From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (qmail 15381 invoked by alias); 14 Jun 2013 12:11:10 -0000 Mailing-List: contact glibc-bugs-help@sourceware.org; run by ezmlm Precedence: bulk List-Id: List-Subscribe: List-Post: List-Help: , Sender: glibc-bugs-owner@sourceware.org Received: (qmail 14613 invoked by uid 55); 14 Jun 2013 12:11:03 -0000 From: "neleai at seznam dot cz" To: glibc-bugs@sourceware.org Subject: [Bug libc/15615] Poor quality output from rand_r Date: Fri, 14 Jun 2013 12:11:00 -0000 X-Bugzilla-Reason: CC X-Bugzilla-Type: changed X-Bugzilla-Watch-Reason: None X-Bugzilla-Product: glibc X-Bugzilla-Component: libc X-Bugzilla-Version: unspecified X-Bugzilla-Keywords: X-Bugzilla-Severity: normal X-Bugzilla-Who: neleai at seznam dot cz X-Bugzilla-Status: NEW X-Bugzilla-Priority: P2 X-Bugzilla-Assigned-To: unassigned at sourceware dot org X-Bugzilla-Target-Milestone: --- X-Bugzilla-Flags: X-Bugzilla-Changed-Fields: Message-ID: In-Reply-To: References: Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: 7bit X-Bugzilla-URL: http://sourceware.org/bugzilla/ Auto-Submitted: auto-generated MIME-Version: 1.0 X-SW-Source: 2013-06/txt/msg00110.txt.bz2 http://sourceware.org/bugzilla/show_bug.cgi?id=15615 --- Comment #3 from Ondrej Bilka --- On Thu, Jun 13, 2013 at 12:38:42PM +0000, bugdal at aerifal dot cx wrote: > http://sourceware.org/bugzilla/show_bug.cgi?id=15615 > > --- Comment #2 from Rich Felker --- > On Thu, Jun 13, 2013 at 08:26:27AM +0000, neleai at seznam dot cz wrote: > > A problem here is that for many users predictability is much more > > important than quality. Developer expects that when he uses rand_r with > > state that he controls will not vary. This can cause extra debbuging hastle > > when > > code mysteriously fails on one machine but not other or desync issues. > > Could you explain better what you're concerned about? By > "predictable", do you mean keeping the same sequence it's had in the > past? Aside from that, any PRNG with 32-bit state and 31-bit output is > equally "predictable". > > > > To fully fix rand_r, the approach of concatenating multiple iterations should > > > be abandoned in favor of a single-LCG-iteration approach followed by an > > > invertable transformation on the output. Obviously a 32-bit cryptographic block > > > cipher would give the best statistical properties, but it would be slow. In > > > > This is false, I have a replacement of this with four rounds of AES. On > > intel using aesenc I performance is better than current, I did not > > propose that due of problems above. I wrote a RFC for random > > replacement on libc-alpha, browse archives. > > AES itself does not use 32-bit blocks, so you must be using a modified > version. Would you care to explain? I searched the archives but could > not find your post. > Here, I wrote a version relevant to random. I did this to see how fast I could get if I employ paralellism and inlining. http://sourceware.org/ml/libc-help/2012-12/msg00005.html To test rand_r equivalent I wrote a simple generator (which is for mostly to test performance, I did not look for quality.) movd (%rdi),%xmm0 movdqa %xmm0,%xmm1 aesenc %xmm0,%xmm1 aesenc %xmm0,%xmm1 aesenc %xmm0,%xmm1 aesenc %xmm0,%xmm1 movd %xmm1, (%rdi) movd %xmm1, %eax shr $1, %eax On sandy bridge this code runs at half of speed of rand_r. -- You are receiving this mail because: You are on the CC list for the bug.