Uniform num broken on Mac OS 12.6 arm64/Vulkan by #2883/#2885

[jdah]

Describe the bug
Shaders which declare a uniform array uniform vec4 u_array[3] now report a num (from bgfx::getUniformInfo) of 1 instead of 3 as expected. I have tested in my own code (by reverting the change) and the issue came from #2883 (PR #2885) where regCount was replaced with num to fix array sizes.

To Reproduce

1. declare an array uniform vec4 ...[n] somewhere
2. check num with bgfx::getUniformUnfo
3. num != n

Expected behavior
num == n

Additional context
Tested on Mac OS Monterey/12.6, M1 Pro/arm64, MoltenVK (Vulkan backend!), latest bgfx

will hopefully come back with further details/proposed fix.

[bkaradzic]

cc @cloudwu

[cloudwu]

I think the reason is that the num hasn't encoded into shader file correctly for Vulkan.

https://github.com/bkaradzic/bgfx/blob/master/tools/shaderc/shaderc_spirv.cpp#L655

It should be un.num = uint16_t(program->getUniformArraySize(ii)); like other backend :

metal : https://github.com/bkaradzic/bgfx/blob/master/tools/shaderc/shaderc_metal.cpp#L455
hlsl : https://github.com/bkaradzic/bgfx/blob/master/tools/shaderc/shaderc_hlsl.cpp#L373
glsl : https://github.com/bkaradzic/bgfx/blob/master/tools/shaderc/shaderc_glsl.cpp#L207

I read the history, and found the PR #2421 changed this behavior. (It's strange that this PR changed Vulkan backend only)
We can revert #2421 to solve this issue, but I think there would be a better solution @bkaradzic (to support uniform num > 255)

---

PR #2885 use num instead of regCount , so bgfx::getUniformUnfo() will returns 1 ( bx::max<uint16_t>(1, _num)) rather than 3.

But if you decalre an array of matrix uniform mat4 ...[3] (reverting PR #2885) , the regCount would be 12. it's not correct, too.

[bkaradzic]

but I think there would be a better solution @bkaradzic (to support uniform num > 255)

Yeah, I need to get some better way of handling this reflection, and figure out how to unit test it.

[bkaradzic]

@jdah What changes you needed to add to bgfx to build with MoltenVK?

[jdah]

None, IIRC - BGFX is set to target Vulkan and then you use the MoltenVK dynamic library (from (MoltenVK Path)/dylib/macOS/libMoltenVK.dylib) alongside the application

[rchen-adb]

Related to the uniform array size limitation, I think there's some slight inconsistent behavior between the Metal and Vulkan backend for predefined uniforms. In Metal, if your array is sized > 255, it is handled because the m_count value comes from the Metal reflection. In Vulkan, the value wraps around because it comes from regCount which is limited to uint8 here, since it is equal to Uniform::num.

Just mentioning this because currently you will see predefined arrays sized > 255 work fine on Metal but break in Vulkan. This happens ie if you define BGFX_CONFIG_MAX_BONES to be 256.