[hlsl-out] account for "vector-relaxed" std140 layout for constant buffers

[teoxoy]

HLSL uses "vector-relaxed" std140 layout for constant buffers, however the current implementation seems to assume std140 layout

The example below shows the issue.

Going from WGSL to HLSL trough naga translates the struct as if it had the same layout in HLSL.
When translating the same WGSL shader to SPIR-V via naga, the offsets are: 0, 8, 16.
However when translating the produced HLSL to SPIR-V via DXC, the offsets are: 0, 4, 12.

WGSL -> HLSL

WGSL -> SPIR-V

HLSL -> SPIR-V

Solution

Insert padding (when necessary) for vector members that are in constant buffers.

References

HLSL Packing Rules for Constant Variables
DirectXShaderCompiler Buffer-Packing Wiki
DirectXShaderCompiler Memory layout rules