Implement Fallback to Vulkan for MoltenVK

[SheepYhangCN]

Implemented fallback_to_vulkan in macOS

[stuartcarnie]

Looks good! One required change to resolve a memory leak.

[clayjohn]

Are there any iOS devices that support MoltenVK, but not our Metal backend?

[bruvzg]

Probably no, it's the opposite if I remember, A11 for Metal and A12 for Vulkan. And on macOS, every Apple Silicon device support both MoltenVK and Metal, and we do not have x86 Metal support.

[stuartcarnie]

That's correct – I did additional work in #99820 ensures A11 Bionic support for Metal, which is better than we had for MoltenVK.

[bruvzg]

So the only use case is fallback from Vulkan to Metal on A11, assuming it will work (most likely MotenVK will crash instead).

[stuartcarnie]

@bruvzg that's a good point! We actually want the reverse; fallback to Metal from Vulkan, although I don't know if MoltenVK fails if the device is below A12?

[bruvzg]

I do not have anything to test it on, but from previous experience it's either crash on init or will start normally and fail during shader compilation. I do not think I have ever seen rendering_context creation fail in any conditions other than completely missing support for specific rendering backend (like no Vulkan on ARM/Windows).

[stuartcarnie]

Presumably we still want the feature; however, the code could be implemented a little differently to match the behaviour of our implementation.

• Metal for iOS has a lower device requirement than MoltenVK, so if Metal fails, MoltenVK will fail too; therefore, the feature should not be necessary for iOS
• Metal is not available or compiled into x86_64 binaries, so it wouldn't even try to initialise a Metal driver; therefore, fallback should always occur.
• All Apple Silicon Macs support Metal, so it should never need to fallback to Vulkan, meaning the feature is not required for Apple Silicon (arm64 builds)

In other words,

• Remove implementation for iOS
• Change implementation for macOS to something like:

rendering_driver = p_rendering_driver;

#if defined(RD_ENABLED)
#if defined(VULKAN_ENABLED)
#if defined(__x86_64__)
	bool fallback_to_vulkan = GLOBAL_GET("rendering/rendering_device/fallback_to_vulkan");
	// Metal rendering driver not available on Intel
	if (fallback_to_vulkan && rendering_driver == "metal") {
		rendering_driver = "vulkan";
		OS::get_singleton()->set_current_rendering_driver_name(rendering_driver);
	}
#endif

	if (rendering_driver == "vulkan") {
...

[SheepYhangCN]

All fixed I think

[stuartcarnie]

LGTM!

[Calinou]

Code looks good to me.

[Calinou]

I'm wondering if we should get this in for 4.4, since the Metal rendering driver was made the default already. Otherwise, projects will stop working on Intel Macs by default if they use Forward+ or Mobile.

[stuartcarnie]

@Calinou I agree – we don't want to break or alienate users on Intel hardware.

[bruvzg]

For the reference, this change should not be necessary in normal conditions:

• If project is edited/exported from Apple Silicon editor, it won't save metal in the project.godot since it default value and default values are omitted, so x86_64 executable will use its default (vulkan) without any additional checks.
• If you try to force Metal render on x86_64 with command line argument (--rendering-driver metal), it will fail during argument validation in the Main and never get to this code.
• But it is possible to manually edit project.godot, and explicitly specify metal, and this code will work as expected.

[akien-mga]

I squashed the commits and clarified in the docs that this is only for macOS x86_64.

[Repiteo]

Thanks!