[macOS] Restore gl_HelperInvocation workaround for Intel macs.

[bruvzg]

Restores workaround removed in #102552

Fixes #107753

If I understand correctly, it's not adding any new defines, so should not create additional shader variants for baker and cause any issues.

[stuartcarnie]

Should be fine for runtime shader generation on Intel Macs, but should be noted that when baking on Linux or Windows, MOLTENVK_USED won't be defined, so it will generate shaders that may not work on Intel macs using MoltenVK.

Is it specific GPUs on Intel Macs that don't work?

[bruvzg]

but should be noted that when baking on Linux or Windows, MOLTENVK_USED won't be defined

Same with NO_IMAGE_ATOMICS which is defined only on macOS and iOS and used in many places, so baking shaders for macOS/iOS from a non-mac system is likely already broken (for both Metal and Vulkan).

Is it specific GPUs on Intel Macs that don't work?

No idea, but the issue was recently encountered on AMD GPU and previously on Intel GPUs, so I assume it's a generic issue with all Intel macs.

[stuartcarnie]

Same with NO_IMAGE_ATOMICS which is defined only on macOS and iOS and used in many places, so baking shaders for macOS/iOS from a non-mac system is likely already broken (for both Metal and Vulkan).

Good point – I need to test NO_IMAGE_ATOMICS again. Some of that was to do with SPIRV-Cross issues, which I haven't revisited.

I noticed @akien-mga is upgrading Vulkan SDKs in #107773. I could do SPIRV-Cross separately and fix the Metal compilation (not hard). @akien-mga has resolved it!

[stuartcarnie]

In order to address that, additional context needs to be passed through the renderer, so that it can bake for different targets / platforms with different feature sets. I already started that in the Metal shader baker code, where I can pass OS + GPU to govern how the shaders are generated and compiled.

[akien-mga]

I believe __x86_64__ should always work, no? So __x86_64 is likely redundant and not necessary.

[bruvzg]

In order to address that, additional context needs to be passed through the renderer, so that it can bake for different targets / platforms with different feature sets. I already started that in the Metal shader baker code, where I can pass OS + GPU to govern how the shaders are generated and compiled.

In case on Metal it seems to be doing it on already compiled SPIRV code, this probably won't work with define (or will be overcomplicated). So it probably should do it by enabling/disabling shader variants on a source level, and ShaderBakerExportPlugin already do have information about target platform, I guess it only need a way to selectively pick variants depending on platform.