There's an autoreleasepool around the per-frame code - which seems to be standard procedure for Metal code - so any object within a render frame that has been created with autorelease will be released at the end of a frame.
Metal has a couple of 'frame transient' objects (like MTLRenderPassDescriptor and MTLRenderCommandEncoder) which have a new instance created in each frame, and I guess the main job of the autoreleasepool is to clean up those transient objects (along with any other 'short-lived-junk' that might be returned from Metal API methods).
And my guess for why this is still needed in the age of ARC: I guess that ARC has a 'autorelease-blindness', e.g. it cannot figure out at compile time what objects are registered with autorelease pools (especially when the objects are passed across DLL boundaries) - it can only add retain/release calls on top. Just speculation on my part though.
Metal has a couple of 'frame transient' objects (like MTLRenderPassDescriptor and MTLRenderCommandEncoder) which have a new instance created in each frame, and I guess the main job of the autoreleasepool is to clean up those transient objects (along with any other 'short-lived-junk' that might be returned from Metal API methods).
And my guess for why this is still needed in the age of ARC: I guess that ARC has a 'autorelease-blindness', e.g. it cannot figure out at compile time what objects are registered with autorelease pools (especially when the objects are passed across DLL boundaries) - it can only add retain/release calls on top. Just speculation on my part though.