
   extern char __stop_##name[] ABSL_ATTRIBUTE_WEAK
 #endif
 #ifndef ABSL_DEFINE_ATTRIBUTE_SECTION_VARS
 #define ABSL_INIT_ATTRIBUTE_SECTION_VARS(name)
 #define ABSL_DEFINE_ATTRIBUTE_SECTION_VARS(name)
 #endif
 
 // ABSL_ATTRIBUTE_SECTION_START
 //
 // Returns `void*` pointers to start/end of a section of code with
 // functions having ABSL_ATTRIBUTE_SECTION(name).
 // Returns 0 if no such functions exist.
 // One must ABSL_DECLARE_ATTRIBUTE_SECTION_VARS(name) for this to compile and
 // link.
 //
 #define ABSL_ATTRIBUTE_SECTION_START(name) \
   (reinterpret_cast<void *>(__start_##name))
 #define ABSL_ATTRIBUTE_SECTION_STOP(name) \
   (reinterpret_cast<void *>(__stop_##name))
 
 #else  // !ABSL_HAVE_ATTRIBUTE_SECTION
 
 #define ABSL_HAVE_ATTRIBUTE_SECTION 0
 
 // provide dummy definitions
 #define ABSL_ATTRIBUTE_SECTION(name)
 #define ABSL_ATTRIBUTE_SECTION_VARIABLE(name)
 #define ABSL_INIT_ATTRIBUTE_SECTION_VARS(name)
 #define ABSL_DEFINE_ATTRIBUTE_SECTION_VARS(name)
 #define ABSL_DECLARE_ATTRIBUTE_SECTION_VARS(name)
 #define ABSL_ATTRIBUTE_SECTION_START(name) (reinterpret_cast<void *>(0))
 #define ABSL_ATTRIBUTE_SECTION_STOP(name) (reinterpret_cast<void *>(0))
 
 #endif  // ABSL_ATTRIBUTE_SECTION
 
 // ABSL_ATTRIBUTE_STACK_ALIGN_FOR_OLD_LIBC
 //
 // Support for aligning the stack on 32-bit x86.
 #if ABSL_HAVE_ATTRIBUTE(force_align_arg_pointer) || \
     (defined(__GNUC__) && !defined(__clang__))
 #if defined(__i386__)
 #define ABSL_ATTRIBUTE_STACK_ALIGN_FOR_OLD_LIBC \
   __attribute__((force_align_arg_pointer))
 #define ABSL_REQUIRE_STACK_ALIGN_TRAMPOLINE (0)
 #elif defined(__x86_64__)
 #define ABSL_REQUIRE_STACK_ALIGN_TRAMPOLINE (1)
 #define ABSL_ATTRIBUTE_STACK_ALIGN_FOR_OLD_LIBC
 #else  // !__i386__ && !__x86_64
 #define ABSL_REQUIRE_STACK_ALIGN_TRAMPOLINE (0)
 #define ABSL_ATTRIBUTE_STACK_ALIGN_FOR_OLD_LIBC
 #endif  // __i386__
 #else
 #define ABSL_ATTRIBUTE_STACK_ALIGN_FOR_OLD_LIBC
 #define ABSL_REQUIRE_STACK_ALIGN_TRAMPOLINE (0)
 #endif
 
 // ABSL_MUST_USE_RESULT
 //
 // Tells the compiler to warn about unused results.
 //
 // For code or headers that are assured to only build with C++17 and up, prefer
 // just using the standard `[[nodiscard]]` directly over this macro.
 //
 // When annotating a function, it must appear as the first part of the
 // declaration or definition. The compiler will warn if the return value from
 // such a function is unused:
 //
 //   ABSL_MUST_USE_RESULT Sprocket* AllocateSprocket();
 //   AllocateSprocket();  // Triggers a warning.
 //
 // When annotating a class, it is equivalent to annotating every function which
 // returns an instance.
 //
 //   class ABSL_MUST_USE_RESULT Sprocket {};
 //   Sprocket();  // Triggers a warning.
 //
 //   Sprocket MakeSprocket();
 //   MakeSprocket();  // Triggers a warning.
 //
 // Note that references and pointers are not instances:
 //
 //   Sprocket* SprocketPointer();
 //   SprocketPointer();  // Does *not* trigger a warning.
 //
 // ABSL_MUST_USE_RESULT allows using cast-to-void to suppress the unused result
 // warning. For that, warn_unused_result is used only for clang but not for gcc.
 // https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66425
 //
 // Note: past advice was to place the macro after the argument list.
 //
 // TODO(b/176172494): Use ABSL_HAVE_CPP_ATTRIBUTE(nodiscard) when all code is
 // compliant with the stricter [[nodiscard]].
 #if defined(__clang__) && ABSL_HAVE_ATTRIBUTE(warn_unused_result)
 #define ABSL_MUST_USE_RESULT __attribute__((warn_unused_result))
 #else
 #define ABSL_MUST_USE_RESULT
 #endif
 
 // ABSL_ATTRIBUTE_HOT, ABSL_ATTRIBUTE_COLD
 //
 // Tells GCC that a function is hot or cold. GCC can use this information to
 // improve static analysis, i.e. a conditional branch to a cold function
 // is likely to be not-taken.
 // This annotation is used for function declarations.
 //
 // Example:
 //
 //   int foo() ABSL_ATTRIBUTE_HOT;
 #if ABSL_HAVE_ATTRIBUTE(hot) || (defined(__GNUC__) && !defined(__clang__))
 #define ABSL_ATTRIBUTE_HOT __attribute__((hot))
 #else
 #define ABSL_ATTRIBUTE_HOT
 #endif
 
 #if ABSL_HAVE_ATTRIBUTE(cold) || (defined(__GNUC__) && !defined(__clang__))
 #define ABSL_ATTRIBUTE_COLD __attribute__((cold))
 #else
 #define ABSL_ATTRIBUTE_COLD
 #endif
 
 // ABSL_XRAY_ALWAYS_INSTRUMENT, ABSL_XRAY_NEVER_INSTRUMENT, ABSL_XRAY_LOG_ARGS
 //
 // We define the ABSL_XRAY_ALWAYS_INSTRUMENT and ABSL_XRAY_NEVER_INSTRUMENT
 // macro used as an attribute to mark functions that must always or never be
 // instrumented by XRay. Currently, this is only supported in Clang/LLVM.
 //
 // For reference on the LLVM XRay instrumentation, see
 // http://llvm.org/docs/XRay.html.
 //
 // A function with the XRAY_ALWAYS_INSTRUMENT macro attribute in its declaration
 // will always get the XRay instrumentation sleds. These sleds may introduce
 // some binary size and runtime overhead and must be used sparingly.
 //
 // These attributes only take effect when the following conditions are met:
 //
 //   * The file/target is built in at least C++11 mode, with a Clang compiler
 //     that supports XRay attributes.
 //   * The file/target is built with the -fxray-instrument flag set for the
 //     Clang/LLVM compiler.
 //   * The function is defined in the translation unit (the compiler honors the
 //     attribute in either the definition or the declaration, and must match).
 //
 // There are cases when, even when building with XRay instrumentation, users
 // might want to control specifically which functions are instrumented for a
 // particular build using special-case lists provided to the compiler. These
 // special case lists are provided to Clang via the
 // -fxray-always-instrument=... and -fxray-never-instrument=... flags. The
 // attributes in source take precedence over these special-case lists.
 //
 // To disable the XRay attributes at build-time, users may define
 // ABSL_NO_XRAY_ATTRIBUTES. Do NOT define ABSL_NO_XRAY_ATTRIBUTES on specific
 // packages/targets, as this may lead to conflicting definitions of functions at
 // link-time.
 //
 // XRay isn't currently supported on Android:
 // https://github.com/android/ndk/issues/368
 #if ABSL_HAVE_CPP_ATTRIBUTE(clang::xray_always_instrument) && \
     !defined(ABSL_NO_XRAY_ATTRIBUTES) && !defined(__ANDROID__)
 #define ABSL_XRAY_ALWAYS_INSTRUMENT [[clang::xray_always_instrument]]
 #define ABSL_XRAY_NEVER_INSTRUMENT [[clang::xray_never_instrument]]
 #if ABSL_HAVE_CPP_ATTRIBUTE(clang::xray_log_args)
 #define ABSL_XRAY_LOG_ARGS(N) \
   [[clang::xray_always_instrument, clang::xray_log_args(N)]]
 #else
 #define ABSL_XRAY_LOG_ARGS(N) [[clang::xray_always_instrument]]
 #endif
 #else
 #define ABSL_XRAY_ALWAYS_INSTRUMENT
 #define ABSL_XRAY_NEVER_INSTRUMENT
 #define ABSL_XRAY_LOG_ARGS(N)
 #endif
 
 // ABSL_ATTRIBUTE_REINITIALIZES
 //
 // Indicates that a member function reinitializes the entire object to a known
 // state, independent of the previous state of the object.
 //
 // The clang-tidy check bugprone-use-after-move allows member functions marked
 // with this attribute to be called on objects that have been moved from;
 // without the attribute, this would result in a use-after-move warning.
 #if ABSL_HAVE_CPP_ATTRIBUTE(clang::reinitializes)
 #define ABSL_ATTRIBUTE_REINITIALIZES [[clang::reinitializes]]
 #else
 #define ABSL_ATTRIBUTE_REINITIALIZES
 #endif
 
 // -----------------------------------------------------------------------------
 // Variable Attributes
 // -----------------------------------------------------------------------------
 
 // ABSL_ATTRIBUTE_UNUSED
 //
 // Prevents the compiler from complaining about variables that appear unused.
 //
 // For code or headers that are assured to only build with C++17 and up, prefer
 // just using the standard '[[maybe_unused]]' directly over this macro.
 //
 // Due to differences in positioning requirements between the old, compiler
 // specific __attribute__ syntax and the now standard [[maybe_unused]], this
 // macro does not attempt to take advantage of '[[maybe_unused]]'.
 #if ABSL_HAVE_ATTRIBUTE(unused) || (defined(__GNUC__) && !defined(__clang__))
 #undef ABSL_ATTRIBUTE_UNUSED
 #define ABSL_ATTRIBUTE_UNUSED __attribute__((__unused__))
 #else
 #define ABSL_ATTRIBUTE_UNUSED
 #endif
 
 // ABSL_ATTRIBUTE_INITIAL_EXEC
 //
 // Tells the compiler to use "initial-exec" mode for a thread-local variable.
 // See http://people.redhat.com/drepper/tls.pdf for the gory details.
 #if ABSL_HAVE_ATTRIBUTE(tls_model) || (defined(__GNUC__) && !defined(__clang__))
 #define ABSL_ATTRIBUTE_INITIAL_EXEC __attribute__((tls_model("initial-exec")))
 #else
 #define ABSL_ATTRIBUTE_INITIAL_EXEC
 #endif
 
 // ABSL_ATTRIBUTE_PACKED
 //
 // Instructs the compiler not to use natural alignment for a tagged data
 // structure, but instead to reduce its alignment to 1.
 //
 // Therefore, DO NOT APPLY THIS ATTRIBUTE TO STRUCTS CONTAINING ATOMICS. Doing
 // so can cause atomic variables to be mis-aligned and silently violate
 // atomicity on x86.
 //
 // This attribute can either be applied to members of a structure or to a
 // structure in its entirety. Applying this attribute (judiciously) to a
 // structure in its entirety to optimize the memory footprint of very
 // commonly-used structs is fine. Do not apply this attribute to a structure in
 // its entirety if the purpose is to control the offsets of the members in the
 // structure. Instead, apply this attribute only to structure members that need
 // it.
 //
 // When applying ABSL_ATTRIBUTE_PACKED only to specific structure members the
 // natural alignment of structure members not annotated is preserved. Aligned
 // member accesses are faster than non-aligned member accesses even if the
 // targeted microprocessor supports non-aligned accesses.
 #if ABSL_HAVE_ATTRIBUTE(packed) || (defined(__GNUC__) && !defined(__clang__))
 #define ABSL_ATTRIBUTE_PACKED __attribute__((__packed__))
 #else
 #define ABSL_ATTRIBUTE_PACKED
 #endif
 
 // ABSL_ATTRIBUTE_FUNC_ALIGN
 //
 // Tells the compiler to align the function start at least to certain
 // alignment boundary
 #if ABSL_HAVE_ATTRIBUTE(aligned) || (defined(__GNUC__) && !defined(__clang__))
 #define ABSL_ATTRIBUTE_FUNC_ALIGN(bytes) __attribute__((aligned(bytes)))
 #else
 #define ABSL_ATTRIBUTE_FUNC_ALIGN(bytes)
 #endif
 
 // ABSL_FALLTHROUGH_INTENDED
 //
 // Annotates implicit fall-through between switch labels, allowing a case to
 // indicate intentional fallthrough and turn off warnings about any lack of a
 // `break` statement. The ABSL_FALLTHROUGH_INTENDED macro should be followed by
 // a semicolon and can be used in most places where `break` can, provided that
 // no statements exist between it and the next switch label.
 //
 // Example:
 //
 //  switch (x) {
 //    case 40:
 //    case 41:
 //      if (truth_is_out_there) {
 //        ++x;
 //        ABSL_FALLTHROUGH_INTENDED;  // Use instead of/along with annotations
 //                                    // in comments
 //      } else {
 //        return x;
 //      }
 //    case 42:
 //      ...
 //
 // Notes: When supported, GCC and Clang can issue a warning on switch labels
 // with unannotated fallthrough using the warning `-Wimplicit-fallthrough`. See
 // clang documentation on language extensions for details:
 // https://clang.llvm.org/docs/AttributeReference.html#fallthrough-clang-fallthrough
 //
 // When used with unsupported compilers, the ABSL_FALLTHROUGH_INTENDED macro has
 // no effect on diagnostics. In any case this macro has no effect on runtime
 // behavior and performance of code.
 
 #ifdef ABSL_FALLTHROUGH_INTENDED
 #error "ABSL_FALLTHROUGH_INTENDED should not be defined."
 #elif ABSL_HAVE_CPP_ATTRIBUTE(fallthrough)
 #define ABSL_FALLTHROUGH_INTENDED [[fallthrough]]
 #elif ABSL_HAVE_CPP_ATTRIBUTE(clang::fallthrough)
 #define ABSL_FALLTHROUGH_INTENDED [[clang::fallthrough]]
 #elif ABSL_HAVE_CPP_ATTRIBUTE(gnu::fallthrough)
 #define ABSL_FALLTHROUGH_INTENDED [[gnu::fallthrough]]
 #else
 #define ABSL_FALLTHROUGH_INTENDED \
   do {                            \
   } while (0)
 #endif
 
 // ABSL_DEPRECATED()
 //
 // Marks a deprecated class, struct, enum, function, method and variable
 // declarations. The macro argument is used as a custom diagnostic message (e.g.
 // suggestion of a better alternative).
 //
 // For code or headers that are assured to only build with C++14 and up, prefer
 // just using the standard `[[deprecated("message")]]` directly over this macro.
 //
 // Examples:
 //
 //   class ABSL_DEPRECATED("Use Bar instead") Foo {...};
 //
 //   ABSL_DEPRECATED("Use Baz() instead") void Bar() {...}
 //
 //   template <typename T>
 //   ABSL_DEPRECATED("Use DoThat() instead")
 //   void DoThis();
 //
 //   enum FooEnum {
 //     kBar ABSL_DEPRECATED("Use kBaz instead"),
 //   };
 //
 // Every usage of a deprecated entity will trigger a warning when compiled with
 // GCC/Clang's `-Wdeprecated-declarations` option. Google's production toolchain
 // turns this warning off by default, instead relying on clang-tidy to report
 // new uses of deprecated code.
 #if ABSL_HAVE_ATTRIBUTE(deprecated)
 #define ABSL_DEPRECATED(message) __attribute__((deprecated(message)))
 #else
 #define ABSL_DEPRECATED(message)
 #endif
 
 // When deprecating Abseil code, it is sometimes necessary to turn off the
 // warning within Abseil, until the deprecated code is actually removed. The
 // deprecated code can be surrounded with these directives to achieve that
 // result.
 //
 // class ABSL_DEPRECATED("Use Bar instead") Foo;
 //
 // ABSL_INTERNAL_DISABLE_DEPRECATED_DECLARATION_WARNING
 // Baz ComputeBazFromFoo(Foo f);
 // ABSL_INTERNAL_RESTORE_DEPRECATED_DECLARATION_WARNING
 #if defined(__GNUC__) || defined(__clang__)
 // Clang also supports these GCC pragmas.
 #define ABSL_INTERNAL_DISABLE_DEPRECATED_DECLARATION_WARNING \
   _Pragma("GCC diagnostic push")             \
   _Pragma("GCC diagnostic ignored \"-Wdeprecated-declarations\"")
 #define ABSL_INTERNAL_RESTORE_DEPRECATED_DECLARATION_WARNING \
   _Pragma("GCC diagnostic pop")
 #elif defined(_MSC_VER)
 #define ABSL_INTERNAL_DISABLE_DEPRECATED_DECLARATION_WARNING \
   _Pragma("warning(push)") _Pragma("warning(disable: 4996)")
 #define ABSL_INTERNAL_RESTORE_DEPRECATED_DECLARATION_WARNING \
   _Pragma("warning(pop)")
 #else
 #define ABSL_INTERNAL_DISABLE_DEPRECATED_DECLARATION_WARNING
 #define ABSL_INTERNAL_RESTORE_DEPRECATED_DECLARATION_WARNING
 #endif  // defined(__GNUC__) || defined(__clang__)
 
 // ABSL_CONST_INIT
 //
 // A variable declaration annotated with the `ABSL_CONST_INIT` attribute will
 // not compile (on supported platforms) unless the variable has a constant
 // initializer. This is useful for variables with static and thread storage
 // duration, because it guarantees that they will not suffer from the so-called
 // "static init order fiasco".
 //
 // This attribute must be placed on the initializing declaration of the
 // variable. Some compilers will give a -Wmissing-constinit warning when this
 // attribute is placed on some other declaration but missing from the
 // initializing declaration.
 //
 // In some cases (notably with thread_local variables), `ABSL_CONST_INIT` can
 // also be used in a non-initializing declaration to tell the compiler that a
 // variable is already initialized, reducing overhead that would otherwise be
 // incurred by a hidden guard variable. Thus annotating all declarations with
 // this attribute is recommended to potentially enhance optimization.
 //
 // Example:
 //
 //   class MyClass {
 //    public:
 //     ABSL_CONST_INIT static MyType my_var;
 //   };
 //
 //   ABSL_CONST_INIT MyType MyClass::my_var = MakeMyType(...);
 //
 // For code or headers that are assured to only build with C++20 and up, prefer
 // just using the standard `constinit` keyword directly over this macro.
 //
 // Note that this attribute is redundant if the variable is declared constexpr.
 #if defined(__cpp_constinit) && __cpp_constinit >= 201907L
 #define ABSL_CONST_INIT constinit
 #elif ABSL_HAVE_CPP_ATTRIBUTE(clang::require_constant_initialization)
 #define ABSL_CONST_INIT [[clang::require_constant_initialization]]
 #else
 #define ABSL_CONST_INIT
 #endif
 
 // ABSL_ATTRIBUTE_PURE_FUNCTION
 //
 // ABSL_ATTRIBUTE_PURE_FUNCTION is used to annotate declarations of "pure"
 // functions. A function is pure if its return value is only a function of its
 // arguments. The pure attribute prohibits a function from modifying the state
 // of the program that is observable by means other than inspecting the
 // function's return value. Declaring such functions with the pure attribute
 // allows the compiler to avoid emitting some calls in repeated invocations of
 // the function with the same argument values.
 //
 // Example:
 //
 //  ABSL_ATTRIBUTE_PURE_FUNCTION std::string FormatTime(Time t);
 #if ABSL_HAVE_CPP_ATTRIBUTE(gnu::pure)
 #define ABSL_ATTRIBUTE_PURE_FUNCTION [[gnu::pure]]
 #elif ABSL_HAVE_ATTRIBUTE(pure)
 #define ABSL_ATTRIBUTE_PURE_FUNCTION __attribute__((pure))
 #else
 // If the attribute isn't defined, we'll fallback to ABSL_MUST_USE_RESULT since
 // pure functions are useless if its return is ignored.
 #define ABSL_ATTRIBUTE_PURE_FUNCTION ABSL_MUST_USE_RESULT
 #endif
 
 // ABSL_ATTRIBUTE_CONST_FUNCTION
 //
 // ABSL_ATTRIBUTE_CONST_FUNCTION is used to annotate declarations of "const"
 // functions. A const function is similar to a pure function, with one
 // exception: Pure functions may return value that depend on a non-volatile
 // object that isn't provided as a function argument, while the const function
 // is guaranteed to return the same result given the same arguments.
 //
 // Example:
 //
 //  ABSL_ATTRIBUTE_CONST_FUNCTION int64_t ToInt64Milliseconds(Duration d);
 #if defined(_MSC_VER) && !defined(__clang__)
 // Put the MSVC case first since MSVC seems to parse const as a C++ keyword.
 #define ABSL_ATTRIBUTE_CONST_FUNCTION ABSL_ATTRIBUTE_PURE_FUNCTION
 #elif ABSL_HAVE_CPP_ATTRIBUTE(gnu::const)
 #define ABSL_ATTRIBUTE_CONST_FUNCTION [[gnu::const]]
 #elif ABSL_HAVE_ATTRIBUTE(const)
 #define ABSL_ATTRIBUTE_CONST_FUNCTION __attribute__((const))
 #else
 // Since const functions are more restrictive pure function, we'll fallback to a
 // pure function if the const attribute is not handled.
 #define ABSL_ATTRIBUTE_CONST_FUNCTION ABSL_ATTRIBUTE_PURE_FUNCTION
 #endif
 
 // ABSL_ATTRIBUTE_LIFETIME_BOUND indicates that a resource owned by a function
 // parameter or implicit object parameter is retained by the return value of the
 // annotated function (or, for a parameter of a constructor, in the value of the
 // constructed object). This attribute causes warnings to be produced if a
 // temporary object does not live long enough.
 //
 // When applied to a reference parameter, the referenced object is assumed to be
 // retained by the return value of the function. When applied to a non-reference
 // parameter (for example, a pointer or a class type), all temporaries
 // referenced by the parameter are assumed to be retained by the return value of
 // the function.
 //
 // See also the upstream documentation:
 // https://clang.llvm.org/docs/AttributeReference.html#lifetimebound
 // https://learn.microsoft.com/en-us/cpp/code-quality/c26816?view=msvc-170
 #if ABSL_HAVE_CPP_ATTRIBUTE(clang::lifetimebound)
 #define ABSL_ATTRIBUTE_LIFETIME_BOUND [[clang::lifetimebound]]
 #elif ABSL_HAVE_CPP_ATTRIBUTE(msvc::lifetimebound)
 #define ABSL_ATTRIBUTE_LIFETIME_BOUND [[msvc::lifetimebound]]
 #elif ABSL_HAVE_ATTRIBUTE(lifetimebound)
 #define ABSL_ATTRIBUTE_LIFETIME_BOUND __attribute__((lifetimebound))
 #else
 #define ABSL_ATTRIBUTE_LIFETIME_BOUND
 #endif
 
 // ABSL_INTERNAL_ATTRIBUTE_VIEW indicates that a type acts like a view i.e. a
 // raw (non-owning) pointer. This enables diagnoses similar to those enabled by
 // ABSL_ATTRIBUTE_LIFETIME_BOUND.
 //
 // See the following links for details:
 // https://reviews.llvm.org/D64448
 // https://lists.llvm.org/pipermail/cfe-dev/2018-November/060355.html
 #if ABSL_HAVE_CPP_ATTRIBUTE(gsl::Pointer)
 #define ABSL_INTERNAL_ATTRIBUTE_VIEW [[gsl::Pointer]]
 #else
 #define ABSL_INTERNAL_ATTRIBUTE_VIEW
 #endif
 
 // ABSL_INTERNAL_ATTRIBUTE_OWNER indicates that a type acts like a smart
 // (owning) pointer. This enables diagnoses similar to those enabled by
 // ABSL_ATTRIBUTE_LIFETIME_BOUND.
 //
 // See the following links for details:
 // https://reviews.llvm.org/D64448
 // https://lists.llvm.org/pipermail/cfe-dev/2018-November/060355.html
 #if ABSL_HAVE_CPP_ATTRIBUTE(gsl::Owner)
 #define ABSL_INTERNAL_ATTRIBUTE_OWNER [[gsl::Owner]]
 #else
 #define ABSL_INTERNAL_ATTRIBUTE_OWNER
 #endif
 
 // ABSL_ATTRIBUTE_TRIVIAL_ABI
 // Indicates that a type is "trivially relocatable" -- meaning it can be
 // relocated without invoking the constructor/destructor, using a form of move
 // elision.
 //
 // From a memory safety point of view, putting aside destructor ordering, it's
 // safe to apply ABSL_ATTRIBUTE_TRIVIAL_ABI if an object's location
 // can change over the course of its lifetime: if a constructor can be run one
 // place, and then the object magically teleports to another place where some
 // methods are run, and then the object teleports to yet another place where it
 // is destroyed. This is notably not true for self-referential types, where the
 // move-constructor must keep the self-reference up to date. If the type changed
 // location without invoking the move constructor, it would have a dangling
 // self-reference.
 //
 // The use of this teleporting machinery means that the number of paired
 // move/destroy operations can change, and so it is a bad idea to apply this to
 // a type meant to count the number of moves.
 //
 // Warning: applying this can, rarely, break callers. Objects passed by value
 // will be destroyed at the end of the call, instead of the end of the
 // full-expression containing the call. In addition, it changes the ABI
 // of functions accepting this type by value (e.g. to pass in registers).
 //
 // See also the upstream documentation:
 // https://clang.llvm.org/docs/AttributeReference.html#trivial-abi
 //
 // b/321691395 - This is currently disabled in open-source builds since
 // compiler support differs. If system libraries compiled with GCC are mixed
 // with libraries compiled with Clang, types will have different ideas about
 // their ABI, leading to hard to debug crashes.
 #define ABSL_ATTRIBUTE_TRIVIAL_ABI
 
 // ABSL_ATTRIBUTE_NO_UNIQUE_ADDRESS
 //
 // Indicates a data member can be optimized to occupy no space (if it is empty)
 // and/or its tail padding can be used for other members.
 //
 // For code that is assured to only build with C++20 or later, prefer using
 // the standard attribute `[[no_unique_address]]` directly instead of this
 // macro.
 //
 // https://devblogs.microsoft.com/cppblog/msvc-cpp20-and-the-std-cpp20-switch/#c20-no_unique_address
 // Current versions of MSVC have disabled `[[no_unique_address]]` since it
 // breaks ABI compatibility, but offers `[[msvc::no_unique_address]]` for
 // situations when it can be assured that it is desired. Since Abseil does not
 // claim ABI compatibility in mixed builds, we can offer it unconditionally.
 #if defined(_MSC_VER) && _MSC_VER >= 1929
 #define ABSL_ATTRIBUTE_NO_UNIQUE_ADDRESS [[msvc::no_unique_address]]
 #elif ABSL_HAVE_CPP_ATTRIBUTE(no_unique_address)
 #define ABSL_ATTRIBUTE_NO_UNIQUE_ADDRESS [[no_unique_address]]
 #else
 #define ABSL_ATTRIBUTE_NO_UNIQUE_ADDRESS
 #endif
 
 // ABSL_ATTRIBUTE_UNINITIALIZED
 //
 // GCC and Clang support a flag `-ftrivial-auto-var-init=<option>` (<option>
 // can be "zero" or "pattern") that can be used to initialize automatic stack
 // variables. Variables with this attribute will be left uninitialized,
 // overriding the compiler flag.
 //
 // See https://clang.llvm.org/docs/AttributeReference.html#uninitialized
 // and https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html#index-uninitialized-variable-attribute
 #if ABSL_HAVE_CPP_ATTRIBUTE(clang::uninitialized)
 #define ABSL_ATTRIBUTE_UNINITIALIZED [[clang::uninitialized]]
 #elif ABSL_HAVE_CPP_ATTRIBUTE(gnu::uninitialized)
 #define ABSL_ATTRIBUTE_UNINITIALIZED [[gnu::uninitialized]]
 #elif ABSL_HAVE_ATTRIBUTE(uninitialized)
 #define ABSL_ATTRIBUTE_UNINITIALIZED __attribute__((uninitialized))
 #else
 #define ABSL_ATTRIBUTE_UNINITIALIZED
 #endif
 
 // ABSL_ATTRIBUTE_WARN_UNUSED
 //
 // Compilers routinely warn about trivial variables that are unused.  For
 // non-trivial types, this warning is suppressed since the
 // constructor/destructor may be intentional and load-bearing, for example, with
 // a RAII scoped lock.
 //
 // For example:
 //
 // class ABSL_ATTRIBUTE_WARN_UNUSED MyType {
 //  public:
 //   MyType();
 //   ~MyType();
 // };
 //
 // void foo() {
 //   // Warns with ABSL_ATTRIBUTE_WARN_UNUSED attribute present.
 //   MyType unused;
 // }
 //
 // See https://clang.llvm.org/docs/AttributeReference.html#warn-unused and
 // https://gcc.gnu.org/onlinedocs/gcc/C_002b_002b-Attributes.html#index-warn_005funused-type-attribute
 #if ABSL_HAVE_CPP_ATTRIBUTE(gnu::warn_unused)
 #define ABSL_ATTRIBUTE_WARN_UNUSED [[gnu::warn_unused]]
 #else
 #define ABSL_ATTRIBUTE_WARN_UNUSED
 #endif
 
 #endif  // ABSL_BASE_ATTRIBUTES_H_

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