[Patch, libfortran] Multi-threaded random_number

[Janne Blomqvist <blomqvist dot janne at gmail dot com>]

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

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