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gamefish

an increase of 0.5ms per frame

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when I rewrite a simple demo using object oriented method, I find the fps droped from 220 to 200, and the average frame time increased 0.5ms. Are member variables really slower than gloables?

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Original post by mrmrcoleman
This belongs in General Programming dude!


i don't know if this due to DX or Class.

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Original post by gamefish
Are member variables really slower than gloables?


Yes, I think. The compiler has to insert a little arithmetic into the code when you access a member variable of a class. How much I'm uncertain of.

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Original post by smr
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Original post by gamefish
Are member variables really slower than gloables?


Yes, I think. The compiler has to insert a little arithmetic into the code when you access a member variable of a class. How much I'm uncertain of.


... and you are right to be uncertain. A variable is a variable. Wether it is in a class or not is not a problem for the compiler. If the variable is on the stack (for example, a local variable), it address is [(stack pointer)+n]. If the variable is an object and you need to access to a member of this object, its adress is [(stack pointer)+n+p]. The main point here is that both n and p are constants. Thus, it finaly becomes [sp+K], where k is a constant and sp is stored in a register.

The same kind of calculus is done if the variable is in the heap.

Now, you can try to compare a heap variable and a stack variable. This comparison is very difficult because you have to take a lot of factors in account: variable locality (it is faster to access to two contiguous memory zone than to two distant ones, because the processor has to flush its data cache and refill it), variable size and alignement, and so on.

smr question is a bit strange: member variables and globals are not related - after all, if a global variable is a class instance, its member variables are also globals.

HTH,

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