# DX12 Unbind UAV, then write to it - DX 11 vs 12

## Recommended Posts

Hello!

I can see that when there's a write to UAVs in CS or PS, and I bind a null ID3D11UnorderedAccessView into a used UAV slot, the GPU won't hang and the writes are silently dropped. I hope I amn't dreaming.

With DX12, I can't seem to emulate this. I reckon it's impossible. The shader just reads the descriptor of the UAV (from a register/offset based on the root signature layout) and does an "image_store" at some offset from the base address. If it's unmapped, bang, we're dead. I tried zeroing out that GPU visible UAV's range in the table, same result. Such an all-zero UAV descriptor doesn't seem very legit. That's expected.

Am I right? How does DX11 do it that it survives this? Does it silently patch the shader or what? Thanks, .P

Edited by pcmaster

##### Share on other sites

How did you zero it out? Did you call ZeroMemory (or similar) on the CPU descriptor handle? That's... not the right way to do it. What you're looking for is a null view. You can call CreateUnorderedAccessView with a null resource, but you must pass a valid view desc. That'll set up the descriptor such that reads return 0 (or last-written data, depending on architecture) and writes are dropped.

##### Share on other sites

Hi, SoL! I was looking exactly for this, is it actually documented anywhere? I'm on an unnamed architecture where I could do a memset... as I say it didn't seem very legit. I'm just trying what you propose.

##### Share on other sites

It does work!

D3D12_UNORDERED_ACCESS_VIEW_DESC dummyUavDesc = {};
dummyUavDesc.ViewDimension = D3D12_UAV_DIMENSION_TEXTURE3D;
dummyUavDesc.Texture3D.FirstWSlice = 0;
dummyUavDesc.Texture3D.MipSlice = 0;
dummyUavDesc.Texture3D.WSize = 2048;
dummyUavDesc.Format = DXGI_FORMAT_R8G8B8A8_SNORM;
pD3D12Device->CreateUnorderedAccessView(nullptr, nullptr, &dummyUavDesc, cpuHandle);

CreateUnorderedAccessView writes all zeroes to the cpuHandle designated memory. CopyDescriptors() copies the zeroes correctly to the contiguous GPU visible descriptor table and the GPU recognises this. All cool. Thank you SoL!

Edited by pcmaster

##### Share on other sites
On 11/28/2017 at 8:02 AM, pcmaster said:

Hi, SoL! I was looking exactly for this, is it actually documented anywhere? I'm on an unnamed architecture where I could do a memset... as I say it didn't seem very legit. I'm just trying what you propose.

It's mentioned in the docs for CreateUnorderedAccessView:

At least one of pResource or pDesc must be provided. A null pResource is used to initialize a null descriptor, which guarantees D3D11-like null binding behavior (reading 0s, writes are discarded), but must have a valid pDesc in order to determine the descriptor type.

It's also mentioned here in the programming guide.

One thing to watch out for is there's no way to have a NULL descriptor for a UAV or SRV  that's bound as a root SRV/UAV parameter. In this case there's really no descriptor (you're just passing a GPU pointer to the buffer data), so you forego any bounds checking on reads or writes. Just like raw pointer access on the CPU, reading or writing out-of-bounds will result in undefined behavior.

##### Share on other sites

Thank you MJP for pointing that out. Our UAVs are in a table, fortunately.

## Create an account

Register a new account

• 9
• 16
• 9
• 13
• 41
• ### Similar Content

• By chiffre
Introduction:
In general my questions pertain to the differences between floating- and fixed-point data. Additionally I would like to understand when it can be advantageous to prefer fixed-point representation over floating-point representation in the context of vertex data and how the hardware deals with the different data-types. I believe I should be able to reduce the amount of data (bytes) necessary per vertex by choosing the most opportune representations for my vertex attributes. Thanks ahead of time if you, the reader, are considering the effort of reading this and helping me.
I found an old topic that shows this is possible in principal, but I am not sure I understand what the pitfalls are when using fixed-point representation and whether there are any hardware-based performance advantages/disadvantages.
(TLDR at bottom)
The Actual Post:
To my understanding HLSL/D3D11 offers not just the traditional floating point model in half-,single-, and double-precision, but also the fixed-point model in form of signed/unsigned normalized integers in 8-,10-,16-,24-, and 32-bit variants. Both models offer a finite sequence of "grid-points". The obvious difference between the two models is that the fixed-point model offers a constant spacing between values in the normalized range of [0,1] or [-1,1], while the floating point model allows for smaller "deltas" as you get closer to 0, and larger "deltas" the further you are away from 0.
To add some context, let me define a struct as an example:
struct VertexData { float[3] position; //3x32-bits float[2] texCoord; //2x32-bits float[3] normals; //3x32-bits } //Total of 32 bytes Every vertex gets a position, a coordinate on my texture, and a normal to do some light calculations. In this case we have 8x32=256bits per vertex. Since the texture coordinates lie in the interval [0,1] and the normal vector components are in the interval [-1,1] it would seem useful to use normalized representation as suggested in the topic linked at the top of the post. The texture coordinates might as well be represented in a fixed-point model, because it seems most useful to be able to sample the texture in a uniform manner, as the pixels don't get any "denser" as we get closer to 0. In other words the "delta" does not need to become any smaller as the texture coordinates approach (0,0). A similar argument can be made for the normal-vector, as a normal vector should be normalized anyway, and we want as many points as possible on the sphere around (0,0,0) with a radius of 1, and we don't care about precision around the origin. Even if we have large textures such as 4k by 4k (or the maximum allowed by D3D11, 16k by 16k) we only need as many grid-points on one axis, as there are pixels on one axis. An unsigned normalized 14 bit integer would be ideal, but because it is both unsupported and impractical, we will stick to an unsigned normalized 16 bit integer. The same type should take care of the normal vector coordinates, and might even be a bit overkill.
struct VertexData { float[3] position; //3x32-bits uint16_t[2] texCoord; //2x16bits uint16_t[3] normals; //3x16bits } //Total of 22 bytes Seems like a good start, and we might even be able to take it further, but before we pursue that path, here is my first question: can the GPU even work with the data in this format, or is all I have accomplished minimizing CPU-side RAM usage? Does the GPU have to convert the texture coordinates back to a floating-point model when I hand them over to the sampler in my pixel shader? I have looked up the data types for HLSL and I am not sure I even comprehend how to declare the vertex input type in HLSL. Would the following work?
struct VertexInputType { float3 pos; //this one is obvious unorm half2 tex; //half corresponds to a 16-bit float, so I assume this is wrong, but this the only 16-bit type I found on the linked MSDN site snorm half3 normal; //same as above } I assume this is possible somehow, as I have found input element formats such as: DXGI_FORMAT_R16G16B16A16_SNORM and DXGI_FORMAT_R16G16B16A16_UNORM (also available with a different number of components, as well as different component lengths). I might have to avoid 3-component vectors because there is no 3-component 16-bit input element format, but that is the least of my worries. The next question would be: what happens with my normals if I try to do lighting calculations with them in such a normalized-fixed-point format? Is there no issue as long as I take care not to mix floating- and fixed-point data? Or would that work as well? In general this gives rise to the question: how does the GPU handle fixed-point arithmetic? Is it the same as integer-arithmetic, and/or is it faster/slower than floating-point arithmetic?
Assuming that we still have a valid and useful VertexData format, how far could I take this while remaining on the sensible side of what could be called optimization? Theoretically I could use the an input element format such as DXGI_FORMAT_R10G10B10A2_UNORM to pack my normal coordinates into a 10-bit fixed-point format, and my verticies (in object space) might even be representable in a 16-bit unsigned normalized fixed-point format. That way I could end up with something like the following struct:
struct VertexData { uint16_t[3] pos; //3x16bits uint16_t[2] texCoord; //2x16bits uint32_t packedNormals; //10+10+10+2bits } //Total of 14 bytes Could I use a vertex structure like this without too much performance-loss on the GPU-side? If the GPU has to execute some sort of unpacking algorithm in the background I might as well let it be. In the end I have a functioning deferred renderer, but I would like to reduce the memory footprint of the huge amount of vertecies involved in rendering my landscape.
TLDR: I have a lot of vertices that I need to render and I want to reduce the RAM-usage without introducing crazy compression/decompression algorithms to the CPU or GPU. I am hoping to find a solution by involving fixed-point data-types, but I am not exactly sure how how that would work.

• Hi.
I wanted to experiment D3D12 development and decided to run some tutorials: Microsoft DirectX-Graphics-Samples, Braynzar Soft, 3dgep...Whatever sample I run, I've got the same crash.
All the initialization process is going well, no error, return codes ok, but as soon as the Present method is invoked on the swap chain, I'm encountering a crash with the following call stack:
The crash is an access violation to a null pointer ( with an offset of 0x80 )
I'm working on a notebook, a toshiba Qosmio x870 with two gpu's: an integrated Intel HD 4000 and a dedicated NVIDIA GTX 670M ( Fermi based ). The HD 4000 is DX11 only and as far as I understand the GTX 670M is DX12 with a feature level 11_0.
I checked that the good adapter was chosen by the sample, and when the D3D12 device is asked in the sample with a 11_0 FL, it is created with no problem. Same for all the required interfaces ( swap chain, command queue...).
I tried a lot of things to solve the problem or get some info, like forcing the notebook to always use the NVIDIA gpu, disabling the debug layer, asking for a different feature level ( by the way 11_0 is the only one that allows me to create the device, any other FL will fail at device creation )...
I have the latest NVIDIA drivers ( 391.35 ), the latest Windows 10 sdk ( 10.0.17134.0 ) and I'm working under
Visual Studio 2017 Community.
Thanks to anybody who can help me find the problem...
• By cozzie
Hi all,
I was wondering it it matters in which order you draw 2D and 3D items, looking at the BeginDraw/EndDraw calls on a D2D rendertarget.
The order in which you do the actual draw calls is clear, 3D first then 2D, means the 2D (DrawText in this case) is in front of the 3D scene.
The question is mainly about when to call the BeginDraw and EndDraw.
Note that I'm drawing D2D stuff through a DXGI surface linked to the 3D RT.
Option 1:
A - Begin frame, clear D3D RT
B - Draw 3D
C - BeginDraw D2D RT
D - Draw 2D
E - EndDraw D2D RT
F - Present
Option 2:
A - Begin frame, clear D3D RT + BeginDraw D2D RT
B - Draw 3D
C - Draw 2D
D - EndDraw D2D RT
E- Present
Would there be a difference (performance/issue?) in using option 2? (versus 1)
Any input is appreciated.

• Do you know any papers that cover custom data structures like lists or binary trees implemented in hlsl without CUDA that work perfectly fine no matter how many threads try to use them at any given time?
• By cozzie
Hi all,
Last week I noticed that when I run my test application(s) in Renderdoc, it crashes when it enable my code that uses D2D/DirectWrite. In Visual Studio no issues occur (debug or release), but when I run the same executable in Renderdoc, it crashes somehow (assert of D2D rendertarget or without any information). Before I spend hours on debugging/ figuring it out, does someone have experience with this symptom and/or know if Renderdoc has known issues with D2D? (if so, that would be bad news for debugging my application in the future );
I can also post some more information on what happens, code and which code commented out, eliminates the problems (when running in RenderDoc).
Any input is appreciated.