Ok I checked, in the version from my original email we have:
callq _swift_retain
movq %rbx, %rdi
callq __TF4main3barFVS_3FooT_
movq %rbx, %rdi
callq _swift_release
Which makes foo obviously non-uniquely referenced inside bar, preventing
COW optimization. So it seems the optimizer somehow doesn't notice it's
dead here and doesn't transfer ownership (?).
On Fri, Apr 1, 2016 at 5:49 PM, Patrick Pijnappel <
patrickpijnap...@gmail.com> wrote:
> The modified version doesn't seem to change any of the results (on -O or
> -Onone). Note that the problem is that it's *not* uniquely referenced
> inside bar where it actually should be – that would mean that ownership is
> currently not directly transferred right?
>
> On Fri, Apr 1, 2016 at 10:27 AM, Joe Groff <jgr...@apple.com> wrote:
>
>>
>> > On Mar 31, 2016, at 4:21 PM, Joe Groff via swift-dev <
>> swift-dev@swift.org> wrote:
>> >
>> >>
>> >> On Mar 31, 2016, at 3:58 PM, Patrick Pijnappel via swift-dev <
>> swift-dev@swift.org> wrote:
>> >>
>> >> In trying to implement a COW type, but I'm running into problems with
>> isUniqueReferenced breaking in even fairly simple cases. For example (with
>> -O) the code below prints "bar: false", commenting out the print in test()
>> makes it print "bar: true", and removing the var parameter var foo: Foo and
>> using var foo = foo instead breaks it again. Am I doing something wrong
>> here?
>> >>
>> >> class FooStorage { var x: Int = 0 }
>> >>
>> >> struct Foo { var storage = FooStorage() }
>> >>
>> >> func bar(var foo: Foo) {
>> >> print("bar: \(isUniquelyReferencedNonObjC(&foo.storage))")
>> >> }
>> >>
>> >> func test() {
>> >> var foo = Foo()
>> >> print("test: \(isUniquelyReferencedNonObjC(&foo.storage))")
>> >> bar(foo)
>> >> }
>> >>
>> >> test()
>> >
>> > You're not doing anything wrong, this is just the ARC optimizer at
>> work. `foo` inside `test` is dead after the call to `bar`, so ownership is
>> transferred directly to `bar`'s parameter.
>>
>> If you want to ensure that `foo` remains alive despite this, you can use
>> `withExtendedLifetime`:
>>
>> class FooStorage { var x: Int = 0 }
>>
>> struct Foo { var storage = FooStorage() }
>>
>> func bar(var foo: Foo) {
>> print("bar: \(isUniquelyReferencedNonObjC(&foo.storage))")
>> }
>>
>> func test() {
>> var foo = Foo()
>> print("test: \(isUniquelyReferencedNonObjC(&foo.storage))")
>> withExtendedLifetime(foo) {
>> bar(foo)
>> }
>> }
>>
>> -Joe
>
>
>
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