Disabling InstanceFlag::VALIDATION_INDIRECT_CALL makes the DX12 backend non-compliant?

[djeedai]

Hi,

Some time ago I asked about the validation step which was auto-injected by wgpu (for the sake of browsers security or stability if I recall) before each indirect dispatch, and was messing with bindings (lots of unnecessary bind calls).

As I recall I was told this would be made optional in a later version (it's mandatory in 24.x). Reading the latest docs in 26.x, I think this is what InstanceFlag::VALIDATION_INDIRECT_CALL corresponds to?

The problem is that there's a section in that doc saying that on the D3D12 backend, disabling this flag makes @builtin(instance_index) not work. I checked the WGSL spec, I didn't see anything that would allow a compliant implementation to choose not to take first_instance into account.

So my understanding is that you have to choose between enabling a debug feature which adds extra dispatches and unnecessary bindings (and so impacts performance), and being able to use the first_instance feature (or re-implementing it yourself). Is that correct, or am I misunderstanding something? Can someone explain what's going on here and why only D3D12 has this issue?

Thanks!

[djeedai]

Closing this because this was a bug report that someone changed into discussion. We have users complaining pipeline switches are very slow, making our library unusable in production, and we can't group calls and avoid pipeline switches because wgpu keeps injecting its own pipeline switches everywhere. And the disabling mechanism breaks several DX12 features.