Replace random(), pg_erand48(), etc with a better PRNG API and algorithm.

Standardize on xoroshiro128** as our basic PRNG algorithm, eliminating
a bunch of platform dependencies as well as fundamentally-obsolete PRNG
code.  In addition, this API replacement will ease replacing the
algorithm again in future, should that become necessary.

xoroshiro128** is a few percent slower than the drand48 family,
but it can produce full-width 64-bit random values not only 48-bit,
and it should be much more trustworthy.  It's likely to be noticeably
faster than the platform's random(), depending on which platform you
are thinking about; and we can have non-global state vectors easily,
unlike with random().  It is not cryptographically strong, but neither
are the functions it replaces.

Fabien Coelho, reviewed by Dean Rasheed, Aleksander Alekseev, and myself

Discussion: https://postgr.es/m/alpine.DEB.2.22.394.2105241211230.165418@pseudo

2 files changed, 19 insertions, 18 deletions

diff --git a/src/backend/optimizer/geqo/geqo_random.c b/src/backend/optimizer/geqo/geqo_random.c
index f21bc047e68..4d9432c0dfe 100644
--- a/src/backend/optimizer/geqo/geqo_random.c
+++ b/src/backend/optimizer/geqo/geqo_random.c
@@ -21,14 +21,7 @@ geqo_set_seed(PlannerInfo *root, double seed)
 {
 	GeqoPrivateData *private = (GeqoPrivateData *) root->join_search_private;
 
-	/*
-	 * XXX. This seeding algorithm could certainly be improved - but it is not
-	 * critical to do so.
-	 */
-	memset(private->random_state, 0, sizeof(private->random_state));
-	memcpy(private->random_state,
-		   &seed,
-		   Min(sizeof(private->random_state), sizeof(seed)));
+	pg_prng_fseed(&private->random_state, seed);
 }
 
 double
@@ -36,5 +29,17 @@ geqo_rand(PlannerInfo *root)
 {
 	GeqoPrivateData *private = (GeqoPrivateData *) root->join_search_private;
 
-	return pg_erand48(private->random_state);
+	return pg_prng_double(&private->random_state);
+}
+
+int
+geqo_randint(PlannerInfo *root, int upper, int lower)
+{
+	GeqoPrivateData *private = (GeqoPrivateData *) root->join_search_private;
+
+	/*
+	 * In current usage, "lower" is never negative so we can just use
+	 * pg_prng_uint64_range directly.
+	 */
+	return (int) pg_prng_uint64_range(&private->random_state, lower, upper);
 }
diff --git a/src/backend/optimizer/geqo/geqo_selection.c b/src/backend/optimizer/geqo/geqo_selection.c
index 66b6c8ae38e..68a2e4e9f15 100644
--- a/src/backend/optimizer/geqo/geqo_selection.c
+++ b/src/backend/optimizer/geqo/geqo_selection.c
@@ -63,10 +63,6 @@ geqo_selection(PlannerInfo *root, Chromosome *momma, Chromosome *daddy,
 	/*
 	 * Ensure we have selected different genes, except if pool size is only
 	 * one, when we can't.
-	 *
-	 * This code was observed to hang up in an infinite loop when the
-	 * platform's implementation of erand48() was broken.  We now always use
-	 * our own version.
 	 */
 	if (pool->size > 1)
 	{
@@ -95,11 +91,11 @@ linear_rand(PlannerInfo *root, int pool_size, double bias)
 	double		max = (double) pool_size;
 
 	/*
-	 * If geqo_rand() returns exactly 1.0 then we will get exactly max from
-	 * this equation, whereas we need 0 <= index < max.  Also it seems
-	 * possible that roundoff error might deliver values slightly outside the
-	 * range; in particular avoid passing a value slightly less than 0 to
-	 * sqrt(). If we get a bad value just try again.
+	 * geqo_rand() is not supposed to return 1.0, but if it does then we will
+	 * get exactly max from this equation, whereas we need 0 <= index < max.
+	 * Also it seems possible that roundoff error might deliver values
+	 * slightly outside the range; in particular avoid passing a value
+	 * slightly less than 0 to sqrt().  If we get a bad value just try again.
 	 */
 	do
 	{