States and Barriers of Aliasing Render Targets

Tue
22
Dec 2020

In my recent post “Initializing DX12 Textures After Allocation and Aliasing” I said that when you use Direct3D 12, you have a render-target or depth-stencil texture “B” aliasing with some other resources “A” in memory and want to use it, you need to initialize it properly every time: 1. issue a barrier of type D3D12_RESOURCE_BARRIER_TYPE_ALIASING, 2. fully initialize its content using Clear, Discard, or Copy, so that garbage data left from previous usage of the memory as overwritten.

That is actually not the full story. There is a third thing that you need to take care of: a transition barrier for this resource. This is because functions: DiscardResource, ClearRenderTargetView, ClearDepthStencilView require a texture to be in a correct state – D3D12_RESOURCE_STATE_RENDER_TARGET or D3D12_RESOURCE_STATE_DEPTH_WRITE. A question arises here: How does resource state tracking interact with memory aliasing?

If you look at error and warning messages from D3D Debug Layer or PIX, you notice that states are tracked per resource, no matter if they alias in memory. This gives us some clue of how we should handle it correctly to tame the Debug Layer and make things formally correct, but it is still not that obvious where to put the barrier. Let's assume the last usage of our texture B is D3D12_RESOURCE_STATE_PIXEL_SHADER_RESOURCE – the texture is used for sampling. Where should we put the barrier to transition it to D3D12_RESOURCE_STATE_RENDER_TARGET, required to do the initializing Discard or Clear after aliasing? I can see 3 solutions here:

1. None at all – skip this barrier, just do the Discard/Clear as if the texture was in the correct RT/DS state. It obviously generates Debug Layer error, but it seems to work fine. My thinking about why this may be fine is: If the Discard/Clear requires the texture to be in the correct RT/DS state and it completely overwrites its content, then it shouldn’t care what initial state the texture is in. Its data are garbage anyway. It will most probably leave the texture initialized properly as for RT/DS state, no matter what.

2. Between the aliasing barrier and Discard/Clear. This would please the Debug Layer, but I have some doubts whether it is correct or necessary. It may not be correct because if the barrier does some transformations of the data, it would transform garbage data left after aliasing, which could be an undefined behavior leading to some persisting corruptions or even to GPU crash. It may not be necessary because we are going to re-initialize the whole content of the texture in a moment anyway with Discard/Clear.

3. After finished working with the texture in the previous frame. The idea is to leave each RT/DS texture that can alias with some other resource in a state that makes it ready for the initializing Discard/Clear next time we want to grab it from the common memory. This will work and will be formally correct, but it also sounds like a well-known and sub-optimal idea of reverting textures to some “base state” after each use. Additional problem appears when your last usage of the texture occurs on the compute queue, because there you cannot transition it to RENDER_TARGET or DEPTH_WRITE state.

Conclusion: While this is a tricky question that has no one simple answer, I would recommend to use [3] if you want to be 100% formally correct or [1] if you want maximum efficiency.

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