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

[Félix Cloutier]

I guess that the example doesn't make a lot of sense outside of collections, and the proposal already says that swap needs to be implemented on collections, but consider the Fisher-Yates shuffle:
-- To shuffle an array a of n elements (indices 0..n-1):
for i from n−1 downto 1 do
     j ← random integer such that 0 ≤ j ≤ i
     exchange a[j] and a[i]
I can come up with other contrived ways to force this into a dynamic analysis situation (but I am not convinced that I would ever see them in real code).

Also, I'm not too solid on how inout works, especially for properties without "physical storage". My understanding is that in some cases, Swift will get a direct pointer to what needs to be modified, and in others, it'll get a pointer to a temporary value and then call a setter with that temporary as the value argument. Does that sound right?

> Le 6 mai 2017 à 10:36, John McCall <rjmccall at apple.com> a écrit :
> 
>> 
>> On May 6, 2017, at 1:11 PM, Félix Cloutier via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> wrote:
>> 
>> I have one concern and one general comment.
>> 
>> General comment: there's a switch to turn off overflow checks for performance reasons. It seems to me that this check is going to be much more expensive than an overflow check, and it's certainly not much more likely to trigger in a release build after some amount of testing, so my opinion would be that there should be a way to turn it off globally. To me, the checking is UBSan behavior more than a language feature.
> 
> -Ounchecked should disable this check, yes.  As for "UBSan" vs. language, the general policy in Swift is to enforce its language rules by default, except in the cases of race conditions (where it would be prohibitive) and explicitly unsafe features (which are intended in part to allow things that are not possible under the normal language rules).
> 
>> Concern: `swap` is quoted a lot for a method that would break under this rule, but as it happens, `swap` with the same value on both sides is (should be) a no-op. Is there a way to not trip the static or dynamic checkers in well-defined cases like that one? Is there any way to check that two inout parameters refer to the same value?
> 
> The only reasonable way to do this is statically, and why would you call 'swap' statically with the same l-value for both arguments?
> 
> John.
> 
>> 
>>> Le 2 mai 2017 à 13:07, Ben Cohen via swift-evolution <swift-evolution at swift.org <mailto:swift-evolution at swift.org>> a écrit :
>>> 
>>> 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?
>>> If you have used other languages or libraries with a similar feature, how do you feel that this proposal compares to those?
>>> How much effort did you put into your review? A glance, a quick reading, or an in-depth study?
>>> More information about the Swift evolution process is available at:
>>> 
>>> 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
>>> 
>>> _______________________________________________
>>> swift-evolution mailing list
>>> swift-evolution at swift.org <mailto:swift-evolution at swift.org>
>>> https://lists.swift.org/mailman/listinfo/swift-evolution <https://lists.swift.org/mailman/listinfo/swift-evolution>
>> 
>> _______________________________________________
>> swift-evolution mailing list
>> swift-evolution at swift.org <mailto:swift-evolution at swift.org>
>> https://lists.swift.org/mailman/listinfo/swift-evolution <https://lists.swift.org/mailman/listinfo/swift-evolution>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <https://lists.swift.org/pipermail/swift-evolution/attachments/20170506/85c2476c/attachment.html>