PR fortran/91414: Improved PRNG

Update the PRNG from xorshift1024* to xoshiro256** by the same
author. For details see

http://prng.di.unimi.it/

and the paper at

https://arxiv.org/abs/1805.01407

Also the seeding is slightly improved, by reading only 8 bytes from
the operating system and using the simple splitmix64 PRNG to fill in
the rest of the PRNG state (as recommended by the xoshiro author),
instead of reading the entire state from the OS.

Regtested on x86_64-pc-linux-gnu, Ok for trunk?

gcc/fortran/ChangeLog:

2019-08-13  Janne Blomqvist  <jb@gcc.gnu.org>

	PR fortran/91414
	* check.c (gfc_check_random_seed): Reduce seed_size.
	* intrinsic.texi (RANDOM_NUMBER): Update to match new PRNG.

gcc/testsuite/ChangeLog:

2019-08-13  Janne Blomqvist  <jb@gcc.gnu.org>

	PR fortran/91414
	* gfortran.dg/random_seed_1.f90: Update to match new seed size.

libgfortran/ChangeLog:

2019-08-13  Janne Blomqvist  <jb@gcc.gnu.org>

	PR fortran/91414
	* intrinsics/random.c (prng_state): Update state struct.
	(master_state): Update to match new size.
	(get_rand_state): Update to match new PRNG.
	(rotl): New function.
	(xorshift1024star): Replace with prng_next.
	(prng_next): New function.
	(jump): Update for new PRNG.
	(lcg_parkmiller): Replace with splitmix64.
	(splitmix64): New function.
	(getosrandom): Fix return value, simplify.
	(init_rand_state): Use getosrandom only to get 8 bytes, splitmix64
	to fill rest of state.
	(random_r4): Update to new function and struct names.
	(random_r8): Likewise.
	(random_r10): Likewise.
	(random_r16): Likewise.
	(arandom_r4): Liekwise.
	(arandom_r8): Likewise.
	(arandom_r10): Likwewise.
	(arandom_r16): Likewise.
	(xor_keys): Reduce size to match new PRNG.
	(random_seed_i4): Update to new function and struct names, remove
	special handling of variable p used in previous PRNG.
	(random_seed_i8): Likewise.

From-SVN: r274361

5 files changed, 22 insertions, 11 deletions

diff --git a/gcc/fortran/ChangeLog b/gcc/fortran/ChangeLog
index 05e5ba5..7131d0f 100644
--- a/gcc/fortran/ChangeLog
+++ b/gcc/fortran/ChangeLog
@@ -1,3 +1,9 @@
+2019-08-13  Janne Blomqvist  <jb@gcc.gnu.org>
+
+        PR fortran/91414
+        * check.c (gfc_check_random_seed): Reduce seed_size.
+        * intrinsic.texi (RANDOM_NUMBER): Update to match new PRNG.
+
 2019-08-12  Thomas Koenig  <tkoenig@gcc.gnu.org>
 
 	PR fortran/91424
diff --git a/gcc/fortran/check.c b/gcc/fortran/check.c
index 370a3c8..2bd8bc3 100644
--- a/gcc/fortran/check.c
+++ b/gcc/fortran/check.c
@@ -6484,9 +6484,8 @@ gfc_check_random_seed (gfc_expr *size, gfc_expr *put, gfc_expr *get)
   mpz_t put_size, get_size;
 
   /* Keep the number of bytes in sync with master_state in
-     libgfortran/intrinsics/random.c. +1 due to the integer p which is
-     part of the state too.  */
-  seed_size = 128 / gfc_default_integer_kind + 1;
+     libgfortran/intrinsics/random.c.  */
+  seed_size = 32 / gfc_default_integer_kind;
 
   if (size != NULL)
     {
diff --git a/gcc/fortran/intrinsic.texi b/gcc/fortran/intrinsic.texi
index f390761..3aa068d 100644
--- a/gcc/fortran/intrinsic.texi
+++ b/gcc/fortran/intrinsic.texi
@@ -11792,10 +11792,10 @@ end program test_random_seed
 Returns a single pseudorandom number or an array of pseudorandom numbers
 from the uniform distribution over the range @math{ 0 \leq x < 1}.
 
-The runtime-library implements the xorshift1024* random number
-generator (RNG). This generator has a period of @math{2^{1024} - 1},
-and when using multiple threads up to @math{2^{512}} threads can each
-generate @math{2^{512}} random numbers before any aliasing occurs.
+The runtime-library implements the xoshiro256** pseudorandom number
+generator (PRNG). This generator has a period of @math{2^{256} - 1},
+and when using multiple threads up to @math{2^{128}} threads can each
+generate @math{2^{128}} random numbers before any aliasing occurs.
 
 Note that in a multi-threaded program (e.g. using OpenMP directives),
 each thread will have its own random number state. For details of the
@@ -11852,7 +11852,7 @@ called either without arguments or with the @var{PUT} argument, the
 given seed is copied into a master seed as well as the seed of the
 current thread. When a new thread uses @code{RANDOM_NUMBER} for the
 first time, the seed is copied from the master seed, and forwarded
-@math{N * 2^{512}} steps to guarantee that the random stream does not
+@math{N * 2^{128}} steps to guarantee that the random stream does not
 alias any other stream in the system, where @var{N} is the number of
 threads that have used @code{RANDOM_NUMBER} so far during the program
 execution.
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index 5b8ed3a..9326a92 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,3 +1,8 @@
+2019-08-13  Janne Blomqvist  <jb@gcc.gnu.org>
+
+        PR fortran/91414
+        * gfortran.dg/random_seed_1.f90: Update to match new seed size.
+
 2019-08-13  Eric Botcazou  <ebotcazou@adacore.com>
 
 	* gnat.dg/discr56.adb, gnat.dg/discr56.ads,
diff --git a/gcc/testsuite/gfortran.dg/random_seed_1.f90 b/gcc/testsuite/gfortran.dg/random_seed_1.f90
index 39c81ce..a97e530 100644
--- a/gcc/testsuite/gfortran.dg/random_seed_1.f90
+++ b/gcc/testsuite/gfortran.dg/random_seed_1.f90
@@ -10,11 +10,12 @@
 PROGRAM random_seed_1
   IMPLICIT NONE
 
-  INTEGER, PARAMETER :: nbytes = 128
+  ! Should match sizeof(master_state) in
+  ! libgfortran/intrinsics/random.c
+  INTEGER, PARAMETER :: nbytes = 32
 
-  ! +1 due to the special 'p' value in xorshift1024*
   ! '+1' to avoid out-of-bounds warnings
-  INTEGER, PARAMETER    :: n = nbytes / KIND(n) + 2
+  INTEGER, PARAMETER    :: n = nbytes / KIND(n) + 1
   INTEGER, DIMENSION(n) :: seed
 
   ! Get seed, array too small