[swift-users] What is "binding" memory?

Andrew Trick atrick at apple.com
Wed Nov 2 17:25:58 CDT 2016

> On Nov 2, 2016, at 12:58 PM, Dave Abrahams via swift-users <swift-users at swift.org> wrote:
>> At the top of the migration guide is a link to the memory model explanation:
>> https://github.com/apple/swift-evolution/blob/master/proposals/0107-unsaferawpointer.md#memory-model-explanation
>> "A memory location's bound type is an abstract, dynamic property of the memory used to formalize
>> type safety.”
>> I’m not sure I like the “prepares the memory” language myself. Binding
>> memory communicates to the compiler that the memory locations are safe
>> for typed access. Nothing happens at runtime--until someone writes a
>> type safety sanitizer. 
> Well, that's a slight overstatement IMO.  Sanitizers aside, the main
> reason for these binding operations is that if you leave them out,
> something different *will* happen at runtime... something that will make
> your code do the wrong thing.
> What I would say is that binding the memory has no immediate runtime
> cost... but it's absolutely required if you want your program to behave
> (and sometimes behaving correctly is a little slower than misbehaving).

Good clarification. I really did not mean to imply that binding memory to a type has no effect on runtime behavior. Taken out of context, “nothing happens at runtime” is quite an understatement.

The original poster seemed to have the impression that the operation of binding memory itself might affect program state, independent of any compiler optimization. I want to make it clear that a call to bindMemory(to:capacity:) has no observable runtime side effects at the point of the call. But I need to throw in an exemption for future sanitizers.

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