|Summary:||Use a better RNG for Math.random()|
|Product:||WebKit||Reporter:||Jan de Mooij <jdemooij>|
|Severity:||Normal||CC:||barraclough, benjamin, cdumez, cmarcelo, commit-queue, ggaren, keith_miller, mark.lam, msaboff, oliver, sbarati, yusukesuzuki|
|Version:||WebKit Nightly Build|
Description Jan de Mooij 2015-11-28 05:51:36 PST
The GameRand-based RNG Safari/JSC uses is very weak. Furthermore, it only uses 32 bits of precision. I blogged about this here: http://jandemooij.nl/blog/2015/11/27/math-random-and-32-bit-precision/ V8 moved to a better RNG and we're switching the one in SpiderMonkey as well. JSC should probably do the same.
Comment 1 Oliver Hunt 2015-11-28 09:55:10 PST
If we really do consider more than 32bits necessary id just move up to a 64bit variant of gamerand which should not have a significant perf impact, then value & (2^53-1) / 2^53 (I can recall if the rand includes 1 as a valid result). Has someone done something to find it's aggressively non-uniform in some space?
Comment 3 Geoffrey Garen 2015-11-30 17:04:17 PST
I kind of wanted to stick with GameRandom in order to troll the internet commentators who keep confusing / conflating RNG algorithm with number of bits provided by RNG interface. But there's literally no downside to upgrading to Xorshift+, so I've done it.
Comment 4 Anders Carlsson 2015-11-30 17:10:54 PST
Comment on attachment 266302 [details] Patch View in context: https://bugs.webkit.org/attachment.cgi?id=266302&action=review > Source/WTF/ChangeLog:10 > + prevents programs that mulitply a random number by a large constant from typo, multiply.
Comment 5 Geoffrey Garen 2015-11-30 19:40:09 PST
Committed r192855: <http://trac.webkit.org/changeset/192855>
Comment 6 Jan de Mooij 2015-12-01 00:27:11 PST
(In reply to comment #3) > I kind of wanted to stick with GameRandom in order to troll the internet > commentators who keep confusing / conflating RNG algorithm with number of > bits provided by RNG interface. It's not just that; GameRand also doesn't do well in statistical tests: http://jandemooij.nl/blog/2015/11/30/testing-math-random-crushing-the-browser/ No big deal probably, but it shows the algorithm has other issues. > But there's literally no downside to upgrading to Xorshift+, so I've done it. \o/