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Pufixas

Variable-length argument list memory question.

3 posts in this topic

Hello.

 

I will try to make this question quick. So there it goes.

 

I want to know if calling a function with Variable-length argument list will allocate new memory? (In java)

 

I mean would it be better to do this:

    public void appendVertexPositionData(float ... vertexPositionData){
        // bla bla bla
    }

    ........

    appendVertexPositionData(1.0f,2.0f,3.0f,4.0f);

Than this?:

public void appendVertexPositionData(float vertexPositionData[]){
        // bla bla bla
    }

    ........

    appendVertexPositionData(new float[] {1.0f,2.0f,3.0f,4.0f});

Or there is not difference?

If there is no difference, then I guess I will have to figure a different way to handle this, because I really don't want to keep allocating new memory in main loop.

 

Thanks in advance.

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Generally Java is pass by value so what I think is happening at run time is that you are pushing a copy of the floats onto the stack and then telling appendVertexPositionData how many via an offset or counter when you use the '...' var args.  The second one you are passing a copy of a pointer to an array onto the stack.  So the first one will duplicate the floats when the method is called, the second will duplicate a pointer.  Memory wise the var args '...' will take up more space.

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Generally Java is pass by value so what I think is happening at run time is that you are pushing a copy of the floats onto the stack and then telling appendVertexPositionData how many via an offset or counter when you use the '...' var args.  The second one you are passing a copy of a pointer to an array onto the stack.  So the first one will duplicate the floats when the method is called, the second will duplicate a pointer.  Memory wise the var args '...' will take up more space.

 

actually, IIRC, the "javac" compiler essentially converts the former case into the latter case.

IOW: it converts the variable arguments list into an array and then passes the array when the method is called.

 

it is then up to the JVM to realize that the lifetime of this temporary array is limited to the method call.

the JVM may allocate the array on the stack (rather than the heap, *1), but this detail is internal to the JVM (generally invisible from the POV of the bytecode or 'javac', which generally goes on the working assumption that all memory is heap memory).

 

this differs significantly from C or C++, which do in-fact directly pass variable argument lists on the stack.

 

 

*1:

IIRC, at least earlier on, it worked something like this:

temporary objects, which are tied to specific method call-stack frames, and will be destroyed when their associated method returns;

young generation, which holds objects which have recently been created;

old generation, which holds objects which have survived a prior GC pass.

 

temporary objects will naturally be short-lived (limited to their original call scope), but will be automatically promoted to 'global' objects (in the young generation) if a reference to them is assigned into an object outside their original scope.

 

generally the GC will focus collection on young generation objects, and when each young object survives the GC, it is promoted to an old-generation object, which will be scanned more rarely.

 

IIRC, later on direct stack allocation was added for objects which can be "proven" not to outlive their original scope.

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Thanks for the answers. But I got my answer by running android DMMS, and checking where memory allocations are occurring. And it showed me that using variable-length argument lists DOES allocate memory. But I changed my code entirely, so I don't even need arrays to pass my vertex data. I just pass it one by one.

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