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Juliean

Shader reflection unused variables in cbuffer

5 posts in this topic

Hello,

 

while implementing skinnig to my engine, I came to notice that the shader-reflection API does not seem to know which variables are unused from a cbuffer and therefore optimized away by the program. For example:

    cbuffer instance
    {
        matrix mWorld;
        matrix mPalette[16];
    }

mPalette is only used in the skinning permutation of this shader, but the reflection still returns 1068 for this cbuffer

void Effect::CreateVCBufferDummies(ID3D11ShaderReflection& reflection)
{
    // Get shader info
    D3D11_SHADER_DESC shaderDesc;
    reflection.GetDesc( &shaderDesc );

    for(size_t i = 0; i < shaderDesc.ConstantBuffers; i++)
    {
        ID3D11ShaderReflectionConstantBuffer* pConstantReflection = reflection.GetConstantBufferByIndex(i);
                    
        D3D11_SHADER_BUFFER_DESC desc;
        pConstantReflection->GetDesc(&desc);

        D3D11_SHADER_INPUT_BIND_DESC bindDesc;
        reflection.GetResourceBindingDescByName(desc.Name, &bindDesc);

// desc.size == 1068 for the shader
        m_cVBuffer[bindDesc.BindPoint] = new d3d::ConstantBuffer(*m_pDevice, desc.Size);
    }
}

The problem with this is that even though the reflection API doesn't notice it, the matrix-array is indeed being optimized away in the shader, and therefor when I try to move the data to the cbuffer via mapping, it crashes in this occasion:

void ConstantBuffer::Overwrite(void* pData) const
{
	D3D11_MAPPED_SUBRESOURCE mappedResource;
	m_pDevice->Map(*m_pBuffer, D3D11_MAP_WRITE_DISCARD, mappedResource );

	// m_size == 1068 for this shader, but the mappedResource.pData only contains space for 64 byte
	memcpy(mappedResource.pData, pData, m_size);

	m_pDevice->Unmap(*m_pBuffer);
}

This also happens with other variable types, and even if the cbuffer-variable isn't used anywhere at all.

 

Does anybody know if there is a way to have the shader-reflection return the size of the cbuffer AFTER the optimizations? Is there any kind of flag for this, or is this maybe a bug, and there possibly exists another solution?

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Apologies, the size is indeed 1088, it mistyped it. The issue is there nevertheless, however I've found a quick workaround by only memcpy'ing the range of the CPU-side buffer that has been written to (so for a non-animated mesh only the first 64 bytes, only when I start to push all the bone-matrices to the buffer the rest is copied). I'll also have a look at querying the variables themselfs, hope this works, because its at least a giant waste of memory (1000+ byte instead of 64 for non-skinned meshes).

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Thanks MJP, I didn't think of this use case before, now it does make sense that it doesn't do that. This also made me think about how this all doesn't make any sense. I mean, I did create the cbuffer from the same value that the shader reflection returned, so they must actually match - I didn't even think about it, my gfx-wrapper takes so much work away that I pretty much asumed that the cbuffers are generated along with the shaders from the DirectX api, lol. The actual cause of the issue was in my material-system. When a material is set to a model, the model takes its effect and resizes its cbuffer to the effects model cbuffers size. Each model can have multiple materials, and the shadow-material (which only had 64 bytes of cbuffer) overwrote the larger cbuffer, since it was added last. I'm still going to need a good solution for when materials are completely replaced, but for now each additional material added can only enlargen the cbuffer but not shrink it, which fixes the crash.

Edited by Juliean
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