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?)

>
> Ian
>

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