> On Jan 3, 2017, at 10:04 PM, Ray Fix via swift-users <swift-users@swift.org> > wrote: > > > There was a great talk at WWDC 2016 about internals and Swift performance. > https://developer.apple.com/videos/play/wwdc2016/416/ > <https://developer.apple.com/videos/play/wwdc2016/416/> > > At one point, Arnold Schwaighofer says, "Copying of large values incurs heap > allocation.” What isn’t clear to me here is if he is still referring to just > existential container types or in just large values in general. > > My understanding is a value can get put on the heap if: > > 1) it is passed through a protocol and exceeds small size (3 words of storage) > 2) it is captured by an escaping closure, and needs to extend lifetime > > Suppose I make a large struct, and pass it as a parameter, is there a point > where the compiler says, “Okay wise-guy, you are going on the heap now.” Can > this be detected with MemoryLayout<BigStruct>.size? I have tried a couple > experiments where I make beefy (1kB) structs in the playground but I never > see the size suddenly dropping to a pointer size. > > Any insights you have are greatly appreciated! 😃✨
No, Swift currently never implicitly puts parts of large structs on the heap. The heap allocation would come from the struct needing to be stored in something heap-allocated, such as a protocol container or capture box, like you said. -Joe
_______________________________________________ swift-users mailing list swift-users@swift.org https://lists.swift.org/mailman/listinfo/swift-users
