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 -- You received this message because you are subscribed to the Google Groups "golang-nuts" group. To unsubscribe from this group and stop receiving emails from it, send an email to golang-nuts+unsubscr...@googlegroups.com. For more options, visit https://groups.google.com/d/optout.
