src/base/compiler.hh

/*
 * Copyright (c) 2012,2017-2018 ARM Limited
 * All rights reserved
 *
 * The license below extends only to copyright in the software and shall
 * not be construed as granting a license to any other intellectual
 * property including but not limited to intellectual property relating
 * to a hardware implementation of the functionality of the software
 * licensed hereunder.  You may use the software subject to the license
 * terms below provided that you ensure that this notice is replicated
 * unmodified and in its entirety in all distributions of the software,
 * modified or unmodified, in source code or in binary form.
 *
 * Copyright (c) 2006 The Regents of The University of Michigan
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are
 * met: redistributions of source code must retain the above copyright
 * notice, this list of conditions and the following disclaimer;
 * redistributions in binary form must reproduce the above copyright
 * notice, this list of conditions and the following disclaimer in the
 * documentation and/or other materials provided with the distribution;
 * neither the name of the copyright holders nor the names of its
 * contributors may be used to endorse or promote products derived from
 * this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
 * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
 * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
 * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
 * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
 * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
 * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
 * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
 * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

#ifndef __BASE_COMPILER_HH__
#define __BASE_COMPILER_HH__

#include <memory>

// http://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html


/*
 * Attributes that become standard in later versions of c++.
 */

// Use M5_FALLTHROUGH to mark when you're intentionally falling through from
// one case to another in a switch statement.
#if __has_cpp_attribute(fallthrough) // Standard in c++17.
#  define M5_FALLTHROUGH [[fallthrough]]
#else
// Not supported, so it's not necessary to avoid warnings.
#  define M5_FALLTHROUGH
#endif

// When the return value of a function should not be discarded, mark it with
// M5_NODISCARD.
#if __has_cpp_attribute(nodiscard) // Standard in c++17, with message in c++20.
#  define M5_NODISCARD [[nodiscard]]
#else
// Not supported, but it's optional so we can just omit it.
#  define M5_NODISCARD
#endif

// When a variable may purposefully not be used, for instance if it's only used
// in debug statements which might be disabled, mark it with M5_VAR_USED.
#if __has_cpp_attribute(maybe_unused) // Standard in c++17.
#  define M5_VAR_USED [[maybe_unused]]
#elif defined(__GNUC__)
// gcc and clang support a custom attribute which is essentially the same
// thing.
#  define M5_VAR_USED [[gnu::unused]]
#else
#  error "Don't know what to do for your compiler."
#endif


/*
 * Compiler specific features.
 */

#if defined(__GNUC__) // clang or gcc.
// Mark a structure as packed, so that no padding is added to its layout. This
// padding might be added to, for instance, ensure certain fields have certain
// alignment.
#  define M5_ATTR_PACKED [[gnu::packed]]

// Prevent a function from being inlined.
#  define M5_NO_INLINE [[gnu::noinline]]

// Set the visibility of a symbol.
#  define M5_PUBLIC [[gnu:visibility("default")]]
#  define M5_LOCAL [[gnu::visibility("hidden")]]
#  define M5_WEAK [[gnu::weak]]

// Force an alignment for a variable.
#  define M5_ALIGNED(alignment) [[gnu::aligned(alignment)]]

// Marker for what should be an unreachable point in the code.
#  define M5_UNREACHABLE __builtin_unreachable()

// To mark a branch condition as likely taken, wrap it's condition with
// M5_LIKELY. To mark it as likely not taken, wrap it's condition with
// M5_UNLIKELY. These can be replaced with the standard attributes [[likely]]
// and [[unlikely]] in c++20, although the syntax is different enough that
// we can't do that with direct substitution.
#  define M5_LIKELY(cond) __builtin_expect(!!(cond), 1)
#  define M5_UNLIKELY(cond) __builtin_expect(!!(cond), 0)
#else
#  error "Don't know what to do for your compiler."
#endif

// When a member variable may be unused, mark it with M5_CLASS_VAR_USED. This
// needs to be limitted to clang only since clang warns on these unused
// variables, and g++ will actually warn if you use this attribute since it
// won't do anything there.
#if defined(__clang__) // clang only.
#  define M5_CLASS_VAR_USED M5_VAR_USED
#else
#  define M5_CLASS_VAR_USED
#endif

#endif // __BASE_COMPILER_HH__