cronet/base/compiler_specific.h

// Copyright 2012 The Chromium Authors
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef BASE_COMPILER_SPECIFIC_H_
#define BASE_COMPILER_SPECIFIC_H_

#include "build/build_config.h"

#if defined(COMPILER_MSVC) && !defined(__clang__)
#error "Only clang-cl is supported on Windows, see https://crbug.com/988071"
#endif

// This is a wrapper around `__has_cpp_attribute`, which can be used to test for
// the presence of an attribute. In case the compiler does not support this
// macro it will simply evaluate to 0.
//
// References:
// https://wg21.link/sd6#testing-for-the-presence-of-an-attribute-__has_cpp_attribute
// https://wg21.link/cpp.cond#:__has_cpp_attribute
#if defined(__has_cpp_attribute)
#define HAS_CPP_ATTRIBUTE(x) __has_cpp_attribute(x)
#else
#define HAS_CPP_ATTRIBUTE(x) 0
#endif

// A wrapper around `__has_attribute`, similar to HAS_CPP_ATTRIBUTE.
#if defined(__has_attribute)
#define HAS_ATTRIBUTE(x) __has_attribute(x)
#else
#define HAS_ATTRIBUTE(x) 0
#endif

// A wrapper around `__has_builtin`, similar to HAS_CPP_ATTRIBUTE.
#if defined(__has_builtin)
#define HAS_BUILTIN(x) __has_builtin(x)
#else
#define HAS_BUILTIN(x) 0
#endif

// Annotate a function indicating it should not be inlined.
// Use like:
//   NOINLINE void DoStuff() { ... }
#if defined(__clang__) && HAS_ATTRIBUTE(noinline)
#define NOINLINE [[clang::noinline]]
#elif defined(COMPILER_GCC) && HAS_ATTRIBUTE(noinline)
#define NOINLINE __attribute__((noinline))
#elif defined(COMPILER_MSVC)
#define NOINLINE __declspec(noinline)
#else
#define NOINLINE
#endif

// Annotate a function indicating it should not be optimized.
#if defined(__clang__) && HAS_ATTRIBUTE(optnone)
#define NOOPT [[clang::optnone]]
#elif defined(COMPILER_GCC) && HAS_ATTRIBUTE(optimize)
#define NOOPT __attribute__((optimize(0)))
#else
#define NOOPT
#endif

#if defined(__clang__) && defined(NDEBUG) && HAS_ATTRIBUTE(always_inline)
#define ALWAYS_INLINE [[clang::always_inline]] inline
#elif defined(COMPILER_GCC) && defined(NDEBUG) && HAS_ATTRIBUTE(always_inline)
#define ALWAYS_INLINE inline __attribute__((__always_inline__))
#elif defined(COMPILER_MSVC) && defined(NDEBUG)
#define ALWAYS_INLINE __forceinline
#else
#define ALWAYS_INLINE inline
#endif

// Annotate a function indicating it should never be tail called. Useful to make
// sure callers of the annotated function are never omitted from call-stacks.
// To provide the complementary behavior (prevent the annotated function from
// being omitted) look at NOINLINE. Also note that this doesn't prevent code
// folding of multiple identical caller functions into a single signature. To
// prevent code folding, see NO_CODE_FOLDING() in base/debug/alias.h.
// Use like:
//   NOT_TAIL_CALLED void FooBar();
#if defined(__clang__) && HAS_ATTRIBUTE(not_tail_called)
#define NOT_TAIL_CALLED [[clang::not_tail_called]]
#else
#define NOT_TAIL_CALLED
#endif

// Specify memory alignment for structs, classes, etc.
// Use like:
//   class ALIGNAS(16) MyClass { ... }
//   ALIGNAS(16) int array[4];
//
// In most places you can use the C++11 keyword "alignas", which is preferred.
//
// Historically, compilers had trouble mixing __attribute__((...)) syntax with
// alignas(...) syntax. However, at least Clang is very accepting nowadays. It
// may be that this macro can be removed entirely.
#if defined(__clang__)
#define ALIGNAS(byte_alignment) alignas(byte_alignment)
#elif defined(COMPILER_MSVC)
#define ALIGNAS(byte_alignment) __declspec(align(byte_alignment))
#elif defined(COMPILER_GCC) && HAS_ATTRIBUTE(aligned)
#define ALIGNAS(byte_alignment) __attribute__((aligned(byte_alignment)))
#endif

// In case the compiler supports it NO_UNIQUE_ADDRESS evaluates to the C++20
// attribute [[no_unique_address]]. This allows annotating data members so that
// they need not have an address distinct from all other non-static data members
// of its class.
//
// References:
// * https://en.cppreference.com/w/cpp/language/attributes/no_unique_address
// * https://wg21.link/dcl.attr.nouniqueaddr
#if defined(COMPILER_MSVC) && HAS_CPP_ATTRIBUTE(msvc::no_unique_address)
// Unfortunately MSVC ignores [[no_unique_address]] (see
// https://devblogs.microsoft.com/cppblog/msvc-cpp20-and-the-std-cpp20-switch/#msvc-extensions-and-abi),
// and clang-cl matches it for ABI compatibility reasons. We need to prefer
// [[msvc::no_unique_address]] when available if we actually want any effect.
#define NO_UNIQUE_ADDRESS [[msvc::no_unique_address]]
#elif HAS_CPP_ATTRIBUTE(no_unique_address)
#define NO_UNIQUE_ADDRESS [[no_unique_address]]
#else
#define NO_UNIQUE_ADDRESS
#endif

// Tells the compiler a function is using a printf-style format string.
// |format_param| is the one-based index of the format string parameter;
// |dots_param| is the one-based index of the "..." parameter.
// For v*printf functions (which take a va_list), pass 0 for dots_param.
// (This is undocumented but matches what the system C headers do.)
// For member functions, the implicit this parameter counts as index 1.
#if (defined(COMPILER_GCC) || defined(__clang__)) && HAS_ATTRIBUTE(format)
#define PRINTF_FORMAT(format_param, dots_param) \
  __attribute__((format(printf, format_param, dots_param)))
#else
#define PRINTF_FORMAT(format_param, dots_param)
#endif

// WPRINTF_FORMAT is the same, but for wide format strings.
// This doesn't appear to yet be implemented in any compiler.
// See http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38308 .
#define WPRINTF_FORMAT(format_param, dots_param)
// If available, it would look like:
//   __attribute__((format(wprintf, format_param, dots_param)))

// Sanitizers annotations.
#if HAS_ATTRIBUTE(no_sanitize)
#define NO_SANITIZE(what) __attribute__((no_sanitize(what)))
#endif
#if !defined(NO_SANITIZE)
#define NO_SANITIZE(what)
#endif

// MemorySanitizer annotations.
#if defined(MEMORY_SANITIZER) && !BUILDFLAG(IS_NACL)
#include <sanitizer/msan_interface.h>

// Mark a memory region fully initialized.
// Use this to annotate code that deliberately reads uninitialized data, for
// example a GC scavenging root set pointers from the stack.
#define MSAN_UNPOISON(p, size) __msan_unpoison(p, size)

// Check a memory region for initializedness, as if it was being used here.
// If any bits are uninitialized, crash with an MSan report.
// Use this to sanitize data which MSan won't be able to track, e.g. before
// passing data to another process via shared memory.
#define MSAN_CHECK_MEM_IS_INITIALIZED(p, size) \
  __msan_check_mem_is_initialized(p, size)
#else  // MEMORY_SANITIZER
#define MSAN_UNPOISON(p, size)
#define MSAN_CHECK_MEM_IS_INITIALIZED(p, size)
#endif  // MEMORY_SANITIZER

// DISABLE_CFI_PERF -- Disable Control Flow Integrity for perf reasons.
#if !defined(DISABLE_CFI_PERF)
#if defined(__clang__) && defined(OFFICIAL_BUILD)
#define DISABLE_CFI_PERF NO_SANITIZE("cfi")
#else
#define DISABLE_CFI_PERF
#endif
#endif

// DISABLE_CFI_ICALL -- Disable Control Flow Integrity indirect call checks.
// Security Note: if you just need to allow calling of dlsym functions use
// DISABLE_CFI_DLSYM.
#if !defined(DISABLE_CFI_ICALL)
#if BUILDFLAG(IS_WIN)
// Windows also needs __declspec(guard(nocf)).
#define DISABLE_CFI_ICALL NO_SANITIZE("cfi-icall") __declspec(guard(nocf))
#else
#define DISABLE_CFI_ICALL NO_SANITIZE("cfi-icall")
#endif
#endif
#if !defined(DISABLE_CFI_ICALL)
#define DISABLE_CFI_ICALL
#endif

// DISABLE_CFI_DLSYM -- applies DISABLE_CFI_ICALL on platforms where dlsym
// functions must be called. Retains CFI checks on platforms where loaded
// modules participate in CFI (e.g. Windows).
#if !defined(DISABLE_CFI_DLSYM)
#if BUILDFLAG(IS_WIN)
// Windows modules register functions when loaded so can be checked by CFG.
#define DISABLE_CFI_DLSYM
#else
#define DISABLE_CFI_DLSYM DISABLE_CFI_ICALL
#endif
#endif
#if !defined(DISABLE_CFI_DLSYM)
#define DISABLE_CFI_DLSYM
#endif

// Macro useful for writing cross-platform function pointers.
#if !defined(CDECL)
#if BUILDFLAG(IS_WIN)
#define CDECL __cdecl
#else  // BUILDFLAG(IS_WIN)
#define CDECL
#endif  // BUILDFLAG(IS_WIN)
#endif  // !defined(CDECL)

// Macro for hinting that an expression is likely to be false.
#if !defined(UNLIKELY)
#if defined(COMPILER_GCC) || defined(__clang__)
#define UNLIKELY(x) __builtin_expect(!!(x), 0)
#else
#define UNLIKELY(x) (x)
#endif  // defined(COMPILER_GCC)
#endif  // !defined(UNLIKELY)

#if !defined(LIKELY)
#if defined(COMPILER_GCC) || defined(__clang__)
#define LIKELY(x) __builtin_expect(!!(x), 1)
#else
#define LIKELY(x) (x)
#endif  // defined(COMPILER_GCC)
#endif  // !defined(LIKELY)

// Compiler feature-detection.
// clang.llvm.org/docs/LanguageExtensions.html#has-feature-and-has-extension
#if defined(__has_feature)
#define HAS_FEATURE(FEATURE) __has_feature(FEATURE)
#else
#define HAS_FEATURE(FEATURE) 0
#endif

#if defined(COMPILER_GCC)
#define PRETTY_FUNCTION __PRETTY_FUNCTION__
#elif defined(COMPILER_MSVC)
#define PRETTY_FUNCTION __FUNCSIG__
#else
// See https://en.cppreference.com/w/c/language/function_definition#func
#define PRETTY_FUNCTION __func__
#endif

#if !defined(CPU_ARM_NEON)
#if defined(__arm__)
#if !defined(__ARMEB__) && !defined(__ARM_EABI__) && !defined(__EABI__) && \
    !defined(__VFP_FP__) && !defined(_WIN32_WCE) && !defined(ANDROID)
#error Chromium does not support middle endian architecture
#endif
#if defined(__ARM_NEON__)
#define CPU_ARM_NEON 1
#endif
#endif  // defined(__arm__)
#endif  // !defined(CPU_ARM_NEON)

#if !defined(HAVE_MIPS_MSA_INTRINSICS)
#if defined(__mips_msa) && defined(__mips_isa_rev) && (__mips_isa_rev >= 5)
#define HAVE_MIPS_MSA_INTRINSICS 1
#endif
#endif

#if defined(__clang__) && HAS_ATTRIBUTE(uninitialized)
// Attribute "uninitialized" disables -ftrivial-auto-var-init=pattern for
// the specified variable.
// Library-wide alternative is
// 'configs -= [ "//build/config/compiler:default_init_stack_vars" ]' in .gn
// file.
//
// See "init_stack_vars" in build/config/compiler/BUILD.gn and
// http://crbug.com/977230
// "init_stack_vars" is enabled for non-official builds and we hope to enable it
// in official build in 2020 as well. The flag writes fixed pattern into
// uninitialized parts of all local variables. In rare cases such initialization
// is undesirable and attribute can be used:
//   1. Degraded performance
// In most cases compiler is able to remove additional stores. E.g. if memory is
// never accessed or properly initialized later. Preserved stores mostly will
// not affect program performance. However if compiler failed on some
// performance critical code we can get a visible regression in a benchmark.
//   2. memset, memcpy calls
// Compiler may replaces some memory writes with memset or memcpy calls. This is
// not -ftrivial-auto-var-init specific, but it can happen more likely with the
// flag. It can be a problem if code is not linked with C run-time library.
//
// Note: The flag is security risk mitigation feature. So in future the
// attribute uses should be avoided when possible. However to enable this
// mitigation on the most of the code we need to be less strict now and minimize
// number of exceptions later. So if in doubt feel free to use attribute, but
// please document the problem for someone who is going to cleanup it later.
// E.g. platform, bot, benchmark or test name in patch description or next to
// the attribute.
#define STACK_UNINITIALIZED [[clang::uninitialized]]
#else
#define STACK_UNINITIALIZED
#endif

// Attribute "no_stack_protector" disables -fstack-protector for the specified
// function.
//
// "stack_protector" is enabled on most POSIX builds. The flag adds a canary
// to each stack frame, which on function return is checked against a reference
// canary. If the canaries do not match, it's likely that a stack buffer
// overflow has occurred, so immediately crashing will prevent exploitation in
// many cases.
//
// In some cases it's desirable to remove this, e.g. on hot functions, or if
// we have purposely changed the reference canary.
#if defined(COMPILER_GCC) || defined(__clang__)
#if HAS_ATTRIBUTE(__no_stack_protector__)
#define NO_STACK_PROTECTOR __attribute__((__no_stack_protector__))
#else
#define NO_STACK_PROTECTOR __attribute__((__optimize__("-fno-stack-protector")))
#endif
#else
#define NO_STACK_PROTECTOR
#endif

// The ANALYZER_ASSUME_TRUE(bool arg) macro adds compiler-specific hints
// to Clang which control what code paths are statically analyzed,
// and is meant to be used in conjunction with assert & assert-like functions.
// The expression is passed straight through if analysis isn't enabled.
//
// ANALYZER_SKIP_THIS_PATH() suppresses static analysis for the current
// codepath and any other branching codepaths that might follow.
#if defined(__clang_analyzer__)

inline constexpr bool AnalyzerNoReturn() __attribute__((analyzer_noreturn)) {
  return false;
}

inline constexpr bool AnalyzerAssumeTrue(bool arg) {
  // AnalyzerNoReturn() is invoked and analysis is terminated if |arg| is
  // false.
  return arg || AnalyzerNoReturn();
}

#define ANALYZER_ASSUME_TRUE(arg) ::AnalyzerAssumeTrue(!!(arg))
#define ANALYZER_SKIP_THIS_PATH() static_cast<void>(::AnalyzerNoReturn())

#else  // !defined(__clang_analyzer__)

#define ANALYZER_ASSUME_TRUE(arg) (arg)
#define ANALYZER_SKIP_THIS_PATH()

#endif  // defined(__clang_analyzer__)

// Use nomerge attribute to disable optimization of merging multiple same calls.
#if defined(__clang__) && HAS_ATTRIBUTE(nomerge)
#define NOMERGE [[clang::nomerge]]
#else
#define NOMERGE
#endif

// Marks a type as being eligible for the "trivial" ABI despite having a
// non-trivial destructor or copy/move constructor. Such types can be relocated
// after construction by simply copying their memory, which makes them eligible
// to be passed in registers. The canonical example is std::unique_ptr.
//
// Use with caution; this has some subtle effects on constructor/destructor
// ordering and will be very incorrect if the type relies on its address
// remaining constant. When used as a function argument (by value), the value
// may be constructed in the caller's stack frame, passed in a register, and
// then used and destructed in the callee's stack frame. A similar thing can
// occur when values are returned.
//
// TRIVIAL_ABI is not needed for types which have a trivial destructor and
// copy/move constructors, such as base::TimeTicks and other POD.
//
// It is also not likely to be effective on types too large to be passed in one
// or two registers on typical target ABIs.
//
// See also:
//   https://clang.llvm.org/docs/AttributeReference.html#trivial-abi
//   https://libcxx.llvm.org/docs/DesignDocs/UniquePtrTrivialAbi.html
#if defined(__clang__) && HAS_ATTRIBUTE(trivial_abi)
#define TRIVIAL_ABI [[clang::trivial_abi]]
#else
#define TRIVIAL_ABI
#endif

// Detect whether a type is trivially relocatable, ie. a move-and-destroy
// sequence can replaced with memmove(). This can be used to optimise the
// implementation of containers. This is automatically true for types that were
// defined with TRIVIAL_ABI such as scoped_refptr.
//
// See also:
//   https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2023/p1144r8.html
//   https://clang.llvm.org/docs/LanguageExtensions.html#:~:text=__is_trivially_relocatable
#if defined(__clang__) && HAS_BUILTIN(__is_trivially_relocatable)
#define IS_TRIVIALLY_RELOCATABLE(t) __is_trivially_relocatable(t)
#else
#define IS_TRIVIALLY_RELOCATABLE(t) false
#endif

// Marks a member function as reinitializing a moved-from variable.
// See also
// https://clang.llvm.org/extra/clang-tidy/checks/bugprone/use-after-move.html#reinitialization
#if defined(__clang__) && HAS_ATTRIBUTE(reinitializes)
#define REINITIALIZES_AFTER_MOVE [[clang::reinitializes]]
#else
#define REINITIALIZES_AFTER_MOVE
#endif

#if defined(__clang__)
#define GSL_OWNER [[gsl::Owner]]
#define GSL_POINTER [[gsl::Pointer]]
#else
#define GSL_OWNER
#define GSL_POINTER
#endif

// Adds the "logically_const" tag to a symbol's mangled name. The "Mutable
// Constants" check [1] detects instances of constants that aren't in .rodata,
// e.g. due to a missing `const`. Using this tag suppresses the check for this
// symbol, allowing it to live outside .rodata without a warning.
//
// [1]:
// https://crsrc.org/c/docs/speed/binary_size/android_binary_size_trybot.md#Mutable-Constants
#if defined(COMPILER_GCC) || defined(__clang__)
#define LOGICALLY_CONST [[gnu::abi_tag("logically_const")]]
#else
#define LOGICALLY_CONST
#endif

// preserve_most clang's calling convention. Reduces register pressure for the
// caller and as such can be used for cold calls. Support for the
// "preserve_most" attribute is limited:
// - 32-bit platforms do not implement it,
// - component builds fail because _dl_runtime_resolve() clobbers registers,
// - there are crashes on arm64 on Windows (https://crbug.com/v8/14065), which
//   can hopefully be fixed in the future.
// Additionally, the initial implementation in clang <= 16 overwrote the return
// register(s) in the epilogue of a preserve_most function, so we only use
// preserve_most in clang >= 17 (see https://reviews.llvm.org/D143425).
// Clang only supports preserve_most on X86-64 and AArch64 for now.
// See https://clang.llvm.org/docs/AttributeReference.html#preserve-most for
// more details.
#if (defined(ARCH_CPU_ARM64) || defined(ARCH_CPU_X86_64)) && \
    !(BUILDFLAG(IS_WIN) && defined(ARCH_CPU_ARM64)) &&       \
    !defined(COMPONENT_BUILD) && defined(__clang__) &&       \
    __clang_major__ >= 17 && HAS_ATTRIBUTE(preserve_most)
#define PRESERVE_MOST __attribute__((preserve_most))
#else
#define PRESERVE_MOST
#endif

// Mark parameters or return types as having a lifetime attached to the class.
//
// When used to mark a method's pointer/reference parameter, the compiler is
// made aware that it will be stored internally in the class and the pointee
// must outlive the class. Typically used on constructor arguments. It should
// appear to the right of the parameter's variable name.
//
// Example:
// ```
// struct S {
//    S(int* p LIFETIME_BOUND) : ptr_(p) {}
//
//    int* ptr_;
// };
// ```
//
// When used on a method with a return value, the compiler is made aware that
// the returned type is/has a pointer to the internals of the class, and must
// not outlive the class object. It should appear after any method qualifiers.
//
// Example:
// ```
// struct S {
//   int* GetPtr() const LIFETIME_BOUND { return i_; };
//
//   int i_;
// };
// ```
//
// This allows the compiler to warn in (a limited set of) cases where the
// pointer would otherwise be left dangling, especially in cases where the
// pointee would be a destroyed temporary.
//
// Docs: https://clang.llvm.org/docs/AttributeReference.html#lifetimebound
#if defined(__clang__)
#define LIFETIME_BOUND [[clang::lifetimebound]]
#else
#define LIFETIME_BOUND
#endif

// Mark a function as pure, meaning that it does not have side effects, meaning
// that it does not write anything external to the function's local variables
// and return value.
//
// WARNING: If this attribute is mis-used it will result in UB and
// miscompilation, as the optimizator may fold multiple calls into one and
// reorder them inappropriately. This shouldn't appear outside of key vocabulary
// types. It allows callers to work with the vocab type directly, and call its
// methods without having to worry about caching things into local variables in
// hot code.
//
// This attribute must not appear on functions that make use of function
// pointers, virtual methods, or methods of templates (including operators like
// comparison), as the "pure" function can not know what those functions do and
// can not guarantee there will never be sideeffects.
#if defined(COMPILER_GCC) || defined(__clang__)
#define PURE_FUNCTION [[gnu::pure]]
#else
#define PURE_FUNCTION
#endif

// Functions should be marked with UNSAFE_BUFFER_USAGE when they lead to
// out-of-bounds bugs when called with incorrect inputs.
//
// Ideally such functions should be paired with a safer version that works with
// safe primitives like `base::span`. Otherwise, another safer coding pattern
// should be documented along side the use of `UNSAFE_BUFFER_USAGE`.
//
// All functions marked with UNSAFE_BUFFER_USAGE should come with a safety
// comment that explains the requirements of the function to prevent an
// out-of-bounds bug. For example:
// ```
// // Function to do things between `input` and `end`.
// //
// // # Safety
// // The `input` must point to an array with size at least 5. The `end` must
// // point within the same allocation of `input` and not come before `input`.
// ```
//
// The requirements described in the safety comment must be sufficient to
// guarantee that the function never goes out of bounds. Annotating a function
// in this way means that all callers will be required to wrap the call in an
// `UNSAFE_BUFFERS()` macro (see below), with a comment justifying how it meets
// the requirements.
#if defined(__clang__) && HAS_ATTRIBUTE(unsafe_buffer_usage)
#define UNSAFE_BUFFER_USAGE [[clang::unsafe_buffer_usage]]
#else
#define UNSAFE_BUFFER_USAGE
#endif

// UNSAFE_BUFFERS() wraps code that violates the -Wunsafe-buffer-usage warning,
// such as:
// - pointer arithmetic,
// - pointer subscripting, and
// - calls to functions annotated with UNSAFE_BUFFER_USAGE.
//
// This indicates code whose bounds correctness cannot be ensured
// systematically, and thus requires manual review.
//
// ** USE OF THIS MACRO SHOULD BE VERY RARE.** This should only be used when
// strictly necessary. Prefer to use `base::span` instead of pointers, or other
// safer coding patterns (like std containers) that avoid the opportunity for
// out-of-bounds bugs to creep into the code. Any use of UNSAFE_BUFFERS() can
// lead to a critical security bug if any assumptions are wrong, or ever become
// wrong in the future.
//
// The macro should be used to wrap the minimum necessary code, to make it clear
// what is unsafe, and prevent accidentally opting extra things out of the
// warning.
//
// All usage of UNSAFE_BUFFERS() should come with a `// SAFETY: ...` comment
// that explains how we have guaranteed that the pointer usage can never go
// out-of-bounds, or that the requirements of the UNSAFE_BUFFER_USAGE function
// are met. The safety comment should allow a reader to check that all
// requirements have been met, using only local invariants. Examples of local
// invariants include:
// - Runtime conditions or CHECKs near the UNSAFE_BUFFERS macros
// - Invariants guaranteed by types in the surrounding code
// - Invariants guaranteed by function calls in the surrounding code
// - Caller requirements, if the containing function is itself marked with
//   UNSAFE_BUFFER_USAGE
//
// The last case should be an option of last resort. It is less safe and will
// require the caller also use the UNSAFE_BUFFERS() macro. Prefer directly
// capturing such invariants in types like `base::span`.
//
// Safety explanations may not rely on invariants that are not fully
// encapsulated close to the UNSAFE_BUFFERS() usage. Instead, use safer coding
// patterns or stronger invariants.
#if defined(__clang__)
// clang-format off
// Formatting is off so that we can put each _Pragma on its own line, as
// recommended by the gcc docs.
#define UNSAFE_BUFFERS(...)                  \
  _Pragma("clang unsafe_buffer_usage begin") \
  __VA_ARGS__                                \
  _Pragma("clang unsafe_buffer_usage end")
// clang-format on
#else
#define UNSAFE_BUFFERS(...) __VA_ARGS__
#endif

// Defines a condition for a function to be checked at compile time if the
// parameter's value is known at compile time. If the condition is failed, the
// function is omitted from the overload set resolution, much like `requires`.
//
// If the parameter is a runtime value, then the condition is unable to be
// checked and the function will be omitted from the overload set resolution.
// This ensures the function can only be called with values known at compile
// time. This is a clang extension.
//
// Example:
// ```
// void f(int a) ENABLE_IF_ATTR(a > 0) {}
// f(1);  // Ok.
// f(0);  // Error: no valid f() found.
// ```
//
// The `ENABLE_IF_ATTR` annotation is preferred over `consteval` with a check
// that breaks compile because metaprogramming does not observe such checks. So
// with `consteval`, the function looks callable to concepts/type_traits but is
// not and will fail to compile even though it reports it's usable. Whereas
// `ENABLE_IF_ATTR` interacts correctly with metaprogramming. This is especially
// painful for constructors. See also
// https://github.com/chromium/subspace/issues/266.
#if defined(__clang__)
#define ENABLE_IF_ATTR(cond, msg) __attribute__((enable_if(cond, msg)))
#else
#define ENABLE_IF_ATTR(cond, msg)
#endif

#endif  // BASE_COMPILER_SPECIFIC_H_