Re: [swift-users] Simultaneous access to global tuple members
Thank you for the clarification.
> On 8. Aug 2017, at 18:39, Guillaume Lessard
> wrote:
>
>
>> On Aug 8, 2017, at 09:10, Martin R wrote:
>>
>> In that example the tuple is a (stored) property of a class, not a global
>> variable. And why does it crash for a global variable, but not for a local
>> variable in a function?
>
> In the case of a local variable in a function, the compiler can statically
> prove that there is no simultaneous access, and using `swap` is allowed. With
> a global variable, the compiler can’t statically prove exclusive access.
> (it seems silly with your simple example, but the system doesn’t try static
> enforcement with global variables.)
>
> Here’s another modification which traps at runtime:
>
> ***
> import Dispatch
>
> func foo() {
> var tuple = (1, 2)
>
> let q = DispatchQueue(label: "queue")
> q.async { swap(&tuple.0, &tuple.1) }
> q.sync {}
>
> print(tuple)
> }
>
> foo()
> ***
>
> Guillaume Lessard
>
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Re: [swift-users] Simultaneous access to global tuple members
> On Aug 8, 2017, at 09:10, Martin R wrote:
>
> In that example the tuple is a (stored) property of a class, not a global
> variable. And why does it crash for a global variable, but not for a local
> variable in a function?
In the case of a local variable in a function, the compiler can statically
prove that there is no simultaneous access, and using `swap` is allowed. With a
global variable, the compiler can’t statically prove exclusive access.
(it seems silly with your simple example, but the system doesn’t try static
enforcement with global variables.)
Here’s another modification which traps at runtime:
***
import Dispatch
func foo() {
var tuple = (1, 2)
let q = DispatchQueue(label: "queue")
q.async { swap(&tuple.0, &tuple.1) }
q.sync {}
print(tuple)
}
foo()
***
Guillaume Lessard
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Re: [swift-users] Simultaneous access to global tuple members
In that example the tuple is a (stored) property of a class, not a global variable. And why does it crash for a global variable, but not for a local variable in a function? (Sorry if I am overlooking something obvious.) Martin > On 8. Aug 2017, at 17:50, Guillaume Lessard > wrote: > > >> On Aug 8, 2017, at 01:11, Martin R via swift-users >> wrote: >> >> SE-0176 Enforce Exclusive Access to Memory >> (https://github.com/apple/swift-evolution/blob/master/proposals/0176-enforce-exclusive-access-to-memory.md) >> states that "Accesses to different stored properties of a struct or >> different elements of a tuple are allowed to overlap." >> >> [snip] >> >> Is there anything special with simultaneous access to members of a global >> tuple, or is this a bug? > > Not a bug. The very next sentence after the one you quoted: “However, note > that modifying part of a value type still requires exclusive access to the > entire value”. The first example in that section > (https://github.com/apple/swift-evolution/blob/master/proposals/0176-enforce-exclusive-access-to-memory.md#value-types) > is similar to your example; it trips the dynamic enforcement mechanism for > the same reason. The second example shows a possible workaround. > > Sincerely, > Guillaume Lessard > ___ swift-users mailing list swift-users@swift.org https://lists.swift.org/mailman/listinfo/swift-users
Re: [swift-users] Simultaneous access to global tuple members
> On Aug 8, 2017, at 01:11, Martin R via swift-users > wrote: > > SE-0176 Enforce Exclusive Access to Memory > (https://github.com/apple/swift-evolution/blob/master/proposals/0176-enforce-exclusive-access-to-memory.md) > states that "Accesses to different stored properties of a struct or > different elements of a tuple are allowed to overlap." > > [snip] > > Is there anything special with simultaneous access to members of a global > tuple, or is this a bug? Not a bug. The very next sentence after the one you quoted: “However, note that modifying part of a value type still requires exclusive access to the entire value”. The first example in that section (https://github.com/apple/swift-evolution/blob/master/proposals/0176-enforce-exclusive-access-to-memory.md#value-types) is similar to your example; it trips the dynamic enforcement mechanism for the same reason. The second example shows a possible workaround. Sincerely, Guillaume Lessard ___ swift-users mailing list swift-users@swift.org https://lists.swift.org/mailman/listinfo/swift-users
[swift-users] Simultaneous access to global tuple members
SE-0176 Enforce Exclusive Access to Memory
(https://github.com/apple/swift-evolution/blob/master/proposals/0176-enforce-exclusive-access-to-memory.md)
states that "Accesses to different stored properties of a struct or different
elements of a tuple are allowed to overlap."
And indeed this compiles and runs as expected (Xcode 9 beta 4):
// main.swift
func foo() {
var tuple = (1, 2)
swap(&tuple.0, &tuple.1)
print(tuple)
}
foo()
However, if the tuple is a global variable, it crashes with a runtime exception:
// main.swift
var tuple = (1, 2)
swap(&tuple.0, &tuple.1)
print(tuple)
Simultaneous accesses to 0x100401cb0, but modification requires exclusive
access.
Previous access (a modification) started at tupleaccess`main + 76
(0x115cc).
Current access (a modification) started at:
0tupleaccess0x000100335b60
swift_beginAccess + 605
1tupleaccess0x00011580 main + 110
2libdyld.dylib 0x7fffa7284234 start + 1
Is there anything special with simultaneous access to members of a global
tuple, or is this a bug?
Regards, Martin
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