Presently I'm working on updating a Windows 11 DX12 desktop app to take advantage of the technologies introduced by DX12 Ultimate (i.e. mesh shaders, VRS & DXR).
All the official samples for Ultimate compile and run on my machine (Core i9/RTX3070 laptop) so as a first step, I wish to begin migrating as much static (i.e. unskinned) geometry over from the conventional (IA-vertex shader) rendering pipeline over to the Amplification->Mesh shader pipeline.
I'm naturally using code from the official samples to facilitate this, and in the process I've encountered a very strange issue which only triggers in my app, but not in the compiled source project.
The specific problem relates to setting up meshlet instancing culling & dynamic LOD selection. When setting descriptors into the mesh shader SRV heap, my app was failing to create a CBV:
// Mesh Info Buffers
D3D12_CONSTANT_BUFFER_VIEW_DESC cbvDesc{};
cbvDesc.BufferLocation = m.MeshInfoResource->GetGPUVirtualAddress();
cbvDesc.SizeInBytes = MeshletUtils::GetAlignedSize<UINT>(sizeof(MeshInfo)); // 256 bytes which is correct
device->CreateConstantBufferView(&cbvDesc, OffsetHandle(i)); // generates error
A CBV into the descriptor range couldn't be generated because the resource's GPU address range was created with only 16 bytes:
D3D12 ERROR: ID3D12Device::CreateConstantBufferView: pDesc->BufferLocation + SizeInBytes - 1 (0x0000000008c1f0ff) exceeds end of the virtual address range of Resource (0x000001BD88FE1BF0:'MeshInfoResource', GPU VA Range: 0x0000000008c1f000 - 0x0000000008c1f00f). [ STATE_CREATION ERROR #649: CREATE_CONSTANT_BUFFER_VIEW_INVALID_RESOURCE]
What made this frustrating was the code is identical to the official sample, but the sample was compiling without issue. But after many hours of trying dumb things, I finally decided to examine the size of the MeshInfo structure, and therein lay the solution.
The MeshInfo struct is defined in the sample's Model class as:
struct MeshInfo
{
uint32_t IndexSize;
uint32_t MeshletCount;
uint32_t LastMeshletVertCount;
uint32_t LastMeshletPrimCount;
};
It is 16 bytes in size, and passed to the resource's description prior to its creation:
auto meshInfoDesc = CD3DX12_RESOURCE_DESC::Buffer(sizeof(MeshInfo));
ThrowIfFailed(device->CreateCommittedResource(&defaultHeap, D3D12_HEAP_FLAG_NONE, &meshInfoDesc, D3D12_RESOURCE_STATE_COPY_DEST, nullptr, IID_PPV_ARGS(&m.MeshInfoResource)));
SetDebugObjectName(m.MeshInfoResource.Get(), L"MeshInfoResource");
But clearly I needed a 256 byte range to conform with D3D12_CONSTANT_BUFFER_DATA_PLACEMENT_ALIGNMENT, so I changed meshInfoDesc to:
auto meshInfoDesc = CD3DX12_RESOURCE_DESC::Buffer(sizeof(MeshInfo) * 16u);
And the project compiles successfully.
So my question is, why isn't this GPU virtual address error also occurring in the sample???
PS: It was necessary to rename Model.h/Model.cpp to MeshletModel.h/MeshletModel.cpp for use in my project, which is based on the DirectX Tool Kit framework, where Model.h/Model.cpp files already exist for the DXTK rigid body animation effect.
The solution was explained in the question, so I will summarize it here as the answer to this post.
When creating a constant buffer view on a D3D12 resource, make sure to allocate enough memory to the resource upon creation. This should be at least 256 bytes to satisfy D3D12_CONSTANT_BUFFER_DATA_PLACEMENT_ALIGNMENT.
I still don't know why the sample code on GitHub could compile without this requirement. Without having delved into the sample's project configuration in detail, it's possible that D3D12 debug layer errors are being dealt with differently, but that's purely speculative.