Constant Buffer size mismatch on PS, fine on VS and GS

Question

I have the following C/C++ struct:

struct ShadowMapCB {
        Math::Matrix4 cropMatrix[4];
        Math::Matrix4 textureMatrix[4];
        float splitPlane[4];
    };

and my HLSL constant buffer:

cbuffer CBShadowMap : register(b4)
{
    matrix g_CropMatrix[4];
    matrix g_TextureMatrix[4];
    float g_fSplitPlane[4];
};

When i bind it to pixel shader i get an error:

D3D11: WARNING: ID3D11DeviceContext::DrawIndexed: The size of the Constant Buffer at slot 4 of the Pixel Shader unit is too small (528 bytes provided, 576 bytes, at least, expected). This is OK, as out-of-bounds reads are defined to return 0. It is also possible the developer knows the missing data will not be used anyway. This is only a problem if the developer actually intended to bind a sufficiently large Constant Buffer for what the shader expects. [ EXECUTION WARNING #351: DEVICE_DRAW_CONSTANT_BUFFER_TOO_SMALL ]

and infact the last 4 floats (array[4]) are messed up, the weird thing is that on the VS and GS it works fine..., and yes i did bind them with PSSetConstantBuffers and update it ofcourse.

What could be causing this?

Answer 1

IIRC, in a constant buffer, each element of an array must start on a 4-float boundary. That didn't affect the matrices since each matrix already starts on a 4-float boundary, but it caused the float g_fSplitPlane[4] to get an extra 3 floats of padding between array elements.

The constant buffer layout rules are the same for all kinds of shaders, so if it doesn't work in the PS, it shouldn't work in the VS and GS either. Was the g_fSplitPlane member not used in the VS and GS, perhaps? If so, the shader compiler might have stripped it out of those, preventing the error from firing.