wgsl interpolate(flat) always results in value of 0

[rukai]

Description
wgsl interpolate(flat) always results in value of 0, I think, at least the repro is very concise
Issue does not occur with the equivalent glsl

Repro steps
rukai@5c29302

Expected vs observed behavior
I expect blue output but get red

Platform
wgpu master, linux, vulkan

[rukai]

I discovered in the wgsl spec that flat cannot be used on u32, as integer types are always flat.
In that case we should make the following changes to naga:

• Error when flat is used on an int.
• Fix the repro provided, naga master seems to be outputting spirv with flat regardless of whether interpolate(flat) is used, so possibly the 0 value is caused by something else.

[kvark]

Does renderdoc capture show anything interesting?

[rukai]

renderdoc is showing the correct value is output by the vertex shader:

But I dont think there is a way to inspect the values received by the fragment shader.

[kvark]

You can debug the fragment shaders.

[rukai]

there was a history and debug button but they were greyed out, I think its not supported yet or something, its strange because the docs indicate it should be supported

[kvark]

I see the same thing, unfortunately :(
What GPU are you testing this on?

[kvark]

This looks like a driver bug. Running your sample code on Windows/AMD shows the correct result.

[kvark]

@rukai are you on NVidia proprietary driver by any chance? What's the GPU?

[kvark]

I submitted this issue to NVidia Vulkan support and already got a weak confirmation that this may be fixed soon 🚀
Unfortunately, no reference number. I'll update once I know more.

[rukai]

Yes the card is Nvidia GTX 960 and I am using the Nvidia proprietary driver.
I had incorrectly assumed it wasnt a driver bug considering it worked with glsl.
Thanks for reporting the issue to Nvidia!