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cozzie

loop counters as class members? (memory usage)

11 posts in this topic

Hi,

 

I'm currently 'implementing' correct usage of const function parameters and class member functions.

During this I'm running into the following:

 

- class 'MyClass' has several (unsigned) int variables, which are used throughout the member functions for for loops, counter etc.

- when I try to make one of these member functions 'const', which is logical looking at the functions itself, but this doesn't work, since the used counter variables are members of the class

 

The solution seems very simple, create local (unsigned) int variables for my loops within the member functions.

My question:

 

- would it be a bad idea, memory/ performance wise, to do this?

For example, my rendermesh and rendersubmesh functions have one or more for loops, using a class member for the for loop.

This would mean I 'make' a new integer in the local functions, for each submesh/ mesh I render in every frame.

 

Any input is really appreciated.

 

Example:

void CD3dmesh::RenderMesh(LPDIRECT3DDEVICE9 pD3ddev, D3DPRIMITIVETYPE pMethod) const
{
	for(sub=0;sub<mSubMeshSize;++sub)
	{
		pD3ddev->DrawIndexedPrimitive(pMethod, 
		 	  	  					  0,
								      mSubMeshTable[sub].VertexStart, 
								      mSubMeshTable[sub].VertexCount, 
								      mSubMeshTable[sub].FaceStart*3,
								      mSubMeshTable[sub].FaceCount); 
	}
}

Where 'sub' now is a class member. I could simply create a int sub (or something) locally in the function.

Maybe a stupid question, but will the memory of the int will be freed everytime the function finishes?

(would it be a good idea performance wise etc.).

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Thanks all, my conclusions:

- making these variables mutable should work, but is tricky (as it is today akready), risk on faulty initializiation etc.
- since they're just simple ints and unsigned ints, no complex / higher level algorithms etc, I will create them all locally within the member functions and cleanup the ones in the class(es)

This also fits perfectly within the given principles.
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Also, remember that with member variables you are effectively doing 'this->var' which, unless the compiler is able to prove can't be affected by other code, it is forced to produce sub-optimal code where it is likely to read-modify-write the value at the start of every loop iteration to ensure any other code looking at it gets the up to date version.

Same applies to end conditions, where accessing a member is going to, more than likely, cause reload from the 'end' variable.

This is why I prefer to structure my loops 'for(unsigned int idx = 0, end = m_count; idx < end; ++idx)' to give the compiler the best chance of removing redundant read/writes and get things into registers.

(member variables for loop counters would also invalidate cache lines where not required causing more memory traffic and core synchronization to happen and in world where memory bandwidth and latency is the biggest issue going this is bad voodoo)
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See http://stackoverflow.com/questions/161053/c-which-is-faster-stack-allocation-or-heap-allocation

 

 

Stack allocation is much faster since all it really does is move the stack pointer.

 

Since loop counters declared locally are allocated on the stack, this is your worst case scenario in terms of performance - a single pointer move (your best case will be just using a register, which your compiler may do automatically for you, or you may explicitly request with the "register" keyword).

 

For memory usage, and leaving aside the fact that you're getting dangerously into micro-optimization territory here, the stack will likewise be more efficient.  If you declare a loop counter as a class member, then the memory used for that loop counter will always be used, even when no loops are running.  Contrast with:

for (int i = 0; i < whatever; ++i)
{
}

In this case the memory is only used within the scope of the loop itself; allocation is (worst-case) just moving the stack pointer, freeing is also (worst case) just moving the stack pointer.  Best case your compiler is just going to take a register for it and just not use that register for anything else within the loop, which means it's effectively totally free: no allocation, no memory usage, fast.

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Thanks, clear.
My thought on optimization was initially that some loops are used each frame and therefor the 'int' counters of the loop would take up that much memory. Not thinking about where the memory was and how quick it's gone, also not thinking about the constant needed communication and synchronization with the class members.

Also for readability I'll go for initializing the counter in the for loop alway, like mhagain showed in the example.
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