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Reitano

DX11 Frame allocator of constant buffers

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Hi,

I am writing a linear allocator of per-frame constants using the DirectX 11.1 API. My plan is to replace the traditional constant allocation strategy, where most of the work is done by the driver behind my back, with a manual one inspired by the DirectX 12 and Vulkan APIs.
In brief, the allocator maintains a list of 64K pages, each page owns a constant buffer managed as a ring buffer. Each page has a history of the N previous frames. At the beginning of a new frame, the allocator retires the frames that have been processed by the GPU and frees up the corresponding space in each page. I use DirectX 11 queries for detecting when a frame is complete and the ID3D11DeviceContext1::VS/PSSetConstantBuffers1 methods for binding constant buffers with an offset.
The new allocator appears to be working but I am not 100% confident it is actually correct. In particular:
1) it relies on queries which I am not too familiar with. Are they 100% reliable ?
2) it maps/unmaps the constant buffer of each page at the beginning of a new frame and then writes the mapped memory as the frame is built. In pseudo code:
BeginFrame:
    page.data = device.Map(page.buffer)
    device.Unmap(page.buffer)
RenderFrame
    Alloc(size, initData)
        ...
        memcpy(page.data + page.start, initData, size)
    Alloc(size, initData)
        ...
        memcpy(page.data + page.start, initData, size)
(Note: calling Unmap at the end of a frame prevents binding the mapped constant buffers and triggers an error in the debug layer)
Is this valid ? 
3) I don't fully understand how many frames I should keep in the history. My intuition says it should be equal to the maximum latency reported by IDXGIDevice1::GetMaximumFrameLatency, which is 3 on my machine. But, this value works fine in an unit test while on a more complex demo I need to manually set it to 5, otherwise the allocator starts overwriting previous frames that have not completed yet. Shouldn't the swap chain Present method block the CPU in this case ?
4) Should I expect this approach to be more efficient than the one managed by the driver ? I don't have meaningful profile data yet.

Is anybody familiar with the approach described above and can answer my questions and discuss the pros and cons of this technique based on his experience ? 
For reference, I've uploaded the (WIP) allocator code at https://paste.ofcode.org/Bq98ujP6zaAuKyjv4X7HSv.  Feel free to adapt it in your engine and please let me know if you spot any mistakes :)

Thanks

Stefano Lanza
 

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Sorry I haven't had time to actually read your code, so quick answers:

1 hour ago, Reitano said:

1) it relies on queries which I am not too familiar with. Are they 100% reliable ?

If you're using them correctly, yes. Event queries are perfect for telling whether the GPU has completed a batch of commands yet or not.

1 hour ago, Reitano said:

    page.data = device.Map(page.buffer)
    device.Unmap(page.buffer)
...
   memcpy(page.data

That's undefined behaviour. The 'page.data' pointer is only valid in-between the call to Map and the call to Unmap. Writing to it after the call to Unmap is not allowed.

You have to map, write in a lot of constants, unmap, then bind and draw.

Yes, this sucks. In GL/D3D12/Vulkan you can do a "persistent map", where there is no Unmap call at all. In D3D11 I don't think persistent mapping is possible, so you've got to jump through hoops to keep the binding API happy. 

I'm not sure if you should restructure things so that you can upload all of your constants long before you start executing any draws, or if you should simply perform a lot more map/unmap calls. I haven't implemented this new cbuffer updating method in D3D11.1 yet because the traditional D3D11 methods have been performing fine for me so far :|

1 hour ago, Reitano said:

I don't fully understand how many frames I should keep in the history.

If you're using queries to track the GPU's progress, then the same number of frames that you're tracking with queries... but I guess that's a circular answer ;)

 To keep a GPU busy you typically need one frame's worth of completed commands queued up while you're working on the next frame's commands, so at least 2. If you want to be even more sure about keeping things smooth, go for 3. Any more than that and you're just adding excessive latency to your game IMHO.

1 hour ago, Reitano said:

the allocator starts overwriting previous frames that have not completed yet

You should use the queries to block the CPU yourself if the CPU is attempting to map a buffer that you know is still potentially in use by the GPU.

Instead of these queries being owned by the buffer system, I like to have the core rendering device own the event queries and use them to give a guarantee about how far behind the CPU the GPU can possibly be. e.g. if your core rendering device promises "the GPU will never be more than two frames behind the CPU", then other systems don't need their own queries -- they can simply make assumptions like "I'm currently preparing frame #8, which means the GPU is either working on frame #8, #7 or #6, so I can safely overwrite data from frame #5 without checking any queries".

[edit] Another lazy approach is to use write-discard instead of write-no-overwrite. This asks the driver to manage garbage collection of old data for you, and you don't have to think about where the GPU is up to... You could implement that for comparison and see how it differs in speed.

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3 hours ago, Reitano said:

3) I don't fully understand how many frames I should keep in the history. My intuition says it should be equal to the maximum latency reported by IDXGIDevice1::GetMaximumFrameLatency, which is 3 on my machine. But, this value works fine in an unit test while on a more complex demo I need to manually set it to 5, otherwise the allocator starts overwriting previous frames that have not completed yet. Shouldn't the swap chain Present method block the CPU in this case ?
4) Should I expect this approach to be more efficient than the one managed by the driver ? I don't have meaningful profile data yet.

For 3, like Hodgman said, if your event queries are working correctly, you shouldn't need to worry about this value. However with that said, the maximum frame latency is not 100% accurate, due to several factors. The drivers are able to override this frame latency, both explicitly as an override if an app never set anything, and implicitly by deferring the actual present operation until after the Present() API has returned. However, on new drivers and new OSes (Windows 10 Anniversary Update with WDDM2.1 drivers at least) using a FLIP_SEQUENTIAL or FLIP_DISCARD swap effect, the maximum frame latency should actually be accurate.

For 4... maybe. At best, you're getting simpler allocation strategies from the drivers because you're allocating large buffers instead of small ones, and are (maybe) running less code to do it. At worst, you're actually doing pretty much the exact same approach the driver would if you were using MAP_WRITE_DISCARD.

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Thank you guys for your replies. Mapping/unmapping and then writing to mapped memory indeed smells of undefined behaviour. So far it works on my machine but I should definitely test it on other GPUs to be more confident.I like this approach as the client code is quite concise, not requiring two calls to Map and Unmap for every constant upload operation. A pity DX11 does not have the concept of persistent mappings.

As for the latency, I am now ignoring the value returned by IDXGIDevice1::GetMaximumFrameLatency  and using instead a conservative latency equal to 5 for the allocator. I will also add a loop to block the CPU in case the number of queued frames goes above this value (which really shouldn't).

@SoldierOfLight

I will read about the new presentation modes. Thanks!

Edited by Reitano
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8 minutes ago, Reitano said:

So far it works on my machine but I should definitely test it on other GPUs to be more confident.

Even if it happens to work on 100 machines that you test on, it may still crash or cause memory corruption on the 101st one... or it may begin to crash/corrupt after the next driver update... 

It's simply luck that it works at all -- apparently your driver is keeping this address range persistently mapped by chance. Even with persistent mapping though, there's typically some kind of "synchronize" API call that you still use in place of "unmap", which ensures that the CPU's write combining buffer has been flushed (i.e. ensure that values that you've written have actually reached RAM before continuing) and instructs the GPU to invalidate this address range from its caches if it happens to be present. Assuming that this trick is actually giving you a persistently mapped buffer in D3D11, without these "synchronize" tasks being performed, it's still unsafe and the GPU may consume out of date data :(

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As alternative to Map/Unmap, you can also use UpdateSubResource1 as described in this article. That particular method also lets you avoid having to manually avoid writing to a buffer that the GPU is currently reading from, which is pretty dodgy to begin with in D3D11 since you don't have explicit submission or fences.

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Thank you all, you've been very helpful.

@Hodgmann

You are so right, I shouldn't even consider code with undefined behavior. I fixed the allocator to always have Map and Unmap calls around a memory write operation. On the API side, client code can use a convenient Upload method to upload small structures like camera data, and  manual Map/Unmap methods to upload potentially large chunks of data, like model instances, lights, materials etc. 

You can find the new code at https://codeshare.io/2p7ZbV

I am planning to refactor the rendering engine at a high level. The idea is to upload ALL constants in a first stage, and only at the end bind them and issue draw calls. This should allow a single call to Map/Unmap per constant buffer like I had originally.

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D3DReadFileToBlob(L"../Shaders/SimplePixelShader.cso", &psBlob); assert(psBlob); hr = g_d3dDevice->CreatePixelShader(psBlob->GetBufferPointer(), psBlob->GetBufferSize(), nullptr, &g_d3dPixelShader); SafeRelease(psBlob); if (FAILED(hr)) { return false; } //Load Vertex Buffer D3D11_BUFFER_DESC vertexBufferDesc{}; vertexBufferDesc.ByteWidth = sizeof(VertexPosColor) * _countof(g_Vertices); vertexBufferDesc.Usage = D3D11_USAGE_DEFAULT; vertexBufferDesc.BindFlags = D3D11_BIND_VERTEX_BUFFER; D3D11_SUBRESOURCE_DATA resourceData{}; resourceData.pSysMem = g_Vertices; hr = g_d3dDevice->CreateBuffer(&vertexBufferDesc, &resourceData, &g_d3dVertexBuffer); if (FAILED(hr)) { return false; } //Load Index Buffer D3D11_BUFFER_DESC indexBufferDesc{}; indexBufferDesc.ByteWidth = sizeof(WORD) * _countof(g_Indicies); indexBufferDesc.Usage = D3D11_USAGE_DEFAULT; indexBufferDesc.BindFlags = D3D11_BIND_INDEX_BUFFER; resourceData.pSysMem = g_Indicies; hr = g_d3dDevice->CreateBuffer(&indexBufferDesc, &resourceData, &g_d3dIndexBuffer); if (FAILED(hr)) { return false; } //Load Constant Buffers D3D11_BUFFER_DESC cBufferDesc{}; cBufferDesc.ByteWidth = sizeof(XMMATRIX); cBufferDesc.Usage = D3D11_USAGE_DEFAULT; cBufferDesc.BindFlags = D3D11_BIND_CONSTANT_BUFFER; for (size_t bufferID = 0; bufferID < NumConstantBuffers; bufferID++) { hr = g_d3dDevice->CreateBuffer(&cBufferDesc, nullptr, &g_d3dConstantBuffers[bufferID]); if (FAILED(hr)) { return false; } } //Setup Projection Matrix RECT client{}; GetClientRect(g_WinHnd, &client); float clientWidth = static_cast<float>(client.right - client.left); float clientHeight = static_cast<float>(client.bottom - client.top); g_ProjectionMatrix = DirectX::XMMatrixPerspectiveFovLH(XMConvertToRadians(45.0f), clientWidth / clientHeight, 0.1f, 100.0f); g_d3dDeviceContext->UpdateSubresource(g_d3dConstantBuffers[CB_Application], 0, nullptr, &g_ProjectionMatrix, 0, 0); return true; } void Update(float deltaTime) { XMVECTOR eyePosition = XMVectorSet(0, 0, -10, 1); XMVECTOR focusPoint = XMVectorSet(0, 0, 0, 1); XMVECTOR upDirection = XMVectorSet(0, 1, 0, 0); g_ViewMatrix = DirectX::XMMatrixLookAtLH(eyePosition, focusPoint, upDirection); g_d3dDeviceContext->UpdateSubresource(g_d3dConstantBuffers[CB_Frame], 0, nullptr, &g_ViewMatrix, 0, 0); static float angle = 0.0f; angle += 90.0f * deltaTime; XMVECTOR rotationAxis = XMVectorSet(0, 1, 1, 0); g_WorldMatrix = DirectX::XMMatrixRotationAxis(rotationAxis, XMConvertToRadians(angle)); g_d3dDeviceContext->UpdateSubresource(g_d3dConstantBuffers[CB_Object], 0, nullptr, &g_WorldMatrix, 0, 0); } void Clear(const FLOAT clearColor[4], FLOAT clearDepth, UINT8 clearStencil) { g_d3dDeviceContext->ClearRenderTargetView(g_d3dRenderTargerView, clearColor); g_d3dDeviceContext->ClearDepthStencilView(g_d3dDepthStencilView, D3D11_CLEAR_DEPTH | D3D11_CLEAR_STENCIL, clearDepth, clearStencil); } void Present(bool vSync) { if (vSync) { g_d3dSwapChain->Present(1, 0); } else { g_d3dSwapChain->Present(0, 0); } } void Render() { assert(g_d3dDevice); assert(g_d3dDeviceContext); Clear(Colors::CornflowerBlue, 1.0f, 0); //IA const UINT vertexStride = sizeof(VertexPosColor); const UINT offset = 0; g_d3dDeviceContext->IASetVertexBuffers(0, 1, &g_d3dVertexBuffer, &vertexStride, &offset); g_d3dDeviceContext->IASetInputLayout(g_d3dInputLayout); g_d3dDeviceContext->IASetIndexBuffer(g_d3dIndexBuffer, DXGI_FORMAT_R16_UINT, 0); g_d3dDeviceContext->IASetPrimitiveTopology(D3D11_PRIMITIVE_TOPOLOGY_TRIANGLELIST); //VS g_d3dDeviceContext->VSSetShader(g_d3dVertexShader, nullptr, 0); g_d3dDeviceContext->VSGetConstantBuffers(0, NumConstantBuffers, g_d3dConstantBuffers); //RS g_d3dDeviceContext->RSSetState(g_d3dRasterizerState); g_d3dDeviceContext->RSSetViewports(1, &g_Viewport); //PS g_d3dDeviceContext->PSSetShader(g_d3dPixelShader, nullptr, 0); //OM g_d3dDeviceContext->OMSetRenderTargets(1, &g_d3dRenderTargerView, g_d3dDepthStencilView); g_d3dDeviceContext->OMSetDepthStencilState(g_d3dDepthStencilState, 1); //draw g_d3dDeviceContext->DrawIndexed(_countof(g_Indicies), 0, 0); Present(g_EnableVSync); } void CleanUp() { SafeRelease(g_d3dVertexShader); SafeRelease(g_d3dPixelShader); SafeRelease(g_d3dVertexBuffer); SafeRelease(g_d3dIndexBuffer); SafeRelease(g_d3dInputLayout); SafeRelease(g_d3dDepthStencilBuffer); for (size_t bufferID = 0; bufferID < NumConstantBuffers; bufferID++) { SafeRelease(g_d3dConstantBuffers[bufferID]); } SafeRelease(g_d3dDepthStencilState); SafeRelease(g_d3dRasterizerState); SafeRelease(g_d3dRenderTargerView); SafeRelease(g_d3dDepthStencilView); SafeRelease(g_d3dSwapChain); SafeRelease(g_d3dDeviceContext); SafeRelease(g_d3dDevice); }  
    • By MarcusAseth
      Hi guys, I'm trying to learn this stuff but running into some problems 😕
      I've compiled my .hlsl into a header file which contains the global variable with the precompiled shader data:
      //... // Approximately 83 instruction slots used #endif const BYTE g_vs[] = { 68, 88, 66, 67, 143, 82, 13, 236, 152, 133, 219, 113, 173, 135, 18, 87, 122, 208, 124, 76, 1, 0, 0, 0, 16, 76, 0, 0, 6, 0, //.... And now following the "Compiling at build time to header files" example at this msdn link , I've included the header files in my main.cpp and I'm trying to create the vertex shader like this:
      hr = g_d3dDevice->CreateVertexShader(g_vs, sizeof(g_vs), nullptr, &g_d3dVertexShader); if (FAILED(hr)) { return -1; } and this is failing, entering the if and returing -1.
      Can someone point out what I'm doing wrong? 😕 
    • By Toastmastern
      Hello everyone,
      After a few years of break from coding and my planet render game I'm giving it a go again from a different angle. What I'm struggling with now is that I have created a Frustum that works fine for now atleast, it does what it's supose to do alltho not perfect. But with the frustum came very low FPS, since what I'm doing right now just to see if the Frustum worked is to recreate the vertex buffer every frame that the camera detected movement. This is of course very costly and not the way to do it. Thats why I'm now trying to learn how to create a dynamic vertexbuffer instead and to map and unmap the vertexes, in the end my goal is to update only part of the vertexbuffer that is needed, but one step at a time ^^

      So below is my code which I use to create the Dynamic buffer. The issue is that I want the size of the vertex buffer to be big enough to handle bigger vertex buffers then just mPlanetMesh.vertices.size() due to more vertices being added later when I start to do LOD and stuff, the first render isn't the biggest one I will need.
      vertexBufferDesc.Usage = D3D11_USAGE_DYNAMIC; vertexBufferDesc.ByteWidth = mPlanetMesh.vertices.size(); vertexBufferDesc.BindFlags = D3D11_BIND_VERTEX_BUFFER; vertexBufferDesc.CPUAccessFlags = D3D11_CPU_ACCESS_WRITE; vertexBufferDesc.MiscFlags = 0; vertexBufferDesc.StructureByteStride = 0; vertexData.pSysMem = &mPlanetMesh.vertices[0]; vertexData.SysMemPitch = 0; vertexData.SysMemSlicePitch = 0; result = device->CreateBuffer(&vertexBufferDesc, &vertexData, &mVertexBuffer); if (FAILED(result)) { return false; } What happens is that the 
      result = device->CreateBuffer(&vertexBufferDesc, &vertexData, &mVertexBuffer); Makes it crash due to Access Violation. When I put the vertices.size() in it works without issues, but when I try to set it to like vertices.size() * 2 it crashes.
      I googled my eyes dry tonight but doesn't seem to find people with the same kind of issue, I've read that the vertex buffer can be bigger if needed. What I'm I doing wrong here?
       
      Best Regards and Thanks in advance
      Toastmastern
    • By yonisi
      Hi,
      I have a terrain engine where the terrain and water are on different grids. So I'm trying to render planar reflections of the terrain into the water grid. After reading some web pages and docs and also trying to learn from the RasterTek reflections demo and the small water bodies demo as well. What I do is as follows:
      1. Create a Reflection view matrix  - Technically I ONLY flip the camera position in the Y direction (Positive Y is up) and add to it 2 * waterLevel. Then I update the View matrix and I save that matrix for later. The code:
      void Camera::UpdateReflectionViewMatrix( float waterLevel ) { mBackupPosition = mPosition; mBackupLook = mLook; mPosition.y = -mPosition.y + 2.0f * waterLevel; //mLook.y = -mLook.y + 2.0f * waterLevel; UpdateViewMatrix(); mReflectionView = View(); } 2. I render the Terrain geometry to a 512x512 sized Render target by using the Reflection view matrix and an opposite culling (My Terrain is using front culling by nature so I'm using back culling for the Reflction render pass). Let me say that I checked with the Graphics debugger and the Reflection Render target looks "OK" at this stage (Picture attached). I don't know if the fact that the terrain is shown only at the top are of the texture is expected or not, but it seems OK.

      3. Render the Reflection texture into the water using projective texturing - I hope this step is OK code wise. Basically I'm sending to the shader the WorldReflectionViewProj matrix that was created at step 1 in order to use it for the projective texture coordinates, I then convert the position in the DS (Water and terrain are drawn with Tessellation) to the projective tex coords using that WorldReflectionViewProj matrix, then I sample the reflection texture after setting up the coordinates in the PS. Here is the code:
      //Send the ReflectionWorldViewProj matrix to the shader: XMStoreFloat4x4(&mPerFrameCB.Data.ReflectionWorldViewProj, XMMatrixTranspose( ( mWorld * pCam->GetReflectedView() ) * mProj )); //Setting up the Projective tex coords in the DS: Output.projTexPosition = mul(float4(worldPos.xyz, 1), g_ReflectionWorldViewProj); //Setting up the coords in the PS and sampling the reflection texture: float2 projTexCoords; projTexCoords.x = input.projTexPosition.x / input.projTexPosition.w / 2.0 + 0.5; projTexCoords.y = -input.projTexPosition.y / input.projTexPosition.w / 2.0 + 0.5; projTexCoords += normal.xz * 0.025; float4 reflectionColor = gReflectionMap.SampleLevel(SamplerClampLinear, projTexCoords, 0); texColor += reflectionColor * 0.25; I'll add that when compiling the PS I'm getting a warning on those dividing by input.projTexPosition.w for a possible float division by 0, I tried to add some offset or some minimum to the dividing term but that still not solved my issue.
      Here is the problem itself. At relatively flat view angles I'm seeing correct reflections (Or at least so it seems), but as I pitch the camera down, I'm seeing those artifacts which I have no idea where are coming from. I'm culling the terrain in the reflection render pass when it's lower than water height (I have heightmaps for that).
       
      Any help will be appreciated because I don't know what is wrong or where else to look.
    • By thmfrnk
      Hi,
      I am looking for a usefull commandline based texture compression tool with the rights to be able to ship with my application. It should have following caps:
      Supports all major image format as source files (jpeg, png, tga, bmp) Export as DDS Compression Formats BC1, BC2, BC3, BC4, BC7 I am actually using the nvdxt tool from Nvidia, but it does not support BC4 (which I need for one-channel 8bit textures). Everything else which I found wasn't really useful.
      Any suggestions?
      Thx
       
    • By trojanfoe
      I have been trying to create a BlendState for my UI text sprites so that they are both alpha-blended (so you can see them) and invert the pixel they are rendered over (again, so you can see them).
      In order to get alpha blending you would need:
      SrcBlend = SRC_ALPHA DestBlend = INV_SRC_ALPHA and in order to have inverted colours you would need something like:
      SrcBlend = INV_DEST_COLOR DestBlend = INV_SRC_COLOR and you can't have both.
      So I have come to the conclusion that it's not possible; am I right?
    • By Royma
      In traditional way, it needs 6 passes for a point light and many passes for cascaded shadow mapping to generate shadow maps. Recently I learnt a method that using a geometry shader to generate all the shadow maps in one pass.I specify a render target and a depth-stencil buffer which are both Texture2dArray in DirectX11.It looks much better than the traditional way I think.But after I implemented it, I found cascaded shadow mapping runs much slower than the traditional way.The fps slow down from 60 to 35.I don't know why.I guess may be I should do some culling or maybe the geometry shader is not efficient.
      I want to know the reason that I reduced the drawcalls from 8 to 1, but it runs slow down.Should I abandon this method or is there any way to optimize this method to run more efficiently than multi-pass rendering?
      Here is the gs code:

      [maxvertexcount(24)]
      void main(
          triangle DepthGsIn input[3] : SV_POSITION,
          inout TriangleStream< DepthPsIn > output
      )
      {
          for (uint k = 0; k < 8; ++k)
          {
              DepthPsIn element;
              element.RTIndex = k;
              for (uint i = 0; i < 3; ++i)
              {
                  float2 shadowSlopeBias = calculateShadowSlopeBias(input.normal, -g_cameras[k].world[1]);
                  float shadowBias = shadowSlopeBias.y * g_cameras[k].shadowMapParameters.x + g_cameras[k].shadowMapParameters.y;
                  element.position = input.position + shadowBias * g_cameras[k].world[1];
                  element.position = mul(element.position, g_cameras[k].viewProjection);
                  element.depth = element.position.z / element.position.w;
                  
                  output.Append(element);
              }
              output.RestartStrip();
          }
      }
       
    • By savail
      Hey,
      There are a few things which confuse me regarding DirectX 11 and HLSL shaders in general. I would be very grateful for your advice!
      1. Let's take for example a scene which invokes 2 totally separate pipeline render passes interchangeably. I understand I need to bind correct shaders for each of the render pass and potentially blend/depth or rasterizer state but what about resources such as Constant Buffers, Shader Resource Views and Unordered Access Views? Assuming that the second render pass uses none of the resources used by the first pass, do I still need to unbind the resources and clean pipeline state after first pass? Or is it ok to leave pipeline with unbound garbage since anything I'd need to bind for second pass would overwrite contents in the appropriate register slots anyway?
      2. Is it a good practice to assign register slots manually to all resources in HLSL?
      3. I thought about assigning manually register slots for every distinct render pass up to the maximum slot limit if neccessary. For example in 1 render pass I invoke 3 CS's, 2 VS's and 2 PS's and for all resources used by those shaders I try to fill as many register slots as neccessary and potentially reuse many times the same slot in shaders sharing the same resource. I was wondering if there is any performance penalty or gain when I bind all of my needed resources at the start of render pass and never gonna have to do it again until next render pass? - this means potentially binding a lot of registers and having excessive number of bound resources for every shader that is run.
      4. Is it a good practice to create a separate include file for every resource that occurs in >= 2 shader files or is it better to duplicate the declarations? In first case, the code is imo easier to maintain and edit but might be harder to read if there's too many includes. I've come up with a compromise between these 2 like this: create a separate include file for every CB that occurs in >= 2 shader files and a separate include file for every sampler I ever need to use. All other resources like srvs and uavs I prefer to duplicate in multiple shaders because they take much less space than CB for example... I'm not sure however if that's a good practice
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