Defers can happen in a loop, so you need a stack for correctness. This change is that the compiler will now automatically inline the defer-ed call so they appear much the same as a direct call at the end of a function if they happen at most once.

The full design doc is here: https://github.com/golang/proposal/blob/master/design/34481-...

> Go 1.13 implements the defer statement by calling into the runtime to push a "defer object" onto the defer chain. Then, in any function that contains defer statements, the compiler inserts a call to runtime.deferreturn at every function exit point to unwind that function's defers.

> We propose optimizing deferred calls in functions where every defer is executed at most once (specifically, a defer may be on a conditional path, but is never in a loop in the control-flow graph). In this optimization, the compiler assigns a bit for every defer site to indicate whether that defer had been reached or not. The defer statement itself simply sets the corresponding bit and stores all necessary arguments in specific stack slots. Then, at every exit point of the function, the compiler open-codes each deferred call, protected by (and clearing) each corresponding bit.

defer-ed calls are (in a very simplified way) pushed on a "stack" of methods to be called just before parent function exists. The mechanics of retrieval-call-repeat_if_more_available are cheap, but not free. They have reduced costs of these interactions.