[PATCH 1/3] libstdc++: Implement std::philox_engine for targets without __int128

[Jonathan Wakely]

On Wed, 17 Dec 2025 at 20:22, Tomasz Kaminski <tkaminsk@redhat.com> wrote:
>
>
>
> On Wed, Dec 17, 2025 at 8:51 PM Jonathan Wakely <jwakely@redhat.com> wrote:
>>
>> This moves the __detail::_Select_uint_least_t<N>::type class to
>> namespace scope and extends it with more 128-bit arithmetic operations,
>> implemented in terms of uint64_t.
>>
>> Now std::philox_engine can use _Select_uint_least_t<w*2>::type instead
>> of __uint128_t, so that it works on targets without 128-bit integers.
>> This also means that targets that do support __uint128_t only use it
>> when actually necessary, so that we use uint64_t when generating a
>> 32-bit result (e.g. with std::philox4x32).
>>
>> libstdc++-v3/ChangeLog:
>>
>>         * include/bits/random.h [!__SIZEOF_INT128__] (__rand_uint128):
>>         Refactor and rename _Select_uint_least_t<128>::type to a new
>>         class. Make all members constexpr. Add new member functions for
>>         additional arithmetic and bitwise operations, and comparisons.
>>         (__detail::_Select_uint_least_t<>::type): Define as an alias of
>>         __rand_uint128.
>>         * include/bits/random.tcc (philox_engine::_M_mulhi): Use
>>         _Select_uint_least_t<w*2>::type instead of __uint128_t.
>>         (philox_engine::_M_transition): Likewise.
>>         * include/bits/version.def (philox_engine): Remove extra_cond.
>>         * include/bits/version.h: Regenerate.
>>         * testsuite/26_numerics/random/philox4x32.cc: Remove
>>         dg-require-cpp-feature-test directive.
>>         * testsuite/26_numerics/random/philox4x64.cc: Likewise.
>>         * testsuite/26_numerics/random/philox_engine/cons/copy.cc:
>>         Likewise.
>>         * testsuite/26_numerics/random/philox_engine/cons/default.cc:
>>         Likewise.
>>         * testsuite/26_numerics/random/philox_engine/cons/seed.cc:
>>         Likewise.
>>         * testsuite/26_numerics/random/philox_engine/operators/equal.cc:
>>         Likewise.
>>         * testsuite/26_numerics/random/philox_engine/operators/serialize.cc:
>>         Likewise.
>>         * testsuite/26_numerics/random/philox_engine/requirements/constants.cc:
>>         Likewise.
>>         * testsuite/26_numerics/random/philox_engine/requirements/typedefs.cc:
>>         Likewise.
>>
>> Co-authored-by: Jakub Jelinek <jakub@redhat.com>
>> ---
>>
>> Tested x86_64-linux
>> Also checked -m32 and -D_GLIBCXX_USE_OLD_GENERATE_CANONICAL
>
> LGTM.
> I haven't fully verified the algorithms part of __int128, but they look ok in general.
> Checked, if additional operations are matching ones that were removed.

Thanks for checking it.

One reason for hoisting this class out of the _Select_uint_least_t
template was that it should be easier to add proper tests for the
operators now. I didn't do that as part of this fix, because the
priority was just to fix the failing tests, and because the tests for
std::uniform_real_distribution and std::linear_congruential_engine
contain checks that the results are as expected, which confirms that
the arithmetic is giving the expected answers for the current uses of
the class.