Recursive structs?

[Alphapage]

Hello,

I can't find any info about nested struct, something like this:

struct MyData {
v1:vec4<f32>,
children: array<MyData>
}

Is it planned to deal with such a structure or is there a workaround to achieve this ?

Thank you in advance for your help.

[dj2]

Just to note, this is a request for recursive structs, nested structs are already available in WGSL.

[Alphapage]

Interesting, I wasn't able to find info about nested struct like this:

struct MyNestedData {
v2:vec4<f32>
}
struct MyData {
v1:vec4<f32>,
nested_datas: array<MyNestedData>
}

If it is possible to set an array of nested struct, why not recursive structs ?

[dj2]

See https://www.w3.org/TR/WGSL/#struct-types where it lists the types of a struct member. The last item: a structure type that has a creation-fixed footprint.

The recursive struct is an error because, currently, the spec says:

It is a shader-creation error if any module scope declaration is recursive. That is, no cycles can exist among the declarations:

[kainino0x]

Even if that were lifted, it would not be possible to create a recursive struct like the one in your example: members of a struct are inline in the struct so the size would be infinite: sizeof(MyData) = sizeof(vec4f) + sizeof(MyData)

A struct can only recurse if the recursive member puts the struct behind a pointer indirection (ptr<> in WGSL). However it is not possible to allocate memory inside a WGSL shader, so the pointer would need to be to either:

• host-shareable memory (uniform buffer or storage buffer) - not possible because ptr types can't be stored in host-shareable memory.
• or a local variable. So you could set up var1 and have it point to var2 and have it point to var3 but you would have to define all of them.
• or, very similarly, an element of an array defined in a local variable (which must be fixed-size because allocation isn't possible).

[dneto0]

+1 to be clear, this choice reflects constraints in the shader languages in underlying APIs: not every underlying shading language allows you to store a pointer in memory. In WGSL this is captured by the fact that ptr<...> types are not storable

[kdashg]

WGSL 2024-06-25 Minutes

• (milestone?)
• DS: Nested structs is already in the spec. Recursive structs…. No.
• DN: Recursive structs is syntactic sugar for storing pointers.
• KG: Have a need for a struct self-reference, like struct Foo { ptr<Foo>
• DN: Order independence at module-scope means it’s already there in terms of identifier resolution.
• JB: Implies malloc?
• AB: No: can have a structure built on the CPU side, and represented in a buffer. (Vulkan buffer reference feature)
• JB: No plans for this.
• AB: This needs a full deliberate design, and it wouldn’t start from this language feature.
• Resolved to close as “no current plans”