Question

Alright, so I’m currently working on a game and ran into a memory issue after refactoring some of the code today.

It uses a component based design and I was modifying how the components were allocated and passed off to entities. Originally some components were allocated as member variables within the entities but now I want to have them allocated elsewhere and passed off to the entity via pointer.

You can see how I implemented this below with sample code from my project. I essentially iterate through all the entities and allocate the components for them. The problem is I’m hitting a access violation at the first line of “starting up” the “instanceObject” on the 6th iteration and have no idea why. Using the debugger, it doesn’t look like any of the variables point to a invalid address.

Here is what I’m doing to create the entities and components.

for (unsigned int i = 0; i < 512; ++i) {
    InstanceObject* _pInstanceObject = new InstanceObject;

    // Initialize temporary variables
    XMFLOAT3 _position, _rotation;
    float _angle = (i / 512.0f) * (2.0f * XM_PI),
          _scale = (float)pResourceManager->numberGenerator.GetInt(50, 5);

    _position.x = 100000.0f * cos(_angle) + pResourceManager->numberGenerator.GetInt(50000, -25000);
    _position.y =(float) pResourceManager->numberGenerator.GetInt(50000, -25000);
    _position.z = 100000.0f * sin(_angle) + pResourceManager->numberGenerator.GetInt(50000, -25000);

    _rotation.x = (XM_PI * 2) * (pResourceManager->numberGenerator.GetInt(100, 0) / 100.0f);
    _rotation.y = (XM_PI * 2) * (pResourceManager->numberGenerator.GetInt(100, 0) / 100.0f);
    _rotation.z = (XM_PI * 2) * (pResourceManager->numberGenerator.GetInt(100, 0) / 100.0f);

    // Set component's state using the temporary variables.
    _pInstanceObject->StartUp(&_position,
        &_rotation,
        &XMFLOAT3(_scale, _scale, _scale),
        &XMFLOAT3(0.0f, 0.0f, 1.0f),
        &XMFLOAT3(1.0f, 0.0f, 0.0f),
        &XMFLOAT3(0.0f, 1.0f, 0.0f)
        );

    // Hand pointer of the component to entity.
    // Entity will handle deallocating component
}

And here is the relevant code from the component.

class InstanceObject {
private:
    XMVECTOR anteriorAxis,
         lateralAxis,
         normalAxis,
         position,
         rotationQuaternion,
         scale;

    XMMATRIX translationMatrix,
         rotationMatrix,
         scaleMatrix;
    void SetAnteriorAxis(const XMFLOAT3 *_anteriorAxis) { anteriorAxis = XMLoadFloat3(_anteriorAxis); }
    void SetLateralAxis(const XMFLOAT3 *_lateralAxis)   { lateralAxis = XMLoadFloat3(_lateralAxis); }
    void SetNormalAxis(const XMFLOAT3 *_normalAxis)     { normalAxis = XMLoadFloat3(_normalAxis); }
public:
    InstanceObject(void)  { }
    InstanceObject(const InstanceObject& _object) : anteriorAxis(_object.anteriorAxis), lateralAxis(_object.lateralAxis),
        normalAxis(_object.normalAxis), position(_object.position), rotationQuaternion(_object.rotationQuaternion), scale(_object.scale),
        translationMatrix(_object.translationMatrix), rotationMatrix(_object.rotationMatrix), scaleMatrix(_object.scaleMatrix) {}
    ~InstanceObject(void) { }

    bool StartUp(const XMFLOAT3 *_position, const XMFLOAT3 *_rotation, const XMFLOAT3 *_scale,
        const XMFLOAT3 *_lookAxis, const XMFLOAT3 *_strafeAxis, const XMFLOAT3 *_upAxis);

    void SetPosition(const XMFLOAT3* _position) { position = XMLoadFloat3(_position); }
    void SetRotationQuaternion(const XMFLOAT3 *_rotation) { rotationQuaternion = XMQuaternionRotationRollPitchYaw(_rotation->x, _rotation->y, _rotation->z); }
    void SetScale(const XMFLOAT3 *_scale) { scale = XMLoadFloat3(_scale); }
}

bool InstanceObject::StartUp(const XMFLOAT3 *_position, const  XMFLOAT3 *_rotation, const XMFLOAT3 *_scale,
        const XMFLOAT3 *_lookAxis, const XMFLOAT3 *_strafeAxis, const XMFLOAT3 *_upAxis) {
    SetPosition(_position);
    SetRotationQuaternion(_rotation);
    SetScale(_scale);   
    SetAnteriorAxis(_lookAxis);
    SetLateralAxis(_strafeAxis);
    SetNormalAxis(_upAxis);

    return true;
}

Any idea of what could be causing this behavior and how should I fix it?

Answer 1

I believe the issue is that the XMVECTOR in your InstanceObject class need to be 16 byte aligned, and the new operator won’t guarantee this for you. You can add a quick check into your code to confirm – check whether the InstanceObject pointer & 0xF is non zero in the iteration in which it crashes.

If that is the case you can write a custom allocator that guarantees the right alignment and use placement new.

It seems to be a fairly common problem with XMVECTOR when used as a member (here’s an existing Connect bug report on this issue, there are a few web pages if you search).

If you just want to fix it quickly to let you get on with other things, you can add a static operator new and delete to your class declaration implemented something like the following snippet:

void* InstanceObject::operator new( size_t size )
{
    // _aligned_malloc is a Microsoft specific method (include malloc.h) but its
    // straightforward to implement if you want to be portable by over-allocating
    // and adjusting the address
    void *result = _aligned_malloc( size, 16 );
    if( result )
        return result;

    throw std::bad_alloc(); 
}

void InstanceObject::operator delete( void* p )
{
    if( p ) _aligned_free(p);
}

If the InstanceObject share a common lifetime you could replace the use of _aligned_malloc with your own aligned arena allocator and make the delete a no-op to improve efficiency.