[Bug libstdc++/94823] modulo arithmetic bug in random.tcc

[cvs-commit at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94823

--- Comment #8 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:3ee44d4c518d61c6bbf75fcf280edc6ce5326ce0

commit r11-3759-g3ee44d4c518d61c6bbf75fcf280edc6ce5326ce0
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Fri Oct 9 16:10:31 2020 +0100

    libstdc++: Fix incorrect results in std::seed_seq::generate [PR 97311]

    This ensures that intermediate results are done in uint32_t values,
    meeting the requirement for operations to be done modulo 2^32.

    If the target doesn't define __UINT32_TYPE__ then substitute uint32_t
    with a class type that uses uint_least32_t and masks the value to
    UINT32_MAX.

    I've also split the first loop that goes from k=0 to k<m into three
    loops, for k=0, [1,s] and [s+1,m). This avoids branching for those three
    cases in the body of the loop, and also avoids the concerns in PR 94823
    regarding the k-1 index when k==0.

    libstdc++-v3/ChangeLog:

            PR libstdc++/97311
            * include/bits/random.tcc (seed_seq::generate): Use uint32_t for
            calculations. Also split the first loop into three loops to
            avoid branching on k on every iteration, resolving PR 94823.
            * testsuite/26_numerics/random/seed_seq/97311.cc: New test.
            * testsuite/26_numerics/random/pr60037-neg.cc: Adjust dg-erro
            line number.