Setting multiple descriptors in a descriptor range in Direct3D 12

Question

First of all, my understanding of a descriptor range is that I can specify multiple buffers (constant buffers in my case) that a shader may use, is that correct? If not, then this is where my misunderstanding is, and the rest of the question will make no sense.

Lets say I want to pass a couple of constant buffers in my vertex shader

// Vertex.hlsl

float value0 : register(b0)
float value1 : register(b1)
...

And for whatever reason I want to use a descriptor range to specify b0 and b1. I fill out a D3D12_DESCRIPTOR_RANGE:

D3D12_DESCRIPTOR_RANGE range;
range.RangeType = D3D12_DESCRIPTOR_RANGE_TYPE_CBV;
range.NumDescriptors = 2;    
range.BaseShaderRegister = 0;
range.RegisterSpace = 0;
range.OffsetInDescriptorsFromTableStart = D3D12_DESCRIPTOR_RANGE_OFFSET_APPEND;

I then go on to shove this into a root parameter

D3D12_ROOT_PARAMETER param;  
param.ParameterType = D3D12_ROOT_PARAMETER_TYPE_DESCRIPTOR_TABLE;
param.DescriptorTable.NumDescriptorRanges = 1; 
param.DescriptorTable.pDescriptorRanges = ⦥
param.ShaderVisibility = D3D12_SHADER_VISIBILITY_VERTEX;

Root parameter goes into my signature description

D3D12_ROOT_SIGNATURE_DESC1 signatureDesc;
signatureDesc.NumParameters = 1;
signatureDesc.pParameters = &param;
signatureDesc.NumStaticSamplers = 0;
signatureDesc.pStaticSamplers = nullptr;
D3D12_ROOT_SIGNATURE_FLAGS = D3D12_ROOT_SIGNATURE_FLAG_ALLOW_INPUT_ASSEMBLER_INPUT_LAYOUT;

After this is create my root signature and so on. I also created a heap for 2 descriptors

D3D12_DESCRIPTOR_HEAP_DESC heapDescCbv;
heapDescCbv.Type = D3D12_DESCRIPTOR_HEAP_TYPE_CBV_SRV_UAV;
heapDescCbv.NumDescriptors = 2;
heapDescCbv.Flags = D3D12_DESCRIPTOR_HEAP_FLAG_SHADER_VISIBLE;
heapDescCbv.NodeMask = 0;
ThrowIfFailed(m_device->CreateDescriptorHeap(&heapDescCbv, IID_PPV_ARGS(&m_cbvHeap)));

I then mapped the respective ID3D12Resource's to get two pointers so I can memcpy my values to them.

void D3D12App::AllocateConstantBuffer(SIZE_T index, size_t dataSize, ID3D12Resource** buffer, void** mappedPtr)
{
    D3D12_HEAP_PROPERTIES heapProp;
    heapProp.CPUPageProperty = D3D12_CPU_PAGE_PROPERTY_UNKNOWN;
    heapProp.MemoryPoolPreference = D3D12_MEMORY_POOL_UNKNOWN;
    heapProp.CreationNodeMask = 1;
    heapProp.VisibleNodeMask = 1;
    heapProp.Type = D3D12_HEAP_TYPE_UPLOAD;
    D3D12_RESOURCE_DESC resDesc;
    resDesc.Dimension = D3D12_RESOURCE_DIMENSION_BUFFER;
    resDesc.Alignment = 0;
    resDesc.Width = (dataSize + 255) & ~255;
    resDesc.Height = 1;
    resDesc.DepthOrArraySize = 1;
    resDesc.MipLevels = 1;
    resDesc.Format = DXGI_FORMAT_UNKNOWN;
    resDesc.SampleDesc.Count = 1;
    resDesc.SampleDesc.Quality = 0;
    resDesc.Layout = D3D12_TEXTURE_LAYOUT_ROW_MAJOR;
    resDesc.Flags = D3D12_RESOURCE_FLAG_NONE;
    ThrowIfFailed(m_device->CreateCommittedResource(&heapProp, D3D12_HEAP_FLAG_NONE,
        &resDesc, D3D12_RESOURCE_STATE_GENERIC_READ, nullptr, IID_PPV_ARGS(buffer)));

    D3D12_CONSTANT_BUFFER_VIEW_DESC cbvDesc;
    cbvDesc.BufferLocation = (*buffer)->GetGPUVirtualAddress();
    cbvDesc.SizeInBytes = (dataSize + 255) & ~255;
    auto cbvHandle = m_cbvHeap->GetCPUDescriptorHandleForHeapStart();
    cbvHandle.ptr += index * m_device->GetDescriptorHandleIncrementSize(D3D12_DESCRIPTOR_HEAP_TYPE_CBV_SRV_UAV);
    m_device->CreateConstantBufferView(&cbvDesc, cbvHandle);

    D3D12_RANGE readRange;
    readRange.Begin = 0;
    readRange.End = 0;
    ThrowIfFailed((*buffer)->Map(0, &readRange, mappedPtr));
}

AllocateConstantBuffer(0, sizeof(m_value0), &m_value0Resource, reinterpret_cast<void**>&m_constPtrvalue0));
AllocateConstantBuffer(1, sizeof(m_value1), &m_value1Resource, reinterpret_cast<void**>&m_constPtrvalue1));

The problem is when I want to feed them to the pipeline. When rendering, I used

auto cbvHandle = m_cbvHeap->GetGPUDescriptorHandleForHeapStart();
m_commandList->SetGraphicsRootDescriptorTable(0, cbvHandle);

The result I get is only register(b0) got the the correct value, and register(b1) remains uninitialized. What did I do wrong?

Answer 1

OK I got it to work. Turned out I need to change the shader a bit:

cbuffer a : register(b0) { float value0; }
cbuffer b : register(b1) { float value1; };

This gave me another question though, according to this link: https://learn.microsoft.com/en-us/windows/win32/direct3dhlsl/dx-graphics-hlsl-constants

the buffer names a and b should be optional, but when I tried that the shaders cannot compile. I guess that is a different question.