[SR-9701] Stack allocated Objective-C no escape closures

[aschwaighofer]

Previous ID	SR-9701
Radar	None
Original Reporter	@aschwaighofer
Type	Improvement

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Improvement
Assignee	None
Priority	Medium

md5: fd60c5edf6ddc3d03a27ac1f2a14393b

Issue Description:

When we pass closures to @NoEscape objective c block arguments the closures are allocated on the heap. We should also stack allocate them.

[belkadan]

The Objective-C convention is actually fine with you stack-allocating all block arguments; it's considered the callee's responsibility to Block_copy them. cc @rjmccall

https://clang.llvm.org/docs/BlockLanguageSpec.html
https://clang.llvm.org/docs/AutomaticReferenceCounting.html#blocks

[rjmccall]

Correct. We don't do this only because we're being cautious about lifetimes. If we had a reliable way of detecting that the copy is unnecessary, it would be a significant win.

Clang has actually implemented an optimization where it eliminates the copying logic from `noescape` blocks entirely, essentially generating them as global blocks that happen to be allocated on the stack.