[Bug libstdc++/85494] implementation of random_device on mingw is useless

[Bug libstdc++/85494] implementation of random_device on mingw is useless

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85494

--- Comment #19 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Jonathan Wakely <redi@gcc.gnu.org>:

https://gcc.gnu.org/g:5f070ba29803c99a5fe94ed7632d7b8c55593df3

commit r11-7867-g5f070ba29803c99a5fe94ed7632d7b8c55593df3
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Fri Mar 26 18:39:49 2021 +0000

    libstdc++: Add PRNG fallback to std::random_device

    This makes std::random_device usable on VxWorks when running on older
    x86 hardware. Since the r10-728 fix for PR libstdc++/85494 the library
    will use the new code unconditionally on x86, but the cpuid checks for
    RDSEED and RDRAND can fail at runtime, depending on the hardware where
    the code is executing. If the OS does not provide /dev/urandom then this
    means the std::random_device constructor always fails. In previous
    releases if /dev/urandom is unavailable then std::mt19937 was used
    unconditionally.

    This patch adds a fallback for the case where the runtime cpuid checks
    for x86 hardware instructions fail, and no /dev/urandom is available.
    When this happens a std::linear_congruential_engine object will be used,
    with a seed based on hashing the engine's address and the current time.
    Distinct std::random_device objects will use different seeds, unless an
    object is created and destroyed and a new object created at the same
    memory location within the clock tick. This is not great, but is better
    than always throwing from the constructor, and better than always using
    std::mt19937 with the same seed (as GCC 9 and earlier do).

    libstdc++-v3/ChangeLog:

            * src/c++11/random.cc (USE_LCG): Define when a pseudo-random
            fallback is needed.
            [USE_LCG] (bad_seed, construct_lcg_at, destroy_lcg_at, __lcg):
            New helper functions and callback.
            (random_device::_M_init): Add 'prng' and 'all' enumerators.
            Replace switch with fallthrough with a series of 'if' statements.
            [USE_LCG]: Construct an lcg_type engine and use __lcg when cpuid
            checks fail.
            (random_device::_M_init_pretr1) [USE_MT19937]: Accept "prng"
            token.
            (random_device::_M_getval): Check for callback unconditionally
            and always pass _M_file pointer.
            * testsuite/26_numerics/random/random_device/85494.cc: Remove
            effective-target check. Use new random_device_available helper.
            * testsuite/26_numerics/random/random_device/94087.cc: Likewise.
            * testsuite/26_numerics/random/random_device/cons/default-cow.cc:
            Remove effective-target check.
            * testsuite/26_numerics/random/random_device/cons/default.cc:
            Likewise.
            * testsuite/26_numerics/random/random_device/cons/token.cc: Use
            new random_device_available helper. Test "prng" token.
            * testsuite/util/testsuite_random.h (random_device_available):
            New helper function.