[Patch, libfortran] Multi-threaded random_number

[Jerry DeLisle]

On 08/08/2016 04:01 AM, Janne Blomqvist wrote:
> PING**2
>

OK, thanks for patch.

Jerry
> On Sun, Jul 24, 2016 at 4:45 PM, Janne Blomqvist
> <blomqvist.janne@gmail.com> wrote:
>> Hi,
>>
>> the attached patch replaces the current random_number / random_seed
>> implementations with an implementation that better supports threads.
>> It's an improved version of the RFC patch I posted earlier at
>> https://gcc.gnu.org/ml/gcc-patches/2015-12/msg02110.html . Please see
>> that earlier message for a longer-winded explanation of what's wrong
>> with the current implementation and how the patch addresses this.
>>
>> In short, with the patch the random number generator state is now
>> per-thread and stored in a per-thread (TLS) variable, enabling a
>> lockless fast-path. This provides up to 2 orders of magnitude better
>> performance on a synthetic benchmark using 4 threads, and provides a
>> more deterministic result as the order that threads are scheduled does
>> not affect the random number streams for each thread.
>>
>> Compared to the RFC patch, a number of minor and not-so-minor bugs
>> have been fixed, so the patch now passes the testsuite (with a few
>> modifications to the suite, part of the patch). Also, for REAL kinds
>> 4, 8, 10 the generated streams are identical (except precision, of
>> course) (like the current implementation), enabling precision
>> comparisons, as requested by Steve Kargl. However, this does not
>> extend to REAL(16) as that would have necessitated doubling the size
>> of the state, along with potential issues of slower escape from a
>> low-entropy state, for a feature that I believe is not used by
>> particularly many users in the end. So if one wants to do precision
>> comparisons with REAL(16) one must employ a wrapper routine.
>>
>> Regtested on x86_64-pc-linux-gnu, Ok for trunk?
>>
>> frontend ChangeLog:
>>
>> 2016-07-27  Janne Blomqvist  <jb@gcc.gnu.org>
>>
>>     * check.c (gfc_check_random_seed): Use new seed size in check.
>>     * intrinsic.texi (RANDOM_NUMBER): Updated documentation.
>>     (RANDOM_SEED): Likewise.
>>
>>
>> testsuite:
>>
>> 2016-07-27  Janne Blomqvist  <jb@gcc.gnu.org>
>>
>>     * gfortran.dg/random_7.f90: Take into account that the last seed
>>     value is the special p value.
>>     * gfortran.dg/random_seed_1.f90: Seed size is now constant.
>>
>>
>> libgfortran:
>> 2016-07-27  Janne Blomqvist  <jb@gcc.gnu.org>
>>
>>     * intrinsics/random.c: Replace KISS with xorshift1024* with
>>     per-thread (TLS) state.
>>     * runtime/main.c (init): Don't call random_seed_i4.
>>
>>
>> --
>> Janne Blomqvist
>
>
>