dosbox-staging/include/compiler.h

/*
 *  SPDX-License-Identifier: GPL-2.0-or-later
 *
 *  Copyright (C) 2019-2021  The DOSBox Staging Team
 *
 *  This program is free software; you can redistribute it and/or modify
 *  it under the terms of the GNU General Public License as published by
 *  the Free Software Foundation; either version 2 of the License, or
 *  (at your option) any later version.
 *
 *  This program is distributed in the hope that it will be useful,
 *  but WITHOUT ANY WARRANTY; without even the implied warranty of
 *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 *  GNU General Public License for more details.
 *
 *  You should have received a copy of the GNU General Public License along
 *  with this program; if not, write to the Free Software Foundation, Inc.,
 *  51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
 */

#ifndef DOSBOX_COMPILER_H
#define DOSBOX_COMPILER_H

// This header wraps compiler-specific features, so they won't need to
// be hacked into the buildsystem.

// Function-like macro __has_cpp_attribute was introduced in C++20, but
// various compilers support it since C++11 as a language extension.
// Thanks to that we can use it for testing support for both language-defined
// and vendor-specific attributes.

#ifndef __has_cpp_attribute // for compatibility with non-supporting compilers
#define __has_cpp_attribute(x) 0
#endif

// Function-like macro __has_attribute was introduced in GCC 5.x and Clang,
// alongside __has_cpp_attribute, and with the same logic.
// See: https://clang.llvm.org/docs/LanguageExtensions.html#has-attribute

#ifdef __has_attribute
#define C_HAS_ATTRIBUTE 1
#else
#define C_HAS_ATTRIBUTE 0
#define __has_attribute(x) 0 // for compatibility with non-supporting compilers
#endif

// When passing the -Wunused flag to GCC or Clang, entities that are unused by
// the program may be diagnosed.  The MAYBE_UNUSED attribute can be used to
// silence such diagnostics when the entity cannot be removed.
//
// The attribute may be applied to the declaration of a class, a typedef,
// a variable, a function or method, a function parameter, an enumeration,
// an enumerator, a non-static data member, or a label.

#if __has_cpp_attribute(maybe_unused)
#define MAYBE_UNUSED [[maybe_unused]]
#elif __has_cpp_attribute(gnu::unused)
#define MAYBE_UNUSED [[gnu::unused]]
#else
#define MAYBE_UNUSED
#endif

// Wrapper for C++17 [[fallthrough]] null statement. Use this to avoid implicit
// fallthrough in switch statements (-Wimplicit-fallthrough flag).

#if __has_cpp_attribute(fallthrough)
#define FALLTHROUGH [[fallthrough]]
#else
#define FALLTHROUGH
#endif

// The __attribute__ syntax is supported by GCC, Clang, and IBM compilers.
//
// Provided for backwards-compatibility with old code; to be gradually
// replaced by new C++ attribute syntax.

#if C_HAS_ATTRIBUTE
#define GCC_ATTRIBUTE(x) __attribute__ ((x))
#else
#define GCC_ATTRIBUTE(x) /* attribute not supported */
#endif

// Wrapper for various compiler extensions for inlining aggresively.
//
// There is NO way to truly FORCE compiler to do inlining, therefore all these
// methods are only strong hints, usually non-preferrable over simple
// 'inline' keyword.
//
// Normal C++ 'inline' is indicator that there might be more than one
// definition for the function (as long as all definitions are the same).
// It's used to define functions in C++ headers. Compiler will automatically
// try to inline (embed compiled code without function call) any function
// defined in a header.
//
// For details about GCC/Clang always_inline, see:
// https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#Common-Function-Attributes
//
// For details about MSVC __forceinline, see:
// https://docs.microsoft.com/en-us/cpp/cpp/inline-functions-cpp#inline-__inline-and-__forceinline

#if __has_attribute(always_inline)
#define INLINE inline __attribute__((always_inline))
#elif defined(_MSC_VER)
#define INLINE __forceinline
#else
#define INLINE inline
#endif

// GCC_LIKELY macro is incorrectly named, because other compilers support
// this feature as well (e.g. Clang, Intel); leave it be for now, at
// least until full support for C++20 [[likely]] attribute will start arriving
// in new compilers.
//
// Note: '!!' trick is used, to convert non-boolean values to 1 or 0
// to prevent accidental incorrect usage (e.g. when user wraps macro
// around a pointer or an integer, expecting usual C semantics).

#if C_HAS_BUILTIN_EXPECT
#define GCC_LIKELY(x)   __builtin_expect(!!(x), 1)
#define GCC_UNLIKELY(x) __builtin_expect(!!(x), 0)
#else
#define GCC_LIKELY
#define GCC_UNLIKELY
#endif

// XSTR and STR macros can be used for turning defines into string literals:
//
// #define FOO 4
// printf("It's a " STR(FOO));  // prints "It's a FOO"
// printf("It's a " XSTR(FOO)); // prints "It's a 4"

#define XSTR(s) STR(s)
#define STR(s)  #s

#endif