[swift-evolution] [swift-evolution-announce] [Review] SE-0176: Enforce Exclusive Access to Memory

[Paul Cantrell]

I like the spirit of the proposal. It leaves me a bit uneasy because the practical ramifications are difficult to fully anticipate. Here follow some questions / concerns for the proposal authors.

(Apologies if these were answered somewhere deep in a discussion thread; it’s the end of the semester and I’ve not kept up with the list.)

• • •

First, have you done any systematic exploration of how impactful this rule is? The proposal states in several places that it hopes/expects violations will be rare, but is that empirical or speculative?

I tried playing around with the implications, thought I’d found a problematic example by writing an in-place Array.shuffle() method — and then was pleased to realize that the proposal already covered my example with swapAt(). In half an hour, I couldn’t come up with something the proposal disallows that it clearly should allow.

Still, the implications of a rule like this are difficult to anticipate. It would be interesting to know how many errors it flags in the Swift source compatibility suite, for example. Is that something we could do before going all-in on a fully robust implementation?

• • •

Second, what are the performance implications of dynamic exclusivity enforcement? Introducing overhead to something as basic as variable access seems dicey. Will it kill “C-like” performance for object properties? For closure vars? Will “accesses underway” flags add to the memory footprint of every object & every closure? You write that your “hope is that…dynamic access-tracking [will be] cheap enough to enable by default;” have you proved this out with a prototype implementation yet?

Or maybe this check is disabled in optimized builds, and I’m fussing over nothing?

Cheers,

Paul


> On May 2, 2017, at 3:07 PM, Ben Cohen <ben_cohen at apple.com> wrote:
> 
> Hello Swift community,
> 
> The review of SE-0176: "Enforce Exclusive Access to Memory" begins now and runs through May 8, 2017.
> 
> The proposal is available here:
> 
> https://github.com/apple/swift-evolution/blob/master/proposals/0176-enforce-exclusive-access-to-memory.md <https://github.com/apple/swift-evolution/blob/master/proposals/0176-enforce-exclusive-access-to-memory.md>
> Reviews are an important part of the Swift evolution process. All reviews should be sent to the swift-evolution mailing list at:
> 
> https://lists.swift.org/mailman/listinfo/swift-evolution <https://lists.swift.org/mailman/listinfo/swift-evolution>
> or, if you would like to keep your feedback private, directly to the review manager. 
> 
> When replying, please try to keep the proposal link at the top of the message:
> 
> Proposal link:
> 
> https://github.com/apple/swift-evolution/blob/master/proposals/0176-enforce-exclusive-access-to-memory.md <https://github.com/apple/swift-evolution/blob/master/proposals/0176-enforce-exclusive-access-to-memory.md>
> Reply text
> 
> Other replies
> 
>  <https://github.com/apple/swift-evolution#what-goes-into-a-review-1>
> What goes into a review?
> 
> The goal of the review process is to improve the proposal under review through constructive criticism and, eventually, determine the direction of Swift. When writing your review, here are some questions you might want to answer in your review:
> 
> What is your evaluation of the proposal?
> Is the problem being addressed significant enough to warrant a change to Swift?
> Does this proposal fit well with the feel and direction of Swift?
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> 
> https://github.com/apple/swift-evolution/blob/master/process.md <https://github.com/apple/swift-evolution/blob/master/process.md>
> 
> Thanks,
> Ben Cohen
> Review Manager
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