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Subject: Re: [boost] [random] new threefry random engine
From: Thijs van den Berg (thijs_at_[hidden])
Date: 2014-04-20 04:37:35

On 20 Apr 2014, at 01:39, Steven Watanabe <watanabesj_at_[hidden]> wrote:

> On 04/19/2014 04:35 PM, Thijs van den Berg wrote:
>> What’s your view on limiting the round to <=20 for the template
> I still don't like it. If all else fails, you can use the
> optimized version for rounds <= 20, and the slow version
> for rounds > 20.
Ok, that’s a good last resort.

>> and providing only the 20 round as a typedef?
> I'd favor providing both. There are plenty of inferior algorithms
> in Boost.Random. I would anticipate that anyone who wants to
> use an algorithm other than mt19937 would have some idea of
> the tradeoffs.

>> I have addressed most other points you’ve mentioned, but the performance issue of a generic rounds version has failed me.
> In theory it could be optimized. What compiler and
> optimization settings are you using? In particular,
> are you using -funroll-loops (GCC)? The version
> you show unrolls the loop 4x. What if you
> unroll 8x and kill the constant arrays? What
> about 20x and eliminate the % 5 in the key addition?
> Or 40x and eliminate both?
Yes, good ideas. I think I’m going to make a template version that eliminated array indices computations.
Right now I was using the default build of the performance tool in random with Clang-503.0.38 on apple dadrwin. I’m now compile and time in manually. One thing I found earlier is that the compiler recognises rotl only with -O3. However, that’s a different issues than the loop/ un-rolled version.
> In Christ,
> Steven Watanabe
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