|Re: [eigen] Cost of a map operation|
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- To: eigen@xxxxxxxxxxxxxxxxxxx
- Subject: Re: [eigen] Cost of a map operation
- From: Gael Guennebaud <gael.guennebaud@xxxxxxxxx>
- Date: Wed, 2 Sep 2009 14:32:43 +0200
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perhaps here you don't want to copy it to an Eigen vector but directly
use *address as an Eigen vector. For that you simply declare:
and use AddressVector as any other Eigen vector, e.g.:
AddressVector *= 2;
that is the same as directly modifying address:
for(int i=0; bla; bla)
address[i] *= 2;
with all Eigen's goodies.
On Wed, Sep 2, 2009 at 2:16 PM, Benoit Jacob<jacob.benoit.1@xxxxxxxxx> wrote:
> This is as efficient as it could be: typically, this expands to
> nothing more than:
> for(int i = 0; i < cordNum; ++i)
> AddressVector[i] = address[i];
> and then 1) Eigen unrolls this if appropriate and 2) Eigen vectorizes
> this if possible.
> Basically, the Map object itself is just an expression, so its
> construction is cheap at least, and in practice the compiler is able
> to avoid actually creating it, everything gets "inlined".
> Tip: you could check the resulting assembly code for yourself, just
> put asm comments around this, asm("#hello"); , and use gcc -S to get
> the assembly...
> 2009/9/2 Rohit Garg <rpg.314@xxxxxxxxx>:
>> What is the cost of this map operation?
>> AddressVector_t AddressVector = Map<AddressVector_t,ForceAligned>(address)
>> where AddressVector_t itself is a typedef for
>> typedef Matrix<int,cordNum,1> AddressVector_t;
>> cordNum is a compile time constant.
>> This map operation occurs in inner loop of my application, so I want
>> to know, how costly is it gonna be? I mean with what use in mind was
>> it written for?
>> Rohit Garg
>> Senior Undergraduate
>> Department of Physics
>> Indian Institute of Technology