Re: [boost] [review] Multiprecision review scheduled for June 8th - 17th, 2012

Subject: Re: [boost] [review] Multiprecision review scheduled for June 8th - 17th, 2012
From: John Maddock (boost.regex_at_[hidden])
Date: 2012-06-06 04:21:06

>>> Tested the time taken to run through all the Boost.Math Bessel function 
>>> tests, with type double, real_concept (a simple wrapper around double 
>>> from Boost.Math's test suite) and mp_number<float_backend<double> > (a 
>>> trivial mp_number backend that wraps a floating point type):
>>>
>>> Time for double = 0.0157549 seconds
>>> Time for real_concept = 0.0166421 seconds
>>> Time for float_backend<double> = 0.0408635 seconds
>>> Time for float_backend<double> - no expression templates = 0.0253047 
>>> seconds
>
> Out of curiosity, what compiler/options? I know from similar experiments 
> that it can play a huge role, and only with the highest optimizations 
> (profile feedback and all) were the performances the same. In other cases 
> the abstraction penalty never disappeared.

It's VC++ 10 with /Ox.

As you can see the abstraction penalty for real_concept (which is about as 
simple a wrapper as you can get) just about but not quite disappears, but is 
still there for the more complex mp_number.

>> Seems like there may be some small opportunity for optimization of the 
>> comparison operators as well (i.e. not evaluating in terms of a compare() 
>> function).
>
> Is this related?
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51938

Yes, though I confess I wasn't expecting compiler to make the optimisation 
you suggest - I'm very impressed that clang can!!

> By the way, compare is required to return int, but sometimes it would be 
> more convenient if I could use long (for the same reason you picked int 
> and not an enum {-1,0,1}). Or even double. Do you do anything with the 
> result of compare that works better because it is an int? I might even 
> want to use a proxy.

Good point - I think the result of my_backend::compare can be any integer or 
enum type - it must be comparable with a literal zero (for the comparison 
ops), and convertible to int (for mp_number::compare).  But I think it would 
be a legitimate optimization for the return type to be the same type as a 
limb in the internal representation.

Cheers, John.