On Thu, Jul 12, 2018 at 11:03 PM, Robert O'Callahan <rob...@ocallahan.org> wrote:
> On Fri, Jul 13, 2018 at 11:40 AM, Steve Fink <sf...@mozilla.com> wrote: > > > On 07/12/2018 04:27 PM, Cameron McCormack wrote: > > > >> On Fri, Jul 13, 2018, at 6:51 AM, Kris Maglione wrote: > >> > >>> I actually have a patch sitting around with helpers to make it super > >>> easy to > >>> use smart pointers as tagged pointers :) I never wound up putting it up > >>> for > >>> review, since my original use case went away, but it you can think of > any > >>> specific cases where it would be useful, I'd be happy to try and get it > >>> landed. > >>> > >> Speaking of tagged pointers, I've used lower one or two bits for tagging > >> a number of times, but I've never tried packing things into the high > bits > >> of a 64 bit pointer. Is that inadvisable for any reason? How many bits > >> can I use, given the 64 bit platforms we need to support? > >> > > > > JS::Value makes use of this. We preserve the bottom 47 bits, but that's > > starting to be problematic as some systems want 48. So, stashing stuff > into > > the high 16 bits is pretty safe! > > > > 57-bit address space support is coming for x86-64. > > Rob > Last I heard the 48-bit assumption had become so pervasive (and useful) that OS devs were planning to only expose this to processes that explicitly opted into it. In the case of linux, you would have to explicitly request high-address pages to start receiving them: https://lwn.net/Articles/717293/ I always assumed firefox simply wouldn't ever opt into high-addresses, unless fission mem-shrink doesn't work out and firefox suddenly needs 300TB of RAM ;p _______________________________________________ dev-platform mailing list dev-platform@lists.mozilla.org https://lists.mozilla.org/listinfo/dev-platform
