Weird behavior with large uniform buffers with the Vulkan backend

[tcdude]

Describe the bug Once uniform buffers in Vulkan get larger than a specific size (around 256 floats in total) the fragment shader stops to render/blend correctly. In the linked repro repo below there are two shader pairs that only differ in two ways:

1. bad.frag.glsl has one more function than good.frag.glsl
2. bad.frag.glsl has a total uniform buffer of 259 floats, while good.frag.glsl has 255

To show that bad.frag.glsl actually renders almost correctly, just fails to set the proper alpha value, one can add a line to the main function setting the alpha of frag_color to 1 and see that every fragment actually has the correct color value, just a wrong alpha value of 0.

Changing the value of this define does not help with the issue.

To Reproduce Steps to reproduce the behavior:

1. Clone this repo
2. Compile the project using the Vulkan graphics backend
3. Run the repro binary
4. Compile using the Direct3D12 graphics backend

Expected behavior Same result from these two draw calls, regardless of total uniform buffer size and (G5) backend. Otherwise possibly a warning/error if the uniform buffer is too large to handle.

Screenshots With Vulkan the output looks like this:

If compiled using Direct3D12 the correct output looks like this:

Execution Environment:

• Host system (where you compile your code): Win 10 and Linux x64
• Target system (where you run your code): Win 10 and Linux x64
• IDE and/or compiler used: msvc/clang
• Kinc revision: cb50ced0
• Kinc build output:

Executing task: C:\code\vulkannodrawrepro/Kinc/make.bat -g vulkan --compile --debug 

Using Kinc (cb50ced0) from C:\code\vulkannodrawrepro\Kinc
kfile found.
Creating Windows project files.
Compiling shader 1 of 7 (bad.frag).
Compiling shader 2 of 7 (bad.vert).
Compiling shader 3 of 7 (good.frag).
Compiling shader 4 of 7 (good.vert).
Compiling shader 5 of 7 (tex.frag).
Compiling shader 6 of 7 (g1.frag).
Compiling shader 7 of 7 (g1.vert).
Compiling...
**********************************************************************
** Visual Studio 2022 Developer Command Prompt v17.5.5
** Copyright (c) 2022 Microsoft Corporation
**********************************************************************
[vcvarsall.bat] Environment initialized for: 'x64'
MSBuild version 17.5.1+f6fdcf537 for .NET Framework
Build time: 0m 7s
Done.

• Application output (if it runs): When running on Windows

-------------------------------------------------------------------
You may only use the C/C++ Extension for Visual Studio Code
with Visual Studio Code, Visual Studio or Visual Studio for Mac
software to help you develop and test your applications.
-------------------------------------------------------------------
Running with Vulkan validation layers enabled.
[OBS] Failed to open pipe
[OBS] graphics-hook.dll loaded against process: ShaderTest.exe
[OBS] (half life scientist) everything..  seems to be in order
The program '[26544] ShaderTest.exe' has exited with code 0 (0x0).

Additional context Since uniform buffer size is a limiting factor on OpenGL/GLES backends regardless, I currently use a texture instead of a uniform buffer to successfully work around this issue. But the behavior difference is interesting and if this can be fixed I would probably switch back to using uniform buffers instead.