Re: Fw: GCC interpretation of C11 atomics (DR 459)

Torvald, thank you for your output, but I think, this discussion gets a little pointless. There is nothing else I can add since gcc folks are reluctant to this change anyway. In my opinion, there is no compelling reason against such an implementation (it is perfectly fine with the standard,

Re: Fw: GCC interpretation of C11 atomics (DR 459)

On Tue, 2018-02-27 at 13:16 +, Ruslan Nikolaev via gcc wrote: > > 3) Torvald pointed out further considerations such as users expecting > > lock-free atomic loads to be faster than stores. > > Is it even true? Is it faster to use some global lock (implemented through > RMW) than a single

Re: Fw: GCC interpretation of C11 atomics (DR 459)

On Tue, 2018-02-27 at 13:04 +, Szabolcs Nagy wrote: > the solutions is to add a language extension I think this only needs a library interface, at least when we're just considering the __atomic builtins. On the C/C++ level, it might amount to just another atomic type, which only has a CAS

Re: Fw: GCC interpretation of C11 atomics (DR 459)

On Tue, 2018-02-27 at 12:56 +, Ruslan Nikolaev via gcc wrote: > But, of course, it is kind of annoying that double-width types (and that also > includes potentially 64-bit on some 32-bit processors, e.g. i586 also has > cmpxchg8b and no official way to read atomically otherwise) need special

Re: Fw: GCC interpretation of C11 atomics (DR 459)

> 1) your proposal would make gcc non-conforming to iso c unless it changes how > static const objects are emitted. I do not think, ISO C requires to put const objects to .rodata. And it is easily solved by not placing it there for _Atomic objects that cannot be safely loaded from read-only

Re: Fw: GCC interpretation of C11 atomics (DR 459)

On 27/02/18 12:56, Ruslan Nikolaev wrote: Formally speaking, either implementation satisfies C11 because the standard allows much leeway in the interpretation here. no, 1) your proposal would make gcc non-conforming to iso c unless it changes how static const objects are emitted. 2) the two

Re: Fw: GCC interpretation of C11 atomics (DR 459)

Formally speaking, either implementation satisfies C11 because the standard allows much leeway in the interpretation here. But, of course, it is kind of annoying that double-width types (and that also includes potentially 64-bit on some 32-bit processors, e.g. i586 also has cmpxchg8b and no

Re: Fw: GCC interpretation of C11 atomics (DR 459)

On Mon, 2018-02-26 at 22:45 +, Ruslan Nikolaev via gcc wrote: > Thanks, everyone, for the output, it is very useful. I am just proposing to > consider the change unless there are clear roadblocks. (Either design choice > is probably OK with respect to the standard formally speaking, but

Re: Fw: GCC interpretation of C11 atomics (DR 459)

On Tue, 2018-02-27 at 10:22 +, Ramana Radhakrishnan wrote: > The way to fix this in AArch64 if there is a > guarantee from the standard that there are no problems with read-only > locations is to implement the change in libatomic. Even though the standard doesn't specify read-only memory, I

Re: Fw: GCC interpretation of C11 atomics (DR 459)

On Mon, 2018-02-26 at 19:39 +, Ruslan Nikolaev via gcc wrote: > Torvald, I definitely do not want to insist on this design choice, but it > makes sense to at least seriuously consider it given the concerns I > described. And especially because IFFUNC in libatomic already redirects to >

Re: Fw: GCC interpretation of C11 atomics (DR 459)

On Mon, Feb 26, 2018 at 10:45 PM, Ruslan Nikolaev via gcc wrote: > Thanks, everyone, for the output, it is very useful. I am just proposing to > consider the change unless there are clear roadblocks. (Either design choice > is probably OK with respect to the standard formally

Re: Fw: GCC interpretation of C11 atomics (DR 459)

Thanks, everyone, for the output, it is very useful. I am just proposing to consider the change unless there are clear roadblocks. (Either design choice is probably OK with respect to the standard formally speaking, but there are some clear advantages also.) I wrote a summary of pros & cons

Re: Fw: GCC interpretation of C11 atomics (DR 459)

Torvald, I definitely do not want to insist on this design choice, but it makes sense to at least seriuously consider it given the concerns I described. And especially because IFFUNC in libatomic already redirects to cmpxchg16b, so it just adds extra cost and indirection. Quite frankly, I do

Re: Fw: GCC interpretation of C11 atomics (DR 459)

On Mon, 2018-02-26 at 07:24 +, Ruslan Nikolaev via gcc wrote: > Alexander, > Thank you for your comments. Please see my response below. I definitely do > not want to fight for or against this change in gcc, but there are definitely > legitimate concerns to consider. I think, it would really

Re: Fw: GCC interpretation of C11 atomics (DR 459)

> I'd say the main issue is that libatomic is not guaranteed to work like that. > Today it relies on IFUNC for redirection, so you may (and not "will") get the > desired behavior on Glibc (implying Linux), not on other OSes, and neither on > Linux with non-GNU libc (nor on bare metal, for that

Re: Fw: GCC interpretation of C11 atomics (DR 459)

On Mon, 26 Feb 2018, Ruslan Nikolaev via gcc wrote: > Well, if libatomic is already doing it when corresponding CPU feature is > available (i.e., effectively implementing operations using cmpxchg16b), I do > not see any problem here. mcx16 implies that you *have* cmpxchg16b, therefore > other code