Re: [boost] [GGL] [geometry] Inexplicable speed benefit when using Visual C++ 2010

Subject: Re: [boost] [GGL] [geometry] Inexplicable speed benefit when using Visual C++ 2010
From: Arash Partow (arash_at_[hidden])
Date: 2010-04-20 20:50:23

On 16/04/2010 11:30 PM, Christian Buchner wrote:
> Hi everybody,
>
> my first attempt to post to this list bounced, so I am trying again.
>
> My employer is an early adopter of the Boost Generic Geometry Library
> [GGL] in an engineering application related to mobile radio
> communicatons. We use it to estimate and optimize the coverage of 4G
> radio networks. Our code uses a lot of multi-polygon unions to
> estimate the amount of ground covered (and not covered) by radio beams
> and iteratively improves the antenna parameters.
>
> We've been compiling and shipping our application with Visual C++ 2008 so far.
>
> We found that GCC 4.4 on Linux was about 100% faster than Visual C++
> 2008 on Linux without modifying the code. This bothered us quite a bit
> as both compilers were allowed to use full optimization. We found that
> by optimizing (globaly overloading) the new and delete operators to
> re-use allocated memory fragments on Windows we were able to get
> nearly 50% speed benefit, so we attributed much of the performance
> difference to sub-optimal memory heap management of Visual C++ 2008.
>
> Then we tried recompiling the project with Visual C++ 2010 Ultimate
> Release Candidate (RC). The speed gain of the algorithm was 900% (not
> joking) and the results still appear to be correct. Now this is
> surreal and no one here in the office has found a reasonable
> explanation yet without going into the metaphysical domain.
>
> Would anyone with knowledge of compiler and runtime internals be able
> to make an educated guess as to how such a speed gain of factor 10 is
> possible? Is anyone else seeing similar speedups in boost or in the
> geometry library when compiling with Visual C++ 2010 RC (HINT: it's a
> free download, so anyone can try it out until end of June 2010).
>

Nothing new here, with msvc10 GPC, which is entirely C (no templates), sees a roughly 6x-7x on fastcode and favour-speed settings over msvc9, with PGO it gets to about 8x-9x and on intel v11 with PGO et al you're looking at around 11x-12x increase over intel v10 or msvc9. The point here is that the increase is mainly centered around new memory allocation mechanisms (as described by Stephan) in the msvc10 backend - not necessarily anything special MS has done wrt c++ specifically. (btw the polygons used range from simple 4-5 corner convex to 100k+ corners concave-disjoint with holes and concentric islands, all operations union, diff, xor). On a side note with intel v11 if the loop unrolling is set correctly for the target processor and if sse4.1 is available, it peaks at around 13x-14x - and this is with a code base that was last touched nearly 8 years ago.