database/postgres
1
0
Fork 0
mirror of https://github.com/postgres/postgres.git synced 2025-06-14 18:42:34 +03:00
Code Activity

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

Browse Source 
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
This commit is contained in:
Tom Lane
2021-11-28 21:32:36 -05:00
parent f44ceb46ec
commit 3804539e48
50 changed files with 543 additions and 480 deletions
Download Patch File Download Diff File Expand all files Collapse all files

20
src/backend/utils/adt/float.c
View File

@ -21,6 +21,7 @@
#include "catalog/pg_type.h"
#include "common/int.h"
#include "common/pg_prng.h"
#include "common/shortest_dec.h"
#include "libpq/pqformat.h"
#include "miscadmin.h"
@ -65,7 +66,7 @@ float8 degree_c_one = 1.0;
/* State for drandom() and setseed() */
static bool drandom_seed_set = false;
static unsigned short drandom_seed[3] = {0, 0, 0};
static pg_prng_state drandom_seed;
/* Local function prototypes */
static double sind_q1(double x);
@ -2762,22 +2763,20 @@ drandom(PG_FUNCTION_ARGS)
* Should that fail for some reason, we fall back on a lower-quality
* seed based on current time and PID.
*/
if (!pg_strong_random(drandom_seed, sizeof(drandom_seed)))
if (unlikely(!pg_prng_strong_seed(&drandom_seed)))
{
TimestampTz now = GetCurrentTimestamp();
uint64 iseed;
/* Mix the PID with the most predictable bits of the timestamp */
iseed = (uint64) now ^ ((uint64) MyProcPid << 32);
drandom_seed[0] = (unsigned short) iseed;
drandom_seed[1] = (unsigned short) (iseed >> 16);
drandom_seed[2] = (unsigned short) (iseed >> 32);
pg_prng_seed(&drandom_seed, iseed);
}
drandom_seed_set = true;
}
/* pg_erand48 produces desired result range [0.0 - 1.0) */
result = pg_erand48(drandom_seed);
/* pg_prng_double produces desired result range [0.0 - 1.0) */
result = pg_prng_double(&drandom_seed);
PG_RETURN_FLOAT8(result);
}
@ -2790,7 +2789,6 @@ Datum
setseed(PG_FUNCTION_ARGS)
{
float8 seed = PG_GETARG_FLOAT8(0);
uint64 iseed;
if (seed < -1 || seed > 1 || isnan(seed))
ereport(ERROR,
@ -2798,11 +2796,7 @@ setseed(PG_FUNCTION_ARGS)
errmsg("setseed parameter %g is out of allowed range [-1,1]",
seed)));
/* Use sign bit + 47 fractional bits to fill drandom_seed[] */
iseed = (int64) (seed * (float8) UINT64CONST(0x7FFFFFFFFFFF));
drandom_seed[0] = (unsigned short) iseed;
drandom_seed[1] = (unsigned short) (iseed >> 16);
drandom_seed[2] = (unsigned short) (iseed >> 32);
pg_prng_fseed(&drandom_seed, seed);
drandom_seed_set = true;
PG_RETURN_VOID();