[Bug target/94087] std::random_device often fails when used from multiple threads

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94087

--- Comment #26 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The releases/gcc-10 branch has been updated by Jonathan Wakely
<redi@gcc.gnu.org>:

https://gcc.gnu.org/g:bbf9d358ae2104a09fc1b61e2b818928b6591831

commit r10-8448-gbbf9d358ae2104a09fc1b61e2b818928b6591831
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Tue May 19 16:49:21 2020 +0100

    libstdc++: Use RDRAND as fallback if RDSEED keeps failing (PR 94087)

    It's not difficult for multiple threads to drain the entropy available
    to the RDSEED instruction, at which point we throw an exception. This
    change will try to use RDRAND after RDSEED fails repeatedly, and only
    throw if RDRAND also fails repeatedly. This doesn't guarantee a random
    value can always be read, but reduces the likelihood of failure when
    using the RDSEED instruction.

            PR libstdc++/94087
            * src/c++11/random.cc (__x86_rdseed): Allow fallback function to be
            passed in.
            (__x86_rdseed_rdrand): New function that uses rdseed with rdrand
            fallback.
            (random_device::_M_init): Use __x86_rdseed_rdrand when both
            instructions are available.
            * testsuite/26_numerics/random/random_device/94087.cc: New test.

    (cherry picked from commit a2d196e75cef95c2b70734ad02e94f9da0e769fe)