1// Copyright 2017 The Abseil Authors.
2//
3// Licensed under the Apache License, Version 2.0 (the "License");
4// you may not use this file except in compliance with the License.
5// You may obtain a copy of the License at
6//
7// https://www.apache.org/licenses/LICENSE-2.0
8//
9// Unless required by applicable law or agreed to in writing, software
10// distributed under the License is distributed on an "AS IS" BASIS,
11// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
12// See the License for the specific language governing permissions and
13// limitations under the License.
14//
15// This header file defines macros for declaring attributes for functions,
16// types, and variables.
17//
18// These macros are used within Abseil and allow the compiler to optimize, where
19// applicable, certain function calls.
20//
21// Most macros here are exposing GCC or Clang features, and are stubbed out for
22// other compilers.
23//
24// GCC attributes documentation:
25// https://gcc.gnu.org/onlinedocs/gcc-4.7.0/gcc/Function-Attributes.html
26// https://gcc.gnu.org/onlinedocs/gcc-4.7.0/gcc/Variable-Attributes.html
27// https://gcc.gnu.org/onlinedocs/gcc-4.7.0/gcc/Type-Attributes.html
28//
29// Most attributes in this file are already supported by GCC 4.7. However, some
30// of them are not supported in older version of Clang. Thus, we check
31// `__has_attribute()` first. If the check fails, we check if we are on GCC and
32// assume the attribute exists on GCC (which is verified on GCC 4.7).
33
34#ifndef ABSL_BASE_ATTRIBUTES_H_
35#define ABSL_BASE_ATTRIBUTES_H_
36
37#include "absl/base/config.h"
38
39// ABSL_HAVE_ATTRIBUTE
40//
41// A function-like feature checking macro that is a wrapper around
42// `__has_attribute`, which is defined by GCC 5+ and Clang and evaluates to a
43// nonzero constant integer if the attribute is supported or 0 if not.
44//
45// It evaluates to zero if `__has_attribute` is not defined by the compiler.
46//
47// GCC: https://gcc.gnu.org/gcc-5/changes.html
48// Clang: https://clang.llvm.org/docs/LanguageExtensions.html
49#ifdef __has_attribute
50#define ABSL_HAVE_ATTRIBUTE(x) __has_attribute(x)
51#else
52#define ABSL_HAVE_ATTRIBUTE(x) 0
53#endif
54
55// ABSL_HAVE_CPP_ATTRIBUTE
56//
57// A function-like feature checking macro that accepts C++11 style attributes.
58// It's a wrapper around `__has_cpp_attribute`, defined by ISO C++ SD-6
59// (https://en.cppreference.com/w/cpp/experimental/feature_test). If we don't
60// find `__has_cpp_attribute`, will evaluate to 0.
61#if defined(__cplusplus) && defined(__has_cpp_attribute)
62// NOTE: requiring __cplusplus above should not be necessary, but
63// works around https://bugs.llvm.org/show_bug.cgi?id=23435.
64#define ABSL_HAVE_CPP_ATTRIBUTE(x) __has_cpp_attribute(x)
65#else
66#define ABSL_HAVE_CPP_ATTRIBUTE(x) 0
67#endif
68
69// -----------------------------------------------------------------------------
70// Function Attributes
71// -----------------------------------------------------------------------------
72//
73// GCC: https://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html
74// Clang: https://clang.llvm.org/docs/AttributeReference.html
75
76// ABSL_PRINTF_ATTRIBUTE
77// ABSL_SCANF_ATTRIBUTE
78//
79// Tells the compiler to perform `printf` format string checking if the
80// compiler supports it; see the 'format' attribute in
81// <https://gcc.gnu.org/onlinedocs/gcc-4.7.0/gcc/Function-Attributes.html>.
82//
83// Note: As the GCC manual states, "[s]ince non-static C++ methods
84// have an implicit 'this' argument, the arguments of such methods
85// should be counted from two, not one."
86#if ABSL_HAVE_ATTRIBUTE(format) || (defined(__GNUC__) && !defined(__clang__))
87#define ABSL_PRINTF_ATTRIBUTE(string_index, first_to_check) \
88 __attribute__((__format__(__printf__, string_index, first_to_check)))
89#define ABSL_SCANF_ATTRIBUTE(string_index, first_to_check) \
90 __attribute__((__format__(__scanf__, string_index, first_to_check)))
91#else
92#define ABSL_PRINTF_ATTRIBUTE(string_index, first_to_check)
93#define ABSL_SCANF_ATTRIBUTE(string_index, first_to_check)
94#endif
95
96// ABSL_ATTRIBUTE_ALWAYS_INLINE
97// ABSL_ATTRIBUTE_NOINLINE
98//
99// Forces functions to either inline or not inline. Introduced in gcc 3.1.
100#if ABSL_HAVE_ATTRIBUTE(always_inline) || \
101 (defined(__GNUC__) && !defined(__clang__))
102#define ABSL_ATTRIBUTE_ALWAYS_INLINE __attribute__((always_inline))
103#define ABSL_HAVE_ATTRIBUTE_ALWAYS_INLINE 1
104#else
105#define ABSL_ATTRIBUTE_ALWAYS_INLINE
106#endif
107
108#if ABSL_HAVE_ATTRIBUTE(noinline) || (defined(__GNUC__) && !defined(__clang__))
109#define ABSL_ATTRIBUTE_NOINLINE __attribute__((noinline))
110#define ABSL_HAVE_ATTRIBUTE_NOINLINE 1
111#else
112#define ABSL_ATTRIBUTE_NOINLINE
113#endif
114
115// ABSL_ATTRIBUTE_NO_TAIL_CALL
116//
117// Prevents the compiler from optimizing away stack frames for functions which
118// end in a call to another function.
119#if ABSL_HAVE_ATTRIBUTE(disable_tail_calls)
120#define ABSL_HAVE_ATTRIBUTE_NO_TAIL_CALL 1
121#define ABSL_ATTRIBUTE_NO_TAIL_CALL __attribute__((disable_tail_calls))
122#elif defined(__GNUC__) && !defined(__clang__) && !defined(__e2k__)
123#define ABSL_HAVE_ATTRIBUTE_NO_TAIL_CALL 1
124#define ABSL_ATTRIBUTE_NO_TAIL_CALL \
125 __attribute__((optimize("no-optimize-sibling-calls")))
126#else
127#define ABSL_ATTRIBUTE_NO_TAIL_CALL
128#define ABSL_HAVE_ATTRIBUTE_NO_TAIL_CALL 0
129#endif
130
131// ABSL_ATTRIBUTE_WEAK
132//
133// Tags a function as weak for the purposes of compilation and linking.
134// Weak attributes did not work properly in LLVM's Windows backend before
135// 9.0.0, so disable them there. See https://bugs.llvm.org/show_bug.cgi?id=37598
136// for further information.
137// The MinGW compiler doesn't complain about the weak attribute until the link
138// step, presumably because Windows doesn't use ELF binaries.
139#if (ABSL_HAVE_ATTRIBUTE(weak) || \
140 (defined(__GNUC__) && !defined(__clang__))) && \
141 (!defined(_WIN32) || (defined(__clang__) && __clang_major__ >= 9)) && \
142 !defined(__MINGW32__)
143#undef ABSL_ATTRIBUTE_WEAK
144#define ABSL_ATTRIBUTE_WEAK __attribute__((weak))
145#define ABSL_HAVE_ATTRIBUTE_WEAK 1
146#else
147#define ABSL_ATTRIBUTE_WEAK
148#define ABSL_HAVE_ATTRIBUTE_WEAK 0
149#endif
150
151// ABSL_ATTRIBUTE_NONNULL
152//
153// Tells the compiler either (a) that a particular function parameter
154// should be a non-null pointer, or (b) that all pointer arguments should
155// be non-null.
156//
157// Note: As the GCC manual states, "[s]ince non-static C++ methods
158// have an implicit 'this' argument, the arguments of such methods
159// should be counted from two, not one."
160//
161// Args are indexed starting at 1.
162//
163// For non-static class member functions, the implicit `this` argument
164// is arg 1, and the first explicit argument is arg 2. For static class member
165// functions, there is no implicit `this`, and the first explicit argument is
166// arg 1.
167//
168// Example:
169//
170// /* arg_a cannot be null, but arg_b can */
171// void Function(void* arg_a, void* arg_b) ABSL_ATTRIBUTE_NONNULL(1);
172//
173// class C {
174// /* arg_a cannot be null, but arg_b can */
175// void Method(void* arg_a, void* arg_b) ABSL_ATTRIBUTE_NONNULL(2);
176//
177// /* arg_a cannot be null, but arg_b can */
178// static void StaticMethod(void* arg_a, void* arg_b)
179// ABSL_ATTRIBUTE_NONNULL(1);
180// };
181//
182// If no arguments are provided, then all pointer arguments should be non-null.
183//
184// /* No pointer arguments may be null. */
185// void Function(void* arg_a, void* arg_b, int arg_c) ABSL_ATTRIBUTE_NONNULL();
186//
187// NOTE: The GCC nonnull attribute actually accepts a list of arguments, but
188// ABSL_ATTRIBUTE_NONNULL does not.
189#if ABSL_HAVE_ATTRIBUTE(nonnull) || (defined(__GNUC__) && !defined(__clang__))
190#define ABSL_ATTRIBUTE_NONNULL(arg_index) __attribute__((nonnull(arg_index)))
191#else
192#define ABSL_ATTRIBUTE_NONNULL(...)
193#endif
194
195// ABSL_ATTRIBUTE_NORETURN
196//
197// Tells the compiler that a given function never returns.
198#if ABSL_HAVE_ATTRIBUTE(noreturn) || (defined(__GNUC__) && !defined(__clang__))
199#define ABSL_ATTRIBUTE_NORETURN __attribute__((noreturn))
200#elif defined(_MSC_VER)
201#define ABSL_ATTRIBUTE_NORETURN __declspec(noreturn)
202#else
203#define ABSL_ATTRIBUTE_NORETURN
204#endif
205
206// ABSL_ATTRIBUTE_NO_SANITIZE_ADDRESS
207//
208// Tells the AddressSanitizer (or other memory testing tools) to ignore a given
209// function. Useful for cases when a function reads random locations on stack,
210// calls _exit from a cloned subprocess, deliberately accesses buffer
211// out of bounds or does other scary things with memory.
212// NOTE: GCC supports AddressSanitizer(asan) since 4.8.
213// https://gcc.gnu.org/gcc-4.8/changes.html
214#if ABSL_HAVE_ATTRIBUTE(no_sanitize_address)
215#define ABSL_ATTRIBUTE_NO_SANITIZE_ADDRESS __attribute__((no_sanitize_address))
216#elif defined(_MSC_VER) && _MSC_VER >= 1928
217// https://docs.microsoft.com/en-us/cpp/cpp/no-sanitize-address
218#define ABSL_ATTRIBUTE_NO_SANITIZE_ADDRESS __declspec(no_sanitize_address)
219#else
220#define ABSL_ATTRIBUTE_NO_SANITIZE_ADDRESS
221#endif
222
223// ABSL_ATTRIBUTE_NO_SANITIZE_MEMORY
224//
225// Tells the MemorySanitizer to relax the handling of a given function. All "Use
226// of uninitialized value" warnings from such functions will be suppressed, and
227// all values loaded from memory will be considered fully initialized. This
228// attribute is similar to the ABSL_ATTRIBUTE_NO_SANITIZE_ADDRESS attribute
229// above, but deals with initialized-ness rather than addressability issues.
230// NOTE: MemorySanitizer(msan) is supported by Clang but not GCC.
231#if ABSL_HAVE_ATTRIBUTE(no_sanitize_memory)
232#define ABSL_ATTRIBUTE_NO_SANITIZE_MEMORY __attribute__((no_sanitize_memory))
233#else
234#define ABSL_ATTRIBUTE_NO_SANITIZE_MEMORY
235#endif
236
237// ABSL_ATTRIBUTE_NO_SANITIZE_THREAD
238//
239// Tells the ThreadSanitizer to not instrument a given function.
240// NOTE: GCC supports ThreadSanitizer(tsan) since 4.8.
241// https://gcc.gnu.org/gcc-4.8/changes.html
242#if ABSL_HAVE_ATTRIBUTE(no_sanitize_thread)
243#define ABSL_ATTRIBUTE_NO_SANITIZE_THREAD __attribute__((no_sanitize_thread))
244#else
245#define ABSL_ATTRIBUTE_NO_SANITIZE_THREAD
246#endif
247
248// ABSL_ATTRIBUTE_NO_SANITIZE_UNDEFINED
249//
250// Tells the UndefinedSanitizer to ignore a given function. Useful for cases
251// where certain behavior (eg. division by zero) is being used intentionally.
252// NOTE: GCC supports UndefinedBehaviorSanitizer(ubsan) since 4.9.
253// https://gcc.gnu.org/gcc-4.9/changes.html
254#if ABSL_HAVE_ATTRIBUTE(no_sanitize_undefined)
255#define ABSL_ATTRIBUTE_NO_SANITIZE_UNDEFINED \
256 __attribute__((no_sanitize_undefined))
257#elif ABSL_HAVE_ATTRIBUTE(no_sanitize)
258#define ABSL_ATTRIBUTE_NO_SANITIZE_UNDEFINED \
259 __attribute__((no_sanitize("undefined")))
260#else
261#define ABSL_ATTRIBUTE_NO_SANITIZE_UNDEFINED
262#endif
263
264// ABSL_ATTRIBUTE_NO_SANITIZE_CFI
265//
266// Tells the ControlFlowIntegrity sanitizer to not instrument a given function.
267// See https://clang.llvm.org/docs/ControlFlowIntegrity.html for details.
268#if ABSL_HAVE_ATTRIBUTE(no_sanitize)
269#define ABSL_ATTRIBUTE_NO_SANITIZE_CFI __attribute__((no_sanitize("cfi")))
270#else
271#define ABSL_ATTRIBUTE_NO_SANITIZE_CFI
272#endif
273
274// ABSL_ATTRIBUTE_NO_SANITIZE_SAFESTACK
275//
276// Tells the SafeStack to not instrument a given function.
277// See https://clang.llvm.org/docs/SafeStack.html for details.
278#if ABSL_HAVE_ATTRIBUTE(no_sanitize)
279#define ABSL_ATTRIBUTE_NO_SANITIZE_SAFESTACK \
280 __attribute__((no_sanitize("safe-stack")))
281#else
282#define ABSL_ATTRIBUTE_NO_SANITIZE_SAFESTACK
283#endif
284
285// ABSL_ATTRIBUTE_RETURNS_NONNULL
286//
287// Tells the compiler that a particular function never returns a null pointer.
288#if ABSL_HAVE_ATTRIBUTE(returns_nonnull)
289#define ABSL_ATTRIBUTE_RETURNS_NONNULL __attribute__((returns_nonnull))
290#else
291#define ABSL_ATTRIBUTE_RETURNS_NONNULL
292#endif
293
294// ABSL_HAVE_ATTRIBUTE_SECTION
295//
296// Indicates whether labeled sections are supported. Weak symbol support is
297// a prerequisite. Labeled sections are not supported on Darwin/iOS.
298#ifdef ABSL_HAVE_ATTRIBUTE_SECTION
299#error ABSL_HAVE_ATTRIBUTE_SECTION cannot be directly set
300#elif (ABSL_HAVE_ATTRIBUTE(section) || \
301 (defined(__GNUC__) && !defined(__clang__))) && \
302 !defined(__APPLE__) && ABSL_HAVE_ATTRIBUTE_WEAK
303#define ABSL_HAVE_ATTRIBUTE_SECTION 1
304
305// ABSL_ATTRIBUTE_SECTION
306//
307// Tells the compiler/linker to put a given function into a section and define
308// `__start_ ## name` and `__stop_ ## name` symbols to bracket the section.
309// This functionality is supported by GNU linker. Any function annotated with
310// `ABSL_ATTRIBUTE_SECTION` must not be inlined, or it will be placed into
311// whatever section its caller is placed into.
312//
313#ifndef ABSL_ATTRIBUTE_SECTION
314#define ABSL_ATTRIBUTE_SECTION(name) \
315 __attribute__((section(#name))) __attribute__((noinline))
316#endif
317
318// ABSL_ATTRIBUTE_SECTION_VARIABLE
319//
320// Tells the compiler/linker to put a given variable into a section and define
321// `__start_ ## name` and `__stop_ ## name` symbols to bracket the section.
322// This functionality is supported by GNU linker.
323#ifndef ABSL_ATTRIBUTE_SECTION_VARIABLE
324#ifdef _AIX
325// __attribute__((section(#name))) on AIX is achived by using the `.csect` psudo
326// op which includes an additional integer as part of its syntax indcating
327// alignment. If data fall under different alignments then you might get a
328// compilation error indicating a `Section type conflict`.
329#define ABSL_ATTRIBUTE_SECTION_VARIABLE(name)
330#else
331#define ABSL_ATTRIBUTE_SECTION_VARIABLE(name) __attribute__((section(#name)))
332#endif
333#endif
334
335// ABSL_DECLARE_ATTRIBUTE_SECTION_VARS
336//
337// A weak section declaration to be used as a global declaration
338// for ABSL_ATTRIBUTE_SECTION_START|STOP(name) to compile and link
339// even without functions with ABSL_ATTRIBUTE_SECTION(name).
340// ABSL_DEFINE_ATTRIBUTE_SECTION should be in the exactly one file; it's
341// a no-op on ELF but not on Mach-O.
342//
343#ifndef ABSL_DECLARE_ATTRIBUTE_SECTION_VARS
344#define ABSL_DECLARE_ATTRIBUTE_SECTION_VARS(name) \
345 extern char __start_##name[] ABSL_ATTRIBUTE_WEAK; \
346 extern char __stop_##name[] ABSL_ATTRIBUTE_WEAK
347#endif
348#ifndef ABSL_DEFINE_ATTRIBUTE_SECTION_VARS
349#define ABSL_INIT_ATTRIBUTE_SECTION_VARS(name)
350#define ABSL_DEFINE_ATTRIBUTE_SECTION_VARS(name)
351#endif
352
353// ABSL_ATTRIBUTE_SECTION_START
354//
355// Returns `void*` pointers to start/end of a section of code with
356// functions having ABSL_ATTRIBUTE_SECTION(name).
357// Returns 0 if no such functions exist.
358// One must ABSL_DECLARE_ATTRIBUTE_SECTION_VARS(name) for this to compile and
359// link.
360//
361#define ABSL_ATTRIBUTE_SECTION_START(name) \
362 (reinterpret_cast<void *>(__start_##name))
363#define ABSL_ATTRIBUTE_SECTION_STOP(name) \
364 (reinterpret_cast<void *>(__stop_##name))
365
366#else // !ABSL_HAVE_ATTRIBUTE_SECTION
367
368#define ABSL_HAVE_ATTRIBUTE_SECTION 0
369
370// provide dummy definitions
371#define ABSL_ATTRIBUTE_SECTION(name)
372#define ABSL_ATTRIBUTE_SECTION_VARIABLE(name)
373#define ABSL_INIT_ATTRIBUTE_SECTION_VARS(name)
374#define ABSL_DEFINE_ATTRIBUTE_SECTION_VARS(name)
375#define ABSL_DECLARE_ATTRIBUTE_SECTION_VARS(name)
376#define ABSL_ATTRIBUTE_SECTION_START(name) (reinterpret_cast<void *>(0))
377#define ABSL_ATTRIBUTE_SECTION_STOP(name) (reinterpret_cast<void *>(0))
378
379#endif // ABSL_ATTRIBUTE_SECTION
380
381// ABSL_ATTRIBUTE_STACK_ALIGN_FOR_OLD_LIBC
382//
383// Support for aligning the stack on 32-bit x86.
384#if ABSL_HAVE_ATTRIBUTE(force_align_arg_pointer) || \
385 (defined(__GNUC__) && !defined(__clang__))
386#if defined(__i386__)
387#define ABSL_ATTRIBUTE_STACK_ALIGN_FOR_OLD_LIBC \
388 __attribute__((force_align_arg_pointer))
389#define ABSL_REQUIRE_STACK_ALIGN_TRAMPOLINE (0)
390#elif defined(__x86_64__)
391#define ABSL_REQUIRE_STACK_ALIGN_TRAMPOLINE (1)
392#define ABSL_ATTRIBUTE_STACK_ALIGN_FOR_OLD_LIBC
393#else // !__i386__ && !__x86_64
394#define ABSL_REQUIRE_STACK_ALIGN_TRAMPOLINE (0)
395#define ABSL_ATTRIBUTE_STACK_ALIGN_FOR_OLD_LIBC
396#endif // __i386__
397#else
398#define ABSL_ATTRIBUTE_STACK_ALIGN_FOR_OLD_LIBC
399#define ABSL_REQUIRE_STACK_ALIGN_TRAMPOLINE (0)
400#endif
401
402// ABSL_MUST_USE_RESULT
403//
404// Tells the compiler to warn about unused results.
405//
406// For code or headers that are assured to only build with C++17 and up, prefer
407// just using the standard `[[nodiscard]]` directly over this macro.
408//
409// When annotating a function, it must appear as the first part of the
410// declaration or definition. The compiler will warn if the return value from
411// such a function is unused:
412//
413// ABSL_MUST_USE_RESULT Sprocket* AllocateSprocket();
414// AllocateSprocket(); // Triggers a warning.
415//
416// When annotating a class, it is equivalent to annotating every function which
417// returns an instance.
418//
419// class ABSL_MUST_USE_RESULT Sprocket {};
420// Sprocket(); // Triggers a warning.
421//
422// Sprocket MakeSprocket();
423// MakeSprocket(); // Triggers a warning.
424//
425// Note that references and pointers are not instances:
426//
427// Sprocket* SprocketPointer();
428// SprocketPointer(); // Does *not* trigger a warning.
429//
430// ABSL_MUST_USE_RESULT allows using cast-to-void to suppress the unused result
431// warning. For that, warn_unused_result is used only for clang but not for gcc.
432// https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66425
433//
434// Note: past advice was to place the macro after the argument list.
435//
436// TODO(b/176172494): Use ABSL_HAVE_CPP_ATTRIBUTE(nodiscard) when all code is
437// compliant with the stricter [[nodiscard]].
438#if defined(__clang__) && ABSL_HAVE_ATTRIBUTE(warn_unused_result)
439#define ABSL_MUST_USE_RESULT __attribute__((warn_unused_result))
440#else
441#define ABSL_MUST_USE_RESULT
442#endif
443
444// ABSL_ATTRIBUTE_HOT, ABSL_ATTRIBUTE_COLD
445//
446// Tells GCC that a function is hot or cold. GCC can use this information to
447// improve static analysis, i.e. a conditional branch to a cold function
448// is likely to be not-taken.
449// This annotation is used for function declarations.
450//
451// Example:
452//
453// int foo() ABSL_ATTRIBUTE_HOT;
454#if ABSL_HAVE_ATTRIBUTE(hot) || (defined(__GNUC__) && !defined(__clang__))
455#define ABSL_ATTRIBUTE_HOT __attribute__((hot))
456#else
457#define ABSL_ATTRIBUTE_HOT
458#endif
459
460#if ABSL_HAVE_ATTRIBUTE(cold) || (defined(__GNUC__) && !defined(__clang__))
461#define ABSL_ATTRIBUTE_COLD __attribute__((cold))
462#else
463#define ABSL_ATTRIBUTE_COLD
464#endif
465
466// ABSL_XRAY_ALWAYS_INSTRUMENT, ABSL_XRAY_NEVER_INSTRUMENT, ABSL_XRAY_LOG_ARGS
467//
468// We define the ABSL_XRAY_ALWAYS_INSTRUMENT and ABSL_XRAY_NEVER_INSTRUMENT
469// macro used as an attribute to mark functions that must always or never be
470// instrumented by XRay. Currently, this is only supported in Clang/LLVM.
471//
472// For reference on the LLVM XRay instrumentation, see
473// http://llvm.org/docs/XRay.html.
474//
475// A function with the XRAY_ALWAYS_INSTRUMENT macro attribute in its declaration
476// will always get the XRay instrumentation sleds. These sleds may introduce
477// some binary size and runtime overhead and must be used sparingly.
478//
479// These attributes only take effect when the following conditions are met:
480//
481// * The file/target is built in at least C++11 mode, with a Clang compiler
482// that supports XRay attributes.
483// * The file/target is built with the -fxray-instrument flag set for the
484// Clang/LLVM compiler.
485// * The function is defined in the translation unit (the compiler honors the
486// attribute in either the definition or the declaration, and must match).
487//
488// There are cases when, even when building with XRay instrumentation, users
489// might want to control specifically which functions are instrumented for a
490// particular build using special-case lists provided to the compiler. These
491// special case lists are provided to Clang via the
492// -fxray-always-instrument=... and -fxray-never-instrument=... flags. The
493// attributes in source take precedence over these special-case lists.
494//
495// To disable the XRay attributes at build-time, users may define
496// ABSL_NO_XRAY_ATTRIBUTES. Do NOT define ABSL_NO_XRAY_ATTRIBUTES on specific
497// packages/targets, as this may lead to conflicting definitions of functions at
498// link-time.
499//
500// XRay isn't currently supported on Android:
501// https://github.com/android/ndk/issues/368
502#if ABSL_HAVE_CPP_ATTRIBUTE(clang::xray_always_instrument) && \
503 !defined(ABSL_NO_XRAY_ATTRIBUTES) && !defined(__ANDROID__)
504#define ABSL_XRAY_ALWAYS_INSTRUMENT [[clang::xray_always_instrument]]
505#define ABSL_XRAY_NEVER_INSTRUMENT [[clang::xray_never_instrument]]
506#if ABSL_HAVE_CPP_ATTRIBUTE(clang::xray_log_args)
507#define ABSL_XRAY_LOG_ARGS(N) \
508 [[clang::xray_always_instrument, clang::xray_log_args(N)]]
509#else
510#define ABSL_XRAY_LOG_ARGS(N) [[clang::xray_always_instrument]]
511#endif
512#else
513#define ABSL_XRAY_ALWAYS_INSTRUMENT
514#define ABSL_XRAY_NEVER_INSTRUMENT
515#define ABSL_XRAY_LOG_ARGS(N)
516#endif
517
518// ABSL_ATTRIBUTE_REINITIALIZES
519//
520// Indicates that a member function reinitializes the entire object to a known
521// state, independent of the previous state of the object.
522//
523// The clang-tidy check bugprone-use-after-move allows member functions marked
524// with this attribute to be called on objects that have been moved from;
525// without the attribute, this would result in a use-after-move warning.
526#if ABSL_HAVE_CPP_ATTRIBUTE(clang::reinitializes)
527#define ABSL_ATTRIBUTE_REINITIALIZES [[clang::reinitializes]]
528#else
529#define ABSL_ATTRIBUTE_REINITIALIZES
530#endif
531
532// -----------------------------------------------------------------------------
533// Variable Attributes
534// -----------------------------------------------------------------------------
535
536// ABSL_ATTRIBUTE_UNUSED
537//
538// Prevents the compiler from complaining about variables that appear unused.
539//
540// For code or headers that are assured to only build with C++17 and up, prefer
541// just using the standard '[[maybe_unused]]' directly over this macro.
542//
543// Due to differences in positioning requirements between the old, compiler
544// specific __attribute__ syntax and the now standard [[maybe_unused]], this
545// macro does not attempt to take advantage of '[[maybe_unused]]'.
546#if ABSL_HAVE_ATTRIBUTE(unused) || (defined(__GNUC__) && !defined(__clang__))
547#undef ABSL_ATTRIBUTE_UNUSED
548#define ABSL_ATTRIBUTE_UNUSED __attribute__((__unused__))
549#else
550#define ABSL_ATTRIBUTE_UNUSED
551#endif
552
553// ABSL_ATTRIBUTE_INITIAL_EXEC
554//
555// Tells the compiler to use "initial-exec" mode for a thread-local variable.
556// See http://people.redhat.com/drepper/tls.pdf for the gory details.
557#if ABSL_HAVE_ATTRIBUTE(tls_model) || (defined(__GNUC__) && !defined(__clang__))
558#define ABSL_ATTRIBUTE_INITIAL_EXEC __attribute__((tls_model("initial-exec")))
559#else
560#define ABSL_ATTRIBUTE_INITIAL_EXEC
561#endif
562
563// ABSL_ATTRIBUTE_PACKED
564//
565// Instructs the compiler not to use natural alignment for a tagged data
566// structure, but instead to reduce its alignment to 1.
567//
568// Therefore, DO NOT APPLY THIS ATTRIBUTE TO STRUCTS CONTAINING ATOMICS. Doing
569// so can cause atomic variables to be mis-aligned and silently violate
570// atomicity on x86.
571//
572// This attribute can either be applied to members of a structure or to a
573// structure in its entirety. Applying this attribute (judiciously) to a
574// structure in its entirety to optimize the memory footprint of very
575// commonly-used structs is fine. Do not apply this attribute to a structure in
576// its entirety if the purpose is to control the offsets of the members in the
577// structure. Instead, apply this attribute only to structure members that need
578// it.
579//
580// When applying ABSL_ATTRIBUTE_PACKED only to specific structure members the
581// natural alignment of structure members not annotated is preserved. Aligned
582// member accesses are faster than non-aligned member accesses even if the
583// targeted microprocessor supports non-aligned accesses.
584#if ABSL_HAVE_ATTRIBUTE(packed) || (defined(__GNUC__) && !defined(__clang__))
585#define ABSL_ATTRIBUTE_PACKED __attribute__((__packed__))
586#else
587#define ABSL_ATTRIBUTE_PACKED
588#endif
589
590// ABSL_ATTRIBUTE_FUNC_ALIGN
591//
592// Tells the compiler to align the function start at least to certain
593// alignment boundary
594#if ABSL_HAVE_ATTRIBUTE(aligned) || (defined(__GNUC__) && !defined(__clang__))
595#define ABSL_ATTRIBUTE_FUNC_ALIGN(bytes) __attribute__((aligned(bytes)))
596#else
597#define ABSL_ATTRIBUTE_FUNC_ALIGN(bytes)
598#endif
599
600// ABSL_FALLTHROUGH_INTENDED
601//
602// Annotates implicit fall-through between switch labels, allowing a case to
603// indicate intentional fallthrough and turn off warnings about any lack of a
604// `break` statement. The ABSL_FALLTHROUGH_INTENDED macro should be followed by
605// a semicolon and can be used in most places where `break` can, provided that
606// no statements exist between it and the next switch label.
607//
608// Example:
609//
610// switch (x) {
611// case 40:
612// case 41:
613// if (truth_is_out_there) {
614// ++x;
615// ABSL_FALLTHROUGH_INTENDED; // Use instead of/along with annotations
616// // in comments
617// } else {
618// return x;
619// }
620// case 42:
621// ...
622//
623// Notes: When supported, GCC and Clang can issue a warning on switch labels
624// with unannotated fallthrough using the warning `-Wimplicit-fallthrough`. See
625// clang documentation on language extensions for details:
626// https://clang.llvm.org/docs/AttributeReference.html#fallthrough-clang-fallthrough
627//
628// When used with unsupported compilers, the ABSL_FALLTHROUGH_INTENDED macro has
629// no effect on diagnostics. In any case this macro has no effect on runtime
630// behavior and performance of code.
631
632#ifdef ABSL_FALLTHROUGH_INTENDED
633#error "ABSL_FALLTHROUGH_INTENDED should not be defined."
634#elif ABSL_HAVE_CPP_ATTRIBUTE(fallthrough)
635#define ABSL_FALLTHROUGH_INTENDED [[fallthrough]]
636#elif ABSL_HAVE_CPP_ATTRIBUTE(clang::fallthrough)
637#define ABSL_FALLTHROUGH_INTENDED [[clang::fallthrough]]
638#elif ABSL_HAVE_CPP_ATTRIBUTE(gnu::fallthrough)
639#define ABSL_FALLTHROUGH_INTENDED [[gnu::fallthrough]]
640#else
641#define ABSL_FALLTHROUGH_INTENDED \
642 do { \
643 } while (0)
644#endif
645
646// ABSL_DEPRECATED()
647//
648// Marks a deprecated class, struct, enum, function, method and variable
649// declarations. The macro argument is used as a custom diagnostic message (e.g.
650// suggestion of a better alternative).
651//
652// For code or headers that are assured to only build with C++14 and up, prefer
653// just using the standard `[[deprecated("message")]]` directly over this macro.
654//
655// Examples:
656//
657// class ABSL_DEPRECATED("Use Bar instead") Foo {...};
658//
659// ABSL_DEPRECATED("Use Baz() instead") void Bar() {...}
660//
661// template <typename T>
662// ABSL_DEPRECATED("Use DoThat() instead")
663// void DoThis();
664//
665// enum FooEnum {
666// kBar ABSL_DEPRECATED("Use kBaz instead"),
667// };
668//
669// Every usage of a deprecated entity will trigger a warning when compiled with
670// GCC/Clang's `-Wdeprecated-declarations` option. Google's production toolchain
671// turns this warning off by default, instead relying on clang-tidy to report
672// new uses of deprecated code.
673#if ABSL_HAVE_ATTRIBUTE(deprecated)
674#define ABSL_DEPRECATED(message) __attribute__((deprecated(message)))
675#else
676#define ABSL_DEPRECATED(message)
677#endif
678
679// ABSL_CONST_INIT
680//
681// A variable declaration annotated with the `ABSL_CONST_INIT` attribute will
682// not compile (on supported platforms) unless the variable has a constant
683// initializer. This is useful for variables with static and thread storage
684// duration, because it guarantees that they will not suffer from the so-called
685// "static init order fiasco".
686//
687// This attribute must be placed on the initializing declaration of the
688// variable. Some compilers will give a -Wmissing-constinit warning when this
689// attribute is placed on some other declaration but missing from the
690// initializing declaration.
691//
692// In some cases (notably with thread_local variables), `ABSL_CONST_INIT` can
693// also be used in a non-initializing declaration to tell the compiler that a
694// variable is already initialized, reducing overhead that would otherwise be
695// incurred by a hidden guard variable. Thus annotating all declarations with
696// this attribute is recommended to potentially enhance optimization.
697//
698// Example:
699//
700// class MyClass {
701// public:
702// ABSL_CONST_INIT static MyType my_var;
703// };
704//
705// ABSL_CONST_INIT MyType MyClass::my_var = MakeMyType(...);
706//
707// For code or headers that are assured to only build with C++20 and up, prefer
708// just using the standard `constinit` keyword directly over this macro.
709//
710// Note that this attribute is redundant if the variable is declared constexpr.
711#if defined(__cpp_constinit) && __cpp_constinit >= 201907L
712#define ABSL_CONST_INIT constinit
713#elif ABSL_HAVE_CPP_ATTRIBUTE(clang::require_constant_initialization)
714#define ABSL_CONST_INIT [[clang::require_constant_initialization]]
715#else
716#define ABSL_CONST_INIT
717#endif
718
719// ABSL_ATTRIBUTE_PURE_FUNCTION
720//
721// ABSL_ATTRIBUTE_PURE_FUNCTION is used to annotate declarations of "pure"
722// functions. A function is pure if its return value is only a function of its
723// arguments. The pure attribute prohibits a function from modifying the state
724// of the program that is observable by means other than inspecting the
725// function's return value. Declaring such functions with the pure attribute
726// allows the compiler to avoid emitting some calls in repeated invocations of
727// the function with the same argument values.
728//
729// Example:
730//
731// ABSL_ATTRIBUTE_PURE_FUNCTION int64_t ToInt64Milliseconds(Duration d);
732#if ABSL_HAVE_CPP_ATTRIBUTE(gnu::pure)
733#define ABSL_ATTRIBUTE_PURE_FUNCTION [[gnu::pure]]
734#elif ABSL_HAVE_ATTRIBUTE(pure)
735#define ABSL_ATTRIBUTE_PURE_FUNCTION __attribute__((pure))
736#else
737#define ABSL_ATTRIBUTE_PURE_FUNCTION
738#endif
739
740// ABSL_ATTRIBUTE_LIFETIME_BOUND indicates that a resource owned by a function
741// parameter or implicit object parameter is retained by the return value of the
742// annotated function (or, for a parameter of a constructor, in the value of the
743// constructed object). This attribute causes warnings to be produced if a
744// temporary object does not live long enough.
745//
746// When applied to a reference parameter, the referenced object is assumed to be
747// retained by the return value of the function. When applied to a non-reference
748// parameter (for example, a pointer or a class type), all temporaries
749// referenced by the parameter are assumed to be retained by the return value of
750// the function.
751//
752// See also the upstream documentation:
753// https://clang.llvm.org/docs/AttributeReference.html#lifetimebound
754#if ABSL_HAVE_CPP_ATTRIBUTE(clang::lifetimebound)
755#define ABSL_ATTRIBUTE_LIFETIME_BOUND [[clang::lifetimebound]]
756#elif ABSL_HAVE_ATTRIBUTE(lifetimebound)
757#define ABSL_ATTRIBUTE_LIFETIME_BOUND __attribute__((lifetimebound))
758#else
759#define ABSL_ATTRIBUTE_LIFETIME_BOUND
760#endif
761
762#endif // ABSL_BASE_ATTRIBUTES_H_
763

source code of include/absl/base/attributes.h