Re: [GD-Windows] memory alignment vs passing by value

[Dan T. <da...@ar...>]

Hmm now I'm confused. I'm going to make some statements and see if I 
can't show my ignorance in such a way that it can be corrected.

It seems weird that aliasing would be a problem - since everything is in 
4 bytes chunks. From what I understood about aliasing, that only 
mattered if you broke dword alignment, e.g.

unsigned char* pData = some_aligned_junk;
return *(pData + 1);   // hit 1 byte off the alignment produced by the 
compiler.

However everything he is dealing with is already in dwords, effectively, 
so this shouldn't be a problem...

Could it be that the compiler recognizes the dprod as const, and as such 
can inline it directly without creating the parameter as a new instance? 
i.e. the code effectively just drops in without any copy constructors 
being called at all, whereas with a reference, it still has to chase the 
pointers, making it a bit slower?

Does this make sense?

-Dan



Jon Watte wrote:

>>I measured the time of computing 100 million bilinear interpolation using
>>dot products, and changing the dprod operation to accept a const reference
>>instead of the const value makes the test run an order of magnitude
>>    
>>
>slower!!
>
>Is this Evil Aliasing, Part II ? You could find out by disassembling
>both versions and comparing. If it's re-loading through the reference
>pointer all the time, then it's probably aliasing related. References
>are really pointers, so they can introduce (false) aliasing hazards.
>
>Cheers,
>
>			/ h+
>
>
>
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