On Tue, Oct 18, 2016 at 3:13 PM, Joshua Liebow-Feeser <he...@joshlf.com> wrote:
>
> On Tue, Oct 18, 2016 at 2:53 PM, Ian Lance Taylor <i...@golang.org> wrote:
>>
>> On Tue, Oct 18, 2016 at 2:45 PM, Joshua Liebow-Feeser <he...@joshlf.com>
>> wrote:
>> >
>> > On Tue, Oct 18, 2016 at 2:40 PM, Joshua Liebow-Feeser <he...@joshlf.com>
>> > wrote:
>> >>
>> >>
>> >>
>> >> On Tue, Oct 18, 2016 at 2:16 PM, Ian Lance Taylor <i...@golang.org>
>> >> wrote:
>> >>>
>> >>> On Tue, Oct 18, 2016 at 12:30 PM, Joshua Liebow-Feeser
>> >>> <he...@joshlf.com>
>> >>> wrote:
>> >>> >
>> >>> > I'm playing around with implementing a wait-free channel in the
>> >>> > runtime
>> >>> > package, and as part of this, it'd be really nice to have
>> >>> > double-word
>> >>> > compare-and-swap (CAS). Barring that, however, for my purposes, it
>> >>> > would
>> >>> > actually be fine to have a one-word value that encodes both a
>> >>> > pointer
>> >>> > and
>> >>> > some extra information using bit packing. The problem, though, is
>> >>> > that
>> >>> > if I
>> >>> > store this value as, for example, a uintptr, the GC may not realize
>> >>> > that
>> >>> > it's a pointer. So my question is: are there any bits in a pointer
>> >>> > which,
>> >>> > when modified, won't mess with the GC? Note that since this is
>> >>> > implemented
>> >>> > in the runtime, I'm totally OK with relying on behavior specific to
>> >>> > the
>> >>> > current GC implementation.
>> >>>
>> >>> See runtime/lfstack*.go.
>> >>
>> >> Awesome, thanks!
>> >
>> >
>> > Actually, quick follow-up. I noticed that the lfstack implementation
>> > side-steps the GC issue by just not keeping pointers. That might work
>> > for me
>> > if I just store runtime.g pointers, but that raises another question:
>> > can
>> > the GC ever free g's, or are they just explicitly freed when a goroutine
>> > quits? That is, is it safe for me to store a pointer/counter hybrid like
>> > in
>> > lfstack - where that pointer is a *g - and assume that the GC won't
>> > collect
>> > the g from out from under me?
>>
>> For the specific case of a g, this is safe at the moment.  The current
>> Go runtime caches all g's and never releases them.  See gfget and
>> gfput in runtime/proc.go.
>
>
> OK great. And they won't ever be moved? (Come to think of it, is pointer
> rewriting only ever a thing on the stack?)

Yes, with the current toolchain, objects in the heap are never moved.

(Obviously no guarantees that this will always be true.)

Ian

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