[Migrated] Initializing workgroup memory

Issue automatically imported from old repo: https://github.com/EmbarkStudios/rust-gpu/issues/1077
Old labels: t: enhancement
Originally creatd by kevinboulain on 2023-07-04T20:07:53Z

As far as I can tell, the workgroup/shared memory (context) can not be declared as to be zero-initialized and one has to do it manually.

https://github.com/KhronosGroup/Vulkan-Docs/issues/1457 has a bunch of interesting links: the extension VK_KHR_zero_initialize_workgroup_memory (promoted to core in Vulkan 1.3) allows to declare workgroup/shared variables with a null initializer. On the SPIR-V side, OpVariable with a Workgroup storage class is allowed to be initialized with OpConstantNull only (like so).

I don't know what the project would prefer, it could be an option to pass to the SpirvBuilder (I assume it could be desirable to always initialize these variables) or another symbol (similarly to how storage_buffer allows extra arguments):

#[spirv(compute(threads(256)))]  
fn compute(
    #[spirv(workgroup, null_initialized = true)] array: &mut [u32; N],
    // ...
) {
  // ...
}

Beginne damit, SpirvBuilder und die bestehende Behandlung von Workgroup-/Shared-Memory nachzuverfolgen, und vergleiche sie anschließend mit den verlinkten Vulkan- und SPIR-V-Regeln für OpConstantNull. Als erledigt gilt die Aufgabe, wenn das Projekt über eine festgelegte API zum Anfordern von nullinitialisiertem Workgroup-Memory verfügt und die entsprechende gültige SPIR-V-Ausgabe unterstützt.