[Patch, libfortran] Multi-threaded random_number

Janne Blomqvist blomqvist.janne@gmail.com
Mon Aug 8 11:01:00 GMT 2016


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

Janne Blomqvist

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