[mpiwg-rma] Atomicity and Pair Types

[maik peterson]

shit, who let you guys out again ? still playing around with rma. that has
really no future...

Am Do., 13. Sep. 2018 um 16:09 Uhr schrieb Mark Hoemmen via mpiwg-rma <
mpiwg-rma at lists.mpi-forum.org>:

> Gmail suggests “sounds good to me” as a standard response and i’m ok with
> that ;-). I think they were more worried about the case of a struct with
> padding bits never making it through a CAS, even though the padding bits
> aren’t part of the value representation.
>
> On Wed, Sep 12, 2018 at 10:40 PM Jeff Hammond <jeff.science at gmail.com>
> wrote:
>
>>
>>
>> > On Sep 12, 2018, at 9:11 PM, Mark Hoemmen via mpiwg-rma <
>> mpiwg-rma at lists.mpi-forum.org> wrote:
>> >
>> > True, it wouldn't affect MPI_COMPARE_AND_SWAP, since it only works on
>> > integer, byte, or logical types.  Never mind then :-)
>> >
>> > MPI_COMPARE_AND_SWAP uses language "if the compare buffer and the
>> > target buffer are identical," which sounds like "bitwise identical" --
>> > the C++ paper I cited is trying to move away from that definition
>> > towards operator==.  It doesn't matter now but it might if people want
>> > to extend it to floating-point types, with -0.0 == +0.0 etc.
>>
>> We will never do that because -0.0 and +0.0 are different numbers and if
>> anybody tries to make the MPI standard say otherwise, I’m going to bludgeon
>> them with a pillowcase full of numerical analysis textbooks.
>>
>> Jeff
>>
>> > mfh
>> >> On Wed, Sep 12, 2018 at 9:42 PM Nathan Hjelm <hjelmn at me.com> wrote:
>> >>
>> >> Interesting but not quite relevant here. The struct types are only
>> valid with minloc and maxloc so we don't have the same issue. The value of
>> the padding bits does not influence the results of the operation.
>> >>
>> >> -Nathan
>> >>
>> >>> On Sep 12, 2018, at 8:43 PM, Mark Hoemmen via mpiwg-rma <
>> mpiwg-rma at lists.mpi-forum.org> wrote:
>> >>>
>> >>> On Wed, Sep 12, 2018 at 2:55 PM Balaji, Pavan via mpiwg-rma
>> >>> <mpiwg-rma at lists.mpi-forum.org> wrote:
>> >>>> Yes, correct.  Although, Nathan had a ticket at some point where he
>> wanted to allow MPI to overwrite the padding bytes in such cases.
>> >>>
>> >>> This issue of padding bytes in atomic updates to a struct came up in
>> >>> the past few C++ meetings:
>> >>>
>> >>> http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p0528r3.html
>> >>>
>> >>> For more explanation, see
>> >>> http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0528r0.html
>> .
>> >>> The goal is that compare-and-exchange shouldn't always return false if
>> >>> the padding bits don't participate in the value representation of a
>> >>> struct.
>> >>>
>> >>> mfh
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