> On May 6, 2017, at 12:36 PM, John McCall via swift-evolution
> <swift-evolution@swift.org <mailto:swift-evolution@swift.org>> wrote:
>
>
>> On May 6, 2017, at 1:11 PM, Félix Cloutier via swift-evolution
>> <swift-evolution@swift.org <mailto:swift-evolution@swift.org>> wrote:
>>
>> 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?
When static analysis can determine that a swap is always a noop, I can’t see
any reason not to flag it as an error.
But Félix’s question was also about the runtime case. And he has a good point.
Here’s a compelling example where allowing the noop swap would make sense:
extension Array {
mutating func shuffle() {
for i in indices {
let j = i + Int(arc4random_uniform(UInt32(count - i)))
swap(&self[i], &self[j])
}
}
}
In this code:
It’s necessary to allow j to sometimes equal i to get a uniform distribution of
random permutations.
Static analysis can’t determine that the noop occurs sometimes but not always.
Adding a check for i == j adds a branch condition to a tight loop. More
importantly, it’s easy to forget this check since static analysis won’t flag
it. It’s just a hidden land mine, which seems contrary to Swift’s “safety
first” aesthetic.
Putting all that together, it would probably be more Swift-like to have swap()
be safe, and if necessary add an unsafeSwap() func that fails on the noop case
and thus avoids the cost of the check in optimized code.
Note that the code above fails even under Swift 3, which explicitly disallows
swapping with a runtime check (“fatal error: swapping a location with itself is
not supported”). So nothing here to undermine the SE-0176; the proposal isn’t
introducing a problem that doesn’t already exist.
Cheers, P
> John.
>
>>
>>> Le 2 mai 2017 à 13:07, Ben Cohen via swift-evolution
>>> <swift-evolution@swift.org <mailto:swift-evolution@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
>>>
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>>
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