Re: [Boost-users] Efficient "[][]" operator overloading (encapsulating a MultiArray)

Subject: Re: [Boost-users] Efficient "[][]" operator overloading (encapsulating a MultiArray)
From: Andreas Schäfer (gentryx_at_[hidden])
Date: 2013-04-09 15:53:25

Jiří-

On 16:36 Tue 09 Apr     , Jiří Vyskočil wrote:
> I am unable to write a minimal test case - if I compare using 
> multi_array directly vs. using this GridArray class in a simple test 
> program (compiled with -O2), the performance is the same. But the real 
> code shows about 20% slowdown - it's a numerical simulaiton, I create 
> several arrays at the beginning and then do computations with them. The 
> rest of the code hasn't been touched - I only chanded the constructors 
> for the arrays and replaced a header which contained some typedefs with 
> a header containing this class.
> 
> Do you have any idea what might be going on? Is there a better way to 
> write an interface for successive [] operators?

I suspect that the compiler isn't able to optimize certain accesses in
the GridArray case. Could you give us some more details on the
numerical kernel you're running? What kind of operation do you run on
the members? Is it some kind of stencil code where you update each
member depending on its neighbors?

I'm asking this as a compiler will typically try to avoid the
potentially costly repetitive address computations and will cache the
results in registers. As compilers are easily confused (e.g. via
possible aliasing), and registers are scarce, even seemingly
inconspicuous code changes may lead to performance degradations.

Best
-Andreas

-- 
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Andreas Schäfer
HPC and Grid Computing
Chair of Computer Science 3
Friedrich-Alexander-Universität Erlangen-Nürnberg, Germany
[phone]
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