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 // ABSL_ATTRIBUTE_TRIVIAL_ABI 763 // Indicates that a type is "trivially relocatable" -- meaning it can be 764 // relocated without invoking the constructor/destructor, using a form of move 765 // elision. 766 // 767 // From a memory safety point of view, putting aside destructor ordering, it's 768 // safe to apply ABSL_ATTRIBUTE_TRIVIAL_ABI if an object's location 769 // can change over the course of its lifetime: if a constructor can be run one 770 // place, and then the object magically teleports to another place where some 771 // methods are run, and then the object teleports to yet another place where it 772 // is destroyed. This is notably not true for self-referential types, where the 773 // move-constructor must keep the self-reference up to date. If the type changed 774 // location without invoking the move constructor, it would have a dangling 775 // self-reference. 776 // 777 // The use of this teleporting machinery means that the number of paired 778 // move/destroy operations can change, and so it is a bad idea to apply this to 779 // a type meant to count the number of moves. 780 // 781 // Warning: applying this can, rarely, break callers. Objects passed by value 782 // will be destroyed at the end of the call, instead of the end of the 783 // full-expression containing the call. In addition, it changes the ABI 784 // of functions accepting this type by value (e.g. to pass in registers). 785 // 786 // See also the upstream documentation: 787 // https://clang.llvm.org/docs/AttributeReference.html#trivial-abi 788 // 789 #if ABSL_HAVE_CPP_ATTRIBUTE(clang::trivial_abi) 790 #define ABSL_ATTRIBUTE_TRIVIAL_ABI [[clang::trivial_abi]] 791 #define ABSL_HAVE_ATTRIBUTE_TRIVIAL_ABI 1 792 #elif ABSL_HAVE_ATTRIBUTE(trivial_abi) 793 #define ABSL_ATTRIBUTE_TRIVIAL_ABI __attribute__((trivial_abi)) 794 #define ABSL_HAVE_ATTRIBUTE_TRIVIAL_ABI 1 795 #else 796 #define ABSL_ATTRIBUTE_TRIVIAL_ABI 797 #endif 798 799 #endif // ABSL_BASE_ATTRIBUTES_H_ 800