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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
This commit is contained in:
Tom Lane
@@ -42,7 +42,6 @@ OBJS = \
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$(PG_CRC32C_OBJS) \
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bsearch_arg.o \
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chklocale.o \
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erand48.o \
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inet_net_ntop.o \
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noblock.o \
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path.o \
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@@ -1,136 +0,0 @@
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/*-------------------------------------------------------------------------
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*
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* erand48.c
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*
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* This file supplies pg_erand48() and related functions, which except
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* for the names are just like the POSIX-standard erand48() family.
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* (We don't supply the full set though, only the ones we have found use
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* for in Postgres. In particular, we do *not* implement lcong48(), so
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* that there is no need for the multiplier and addend to be variable.)
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*
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* We used to test for an operating system version rather than
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* unconditionally using our own, but (1) some versions of Cygwin have a
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* buggy erand48() that always returns zero and (2) as of 2011, glibc's
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* erand48() is strangely coded to be almost-but-not-quite thread-safe,
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* which doesn't matter for the backend but is important for pgbench.
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*
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* Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
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*
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* Portions Copyright (c) 1993 Martin Birgmeier
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* All rights reserved.
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*
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* You may redistribute unmodified or modified versions of this source
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* code provided that the above copyright notice and this and the
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* following conditions are retained.
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*
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* This software is provided ``as is'', and comes with no warranties
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* of any kind. I shall in no event be liable for anything that happens
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* to anyone/anything when using this software.
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*
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* IDENTIFICATION
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* src/port/erand48.c
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*
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*-------------------------------------------------------------------------
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*/
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#include "c.h"
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#include <math.h>
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/* These values are specified by POSIX */
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#define RAND48_MULT UINT64CONST(0x0005deece66d)
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#define RAND48_ADD UINT64CONST(0x000b)
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/* POSIX specifies 0x330e's use in srand48, but the other bits are arbitrary */
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#define RAND48_SEED_0 (0x330e)
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#define RAND48_SEED_1 (0xabcd)
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#define RAND48_SEED_2 (0x1234)
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static unsigned short _rand48_seed[3] = {
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RAND48_SEED_0,
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RAND48_SEED_1,
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RAND48_SEED_2
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};
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/*
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* Advance the 48-bit value stored in xseed[] to the next "random" number.
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*
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* Also returns the value of that number --- without masking it to 48 bits.
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* If caller uses the result, it must mask off the bits it wants.
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*/
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static uint64
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_dorand48(unsigned short xseed[3])
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{
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/*
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* We do the arithmetic in uint64; any type wider than 48 bits would work.
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*/
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uint64 in;
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uint64 out;
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in = (uint64) xseed[2] << 32 | (uint64) xseed[1] << 16 | (uint64) xseed[0];
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out = in * RAND48_MULT + RAND48_ADD;
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xseed[0] = out & 0xFFFF;
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xseed[1] = (out >> 16) & 0xFFFF;
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xseed[2] = (out >> 32) & 0xFFFF;
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return out;
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}
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/*
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* Generate a random floating-point value using caller-supplied state.
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* Values are uniformly distributed over the interval [0.0, 1.0).
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*/
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double
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pg_erand48(unsigned short xseed[3])
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{
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uint64 x = _dorand48(xseed);
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return ldexp((double) (x & UINT64CONST(0xFFFFFFFFFFFF)), -48);
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}
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/*
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* Generate a random non-negative integral value using internal state.
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* Values are uniformly distributed over the interval [0, 2^31).
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*/
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long
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pg_lrand48(void)
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{
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uint64 x = _dorand48(_rand48_seed);
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return (x >> 17) & UINT64CONST(0x7FFFFFFF);
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}
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/*
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* Generate a random signed integral value using caller-supplied state.
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* Values are uniformly distributed over the interval [-2^31, 2^31).
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*/
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long
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pg_jrand48(unsigned short xseed[3])
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{
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uint64 x = _dorand48(xseed);
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return (int32) ((x >> 16) & UINT64CONST(0xFFFFFFFF));
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}
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/*
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* Initialize the internal state using the given seed.
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*
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* Per POSIX, this uses only 32 bits from "seed" even if "long" is wider.
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* Hence, the set of possible seed values is smaller than it could be.
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* Better practice is to use caller-supplied state and initialize it with
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* random bits obtained from a high-quality source of random bits.
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*
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* Note: POSIX specifies a function seed48() that allows all 48 bits
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* of the internal state to be set, but we don't currently support that.
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*/
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void
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pg_srand48(long seed)
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{
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_rand48_seed[0] = RAND48_SEED_0;
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_rand48_seed[1] = (unsigned short) seed;
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_rand48_seed[2] = (unsigned short) (seed >> 16);
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}
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@@ -1,25 +0,0 @@
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/*-------------------------------------------------------------------------
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*
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* random.c
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* random() wrapper
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*
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* Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
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* Portions Copyright (c) 1994, Regents of the University of California
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*
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*
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* IDENTIFICATION
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* src/port/random.c
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*
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*-------------------------------------------------------------------------
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*/
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#include "c.h"
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#include <math.h>
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long
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random(void)
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{
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return pg_lrand48();
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}
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@@ -1,25 +0,0 @@
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/*-------------------------------------------------------------------------
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*
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* srandom.c
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* srandom() wrapper
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*
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* Portions Copyright (c) 1996-2021, PostgreSQL Global Development Group
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* Portions Copyright (c) 1994, Regents of the University of California
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*
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*
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* IDENTIFICATION
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* src/port/srandom.c
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*
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*-------------------------------------------------------------------------
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*/
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#include "c.h"
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#include <math.h>
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void
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srandom(unsigned int seed)
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{
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pg_srand48((long int) seed);
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}
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