libcruft-util/cruft/util/rand/rdrand.cpp

/*
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/.
 *
 * Copyright 2019 Danny Robson <danny@nerdcruft.net>
 */

#include "rdrand.hpp"

using cruft::rand::rdrand;


///////////////////////////////////////////////////////////////////////////////
rdrand::rdrand ()
{ ; }


//---------------------------------------------------------------------------------------
rdrand::rdrand (std::string const &)
    : rdrand ()
{ ; }


///////////////////////////////////////////////////////////////////////////////
rdrand::result_type
rdrand::operator() ()
{
    uint64_t res;
    char success;

    __asm__ __volatile__ (
        "rdrand %0;"
        "setc %1;"

        : "=r" (res)
        , "=qm" (success)
    );

    if (!__builtin_expect(success, 1))
        throw std::runtime_error ("no value available for rdrand");

    // As long as the u64 we just grabbed is of a type we can truncate then
    // we'll do just that and return it.
    static_assert (sizeof (result_type) <= sizeof (res));
    static_assert (std::is_signed_v<result_type> == std::is_signed_v<decltype(res)>);
    return static_cast<result_type> (res);
}