On 10/11/2019 4:32:11 PM, Richard Elling <richard.ell...@richardelling.com> wrote:
On Oct 11, 2019, at 2:50 PM, Garrett D'Amore <garr...@damore.org [mailto:garr...@damore.org]> wrote: The issue is that you can't just arbitrarily throw a mutex out there -- you have to have a place to *store* that, and you can't fit it inside the 64-bit value. With a 64-bit ISA this isn't usually a problem, but with 32-bit ISAs it is. I'm not sure how this affects the compiler builtin atomics since they don't add mutexes. Garrett D'Amore: You're missing the point. If you have a 32-bit ISA that doesn't offer a 64-bit atomic operation, then you have to fabricate one. Fabricating one requires a mutex, spinlock, or some other value. What this means is that compiler builtins can't be used to solve the problem of supporting a 64-bit atomic when the underlying platform lacks support. So you have to solve it in software typically, which is what I think this whole discussion is about. - Garrett -- richard The only way to store the mutex (which could just be a spinlock) is to have some other place that has it -- typically in library code. Allocation of other objects like that normally falls outside the scope of a compiler builtin (modulo bringing in a separate runtime object file, which can work for user programs but generally not for kernels.) On 10/11/2019 11:38:57 AM, Richard Elling <richard.ell...@richardelling.com [mailto:richard.ell...@richardelling.com]> wrote: On Oct 9, 2019, at 11:41 AM, Garrett D'Amore <garr...@damore.org [mailto:garr...@damore.org]> wrote: I don't think 32-bit compilers generally offer builtins for 64-bit atomics. Frankly, they can't really unless the underlying ISA provides some additional support for this in particular. Yes, that is why the builtins exist... the underlying ISA may have a method that is not part of the C language. Worst case, mutex protection will work... slowly. -- richard To implement a 64-bit atomic on a 32-bit architecture, you generally needs some additional state somewhere -- typically some sort of mutex or spinlock. That has to live somewhere. (You *might* be able to have a compiler builtin that provides this along with a compiler runtime which provides an instance of the spinlock somewhere in the program's data section. I think this sort of "builtin" (which isn't really builtin at all) generally can't be used in operating system kernels -- e.g. with --freestanding.) On 10/9/2019 11:26:40 AM, Richard Elling <richard.ell...@richardelling.com [mailto:richard.ell...@richardelling.com]> wrote: If it is possible to specify a compiler version, it might be easier to use the compiler builtin atomics. Just sayin' -- richard ------------------------------------------ openzfs: openzfs-developer Permalink: https://openzfs.topicbox.com/groups/developer/T3ee8a81d5f09f2ec-Mabd6346845b79e16d16f57c2 [https://openzfs.topicbox.com/groups/developer/T3ee8a81d5f09f2ec-Mabd6346845b79e16d16f57c2] Delivery options: https://openzfs.topicbox.com/groups/developer/subscription [https://openzfs.topicbox.com/groups/developer/subscription] openzfs [https://openzfs.topicbox.com/latest] / openzfs-developer / see discussions [https://openzfs.topicbox.com/groups/developer] + participants [https://openzfs.topicbox.com/groups/developer/members] + delivery options [https://openzfs.topicbox.com/groups/developer/subscription] Permalink [https://openzfs.topicbox.com/groups/developer/T3ee8a81d5f09f2ec-M51858eebfe98f256ff2d7f42] ------------------------------------------ openzfs: openzfs-developer Permalink: https://openzfs.topicbox.com/groups/developer/T3ee8a81d5f09f2ec-M620340096d08af342b9b67d2 Delivery options: https://openzfs.topicbox.com/groups/developer/subscription
