[gfortran] PATCH -- Fix for PR 15619

[Paul Brook]

> Sigh.  I really didn't want to become an expert on PRNG.  I only
> want to use them. :-)  So, here's my current solution.

In leu of  approach, However:

+#ifdef __FreeBSD__
+#  include <sys/uio.h>
+#  include <unistd.h>
+#endif

Definitely not. Why is this needed? It should be a proper configure check.

+const GFC_INTEGER_4
+  kiss_default_seed[4] = {123456789, 362436069, 521288629, 916191069};
+static GFC_INTEGER_4
+  kiss_seed[4]         = {123456789, 362436069, 521288629, 916191069};

Shouldn't these seed values use unsigned types? You're doing right shifts on 
the, so it will affect the value.

+/* kiss_random_kernel() returns an integer value in the range of
+   [- GFC_INTEGER_4_HUGE, GFC_INTEGER_4_HUGE].  The distribution of 
+   pseudorandom numbers should be uniform.  */

I suspect this is wrong. DOesn't it return a value in the range
[- (GFC_INTEGER_4_HUGE + 1), GFC_INTEGER_4_HUGE] ?

+void prefix(random_r4) (GFC_REAL_4 *x)
+{
+
+  *x = (GFC_REAL_4) kiss_random_kernel();
+  *x = ((*x < 0) ? - *x : *x) / ((GFC_REAL_4) (GFC_INTEGER_4_HUGE) + 1.);
+}

This is wrong.  (-x == x && x < 0) when x = 1u<<31.

Also ((GFC_REAL_4) (GFC_INTEGER_4_HUGE) + 1.) == GFC_INTEGER_4_HUGE
because of limited precision, so it can return 1.0. I'd prefer it if you 
used/copied the existing routines to do this.

These also return in the range [0, 1], but that can be fixed by iterating 
until we generate a value <1.0.

Please add a comment saying why we #if 0'd the existing implementation. ie. 
randomness of values depends on seed quality, get/put don't work properly, 
mention PR15619.

You also seem to have included the remnants of a previous patch, and some 
debugging output.

Paul