[PATCH, libgfortran] Always seed the PRNG from the OS

Janne Blomqvist blomqvist.janne@gmail.com
Tue Aug 16 08:19:00 GMT 2016


Hi,

now that I have changed the PRNG algorithm behind RANDOM_NUMBER, and
thus breaking compatibility (in the weak sense that the stream of
random numbers will be different), one might also do a few other minor
improvements.

The attached patch makes sure to always initialize the PRNG seed from
the OS (/dev/urandom or falling back to gettimeofday ^ getpid) so that
the random generator state is different every time one runs a program.
This is probably closer to what people intuitively expect, and if one
wants a constant seed, it's easy enough to do by explicitly calling
RANDOM_SEED.

Unless there are objections, I intend to commit this in a few days. Regtested on
x86_64-pc-linux-gnu.

libgfortran:

2016-08-16  Janne Blomqvist  <jb@gcc.gnu.org>

    * intrinsics/random.c (master_init): New variable.
    (init_rand_state): Move below getosrandom (), maybe initialize
    master_state.
    (random_seed_i4): If called with no arguments, set master_init to
    false, and reinitialize. If called with PUT=, set master_init to
    true.
    (random_seed_i8): Likewise.

testsuite:

2016-08-16  Janne Blomqvist  <jb@gcc.gnu.org>

    * gfortran.dg/random_4.f90: Initialize seed before using, handle
    the last special seed value.
    * gfortran.dg/random_7.f90: Use size for last array member instead
    of hardcoded value.


-- 
Janne Blomqvist
-------------- next part --------------
diff --git a/gcc/testsuite/gfortran.dg/random_4.f90 b/gcc/testsuite/gfortran.dg/random_4.f90
index 416b17c..e60698f 100644
--- a/gcc/testsuite/gfortran.dg/random_4.f90
+++ b/gcc/testsuite/gfortran.dg/random_4.f90
@@ -6,8 +6,11 @@ program trs
   integer, allocatable, dimension(:) :: seed, check
   call test_random_seed(size)
   allocate(seed(size),check(size))
+  seed = 42
   call test_random_seed(put=seed)
   call test_random_seed(get=check)
+  ! With xorshift1024* the last seed value is special
+  seed(size) = check(size)
   if (any (seed /= check)) call abort
 contains
   subroutine test_random_seed(size, put, get)
diff --git a/gcc/testsuite/gfortran.dg/random_7.f90 b/gcc/testsuite/gfortran.dg/random_7.f90
index aafe346..8cd9c43 100644
--- a/gcc/testsuite/gfortran.dg/random_7.f90
+++ b/gcc/testsuite/gfortran.dg/random_7.f90
@@ -10,8 +10,9 @@ program trs
   seed(:) = huge(seed) / 17
   call test_random_seed(put=seed)
   call test_random_seed(get=check)
-  ! In the current implementation seed(17) is special
-  seed(17) = check(17)
+  ! In the current xorshift1024* implementation the last seed value is
+  ! special
+  seed(size) = check(size)
   if (any (seed /= check)) call abort
 contains
   subroutine test_random_seed(size, put, get)
diff --git a/libgfortran/intrinsics/random.c b/libgfortran/intrinsics/random.c
index 3b91389..35c7611 100644
--- a/libgfortran/intrinsics/random.c
+++ b/libgfortran/intrinsics/random.c
@@ -193,9 +193,10 @@ typedef struct
 xorshift1024star_state;
 
 
-/* How many times we have jumped. This and master_state are the only
-   variables protected by random_lock.  */
-static unsigned njumps;
+/* master_init, njumps, and master_state are the only variables
+   protected by random_lock.  */
+static bool master_init;
+static unsigned njumps; /* How many times we have jumped.  */
 static uint64_t master_state[] = {
   0xad63fa1ed3b55f36ULL, 0xd94473e78978b497ULL, 0xbc60592a98172477ULL,
   0xa3de7c6e81265301ULL, 0x586640c5e785af27ULL, 0x7a2a3f63b67ce5eaULL,
@@ -272,24 +273,6 @@ jump (xorshift1024star_state* rs)
 }
 
 
-/* Initialize the random number generator for the current thread,
-   using the master state and the number of times we must jump.  */
-
-static void
-init_rand_state (xorshift1024star_state* rs, const bool locked)
-{
-  if (!locked)
-    __gthread_mutex_lock (&random_lock);
-  memcpy (&rs->s, master_state, sizeof (master_state));
-  unsigned n = njumps++;
-  if (!locked)
-    __gthread_mutex_unlock (&random_lock);
-  for (unsigned i = 0; i < n; i++)
-    jump (rs);
-  rs->init = true;
-}
-
-
 /* Super-simple LCG generator used in getosrandom () if /dev/urandom
    doesn't exist.  */
 
@@ -359,6 +342,30 @@ getosrandom (void *buf, size_t buflen)
 }
 
 
+/* Initialize the random number generator for the current thread,
+   using the master state and the number of times we must jump.  */
+
+static void
+init_rand_state (xorshift1024star_state* rs, const bool locked)
+{
+  if (!locked)
+    __gthread_mutex_lock (&random_lock);
+  if (!master_init)
+    {
+      getosrandom (master_state, sizeof (master_state));
+      njumps = 0;
+      master_init = true;
+    }
+  memcpy (&rs->s, master_state, sizeof (master_state));
+  unsigned n = njumps++;
+  if (!locked)
+    __gthread_mutex_unlock (&random_lock);
+  for (unsigned i = 0; i < n; i++)
+    jump (rs);
+  rs->init = true;
+}
+
+
 /*  This function produces a REAL(4) value from the uniform distribution
     with range [0,1).  */
 
@@ -791,8 +798,7 @@ random_seed_i4 (GFC_INTEGER_4 *size, gfc_array_i4 *put, gfc_array_i4 *get)
      a processor-dependent value to the seed."  */
   if (size == NULL && put == NULL && get == NULL)
     {
-      getosrandom (master_state, sizeof (master_state));
-      njumps = 0;
+      master_init = false;
       init_rand_state (rs, true);
     }
 
@@ -816,6 +822,7 @@ random_seed_i4 (GFC_INTEGER_4 *size, gfc_array_i4 *put, gfc_array_i4 *get)
 	 provide seeds with quality only in the lower or upper part.  */
       scramble_seed ((unsigned char *) master_state, seed, sizeof seed);
       njumps = 0;
+      master_init = true;
       init_rand_state (rs, true);
 
       rs->p = put->base_addr[SZ * GFC_DESCRIPTOR_STRIDE(put, 0)] & 15;
@@ -873,8 +880,7 @@ random_seed_i8 (GFC_INTEGER_8 *size, gfc_array_i8 *put, gfc_array_i8 *get)
      a processor-dependent value to the seed."  */
   if (size == NULL && put == NULL && get == NULL)
     {
-      getosrandom (master_state, sizeof (master_state));
-      njumps = 0;
+      master_init = false;
       init_rand_state (rs, true);
     }
 
@@ -894,6 +900,7 @@ random_seed_i8 (GFC_INTEGER_8 *size, gfc_array_i8 *put, gfc_array_i8 *get)
 		sizeof (GFC_UINTEGER_8));
 
       njumps = 0;
+      master_init = true;
       init_rand_state (rs, true);
       rs->p = put->base_addr[SZ * GFC_DESCRIPTOR_STRIDE(put, 0)] & 15;
      }


More information about the Gcc-patches mailing list