Root/Push constants

[alelievr]

Hello,

I added D3D12 Root constant and Vulkan's push constant to the command buffer API. I don't really have experience writing metal code so I'm not sure what I did works (and I can't test either).

[andrejnau]

I don't really have experience writing metal code so I'm not sure what I did works (and I can't test either).

No need to worry about metal, just place assert(false) there instead of implementation. I will handle it later.

If you're interested, there are 2 problems with the metal

1. SPIRV-Cross does not automatically emit push constants blocks, when [[vk::push_constant]] is used in hlsl.
2. You cannot just create new MTLRenderCommandEncoder/MTLComputeCommandEncoder, set constants, and endEncoding. You need to track constants changes and apply before echo draw/dispatch.

[andrejnau]

It is better to use PushConstants, to match it with [[vk::push_constant]] in hlsl.
void PushConstants(ShaderType shader_type, uint32_t dst_offset, const void* data, uint32_t size)

In Vulkan we can have at most 1 push_constant block in each shader, I suggest to add the same limitation in FlyCube. In this case ShaderType + active Pipeline will probably be enough to find root_parameter_index in DirectX12 implementation.

In my opinion PushConstants is enough to for all cases, but I don't mind having PushConstant as well.
void PushConstant(ShaderType shader_type, uint32_t dst_offset, uint32_t value)

[andrejnau]

In the current implementation, how do you know which root_parameter_index value you need to pass?

[alelievr]

The root parameter index is the index of the binding set/layout set in the list given to CreateBindingSetLayout or CreateBindingSet.

[andrejnau]

You also need to specify pipeline_layout_info.pPushConstantRanges in VKBindingSetLayout. But it probably won't work without information about push constant size.

[andrejnau]

I suggest to use ViewType::kPushConstant instead.

[andrejnau]

Actually no, it's better to use ViewType::kConstantBuffer. Because ShaderReflection must be able to distinguish ViewType::kPushConstant from ViewType::kConstantBuffer, but there is no way to do so in dxil, since dxc ignore [[vk::push_constant]] when compile to dxil.

But since we still need to pass the size for push_constant, it is better to pass it separately from the BindKey.

[alelievr]

If I use ViewType::kConstantBuffer instead of this boolean, how can I know if the root BindKey should go to a root constant?

[andrejnau]

For example, we can separate them.

struct PushConstant {
    ShaderType shader_type = ShaderType::kUnknown;
    uint32_t size = 0;
};

std::shared_ptr<BindingSetLayout> CreateBindingSetLayout(const std::vector<BindKey>& descs, const std::vector<PushConstant>& push_constants = {});

[andrejnau]

Not sure what is best to use, just ShaderType or BindKey.

1. ShaderType
   At most 1 push constant per shader stage to match Vulkan limitation.
   Slot and space are 0 because spirv do not save bindings when [[vk::push_constant]] is used.
2. BindKey
   Allow multiple push constant in DirectX12 and Metal, but Vulkan still supports at most 1 push constant per shader stage at slot/space 0 with using [[vk::push_constant]].
   More flexible, but harder to use in Vulkan.

[andrejnau]

1. SPIRV-Cross does not automatically emit push constants blocks, when [[vk::push_constant]] is used in hlsl.

According to the documentation, MTL*CommandEncoder::set*Bytes implicitly creates a buffer and stores the transferred data into it. It seems that there is no equivalent of push_constant in MSL, so it is correct that SPIRV-Cross generates a buffer binding for [[vk::push_constant]].