[mpiwg-abi] MPI-5 ABI proposal

[Jeff Hammond]

I think we have to make the ABI change opt-in, because not breaking ABI
backwards compatibility with existing implementations is one of the big
reasons we don't already have an ABI standard.

I share Christoph's concern here and prefer not to have a separate header
file if we can avoid it.  However, if we don't have a new header, we cannot
interoperate between old and new ABIs.  With a new header, the option
exists, although we might not take it.

There are other ways to deal with the header situation, such as a separate
path, preprocessor macros, etc.  There is no best option here.  Every
variety will cause a problem for somebody, although hopefully we can find
an option that only causes minor problems in the worst case.

Jeff

On Tue, Jan 24, 2023 at 7:06 PM Holmes, Daniel John <
daniel.john.holmes at intel.com> wrote:

> Hi Christoph,
>
> I think the idea of "mpi_abi.h" is that everyone in the world immediately
> adds 4 characters to every file that currently has "mpi.h" in it. An opt-in
> approach.
>
> Another possibility is that "mpi.h" becomes the ABI version of the header
> and we specify "mpi_impl.h" as the implementation-specific header currently
> exposed by the implementation. An opt-out approach.
>
> If it's done right, no-one will (need/want to) opt out.
>
> Best wishes,
> Dan.
>
> -----Original Message-----
> From: Christoph Niethammer <niethammer at hlrs.de>
> Sent: 24 January 2023 16:53
> To: Holmes, Daniel John <daniel.john.holmes at intel.com>
> Cc: Jeff Hammond <jeff.science at gmail.com>; mpiwg-abi at lists.mpi-forum.org
> Subject: Re: [mpiwg-abi] MPI-5 ABI proposal
>
> Hi Dan, Heff,
>
> I am not sure if I like the idea of having another possible header file
> for *library-only* backward compatibility.
> From a software installation perspective this looks like a nightmare to me
> to maintain in parallel. We are just getting rid of one header in Fortran
> currently. ;)
>
> Regarding the issue Dan brought up:
> I think it would be good to have at least a chance to check if MPI library
> and MPI header fit together.
> One idea here would be to introduce a library routine "MPI_Abi_version()"
> that returns the API version and having a macro constant MPI_ABI_VERSION in
> the header, that can be compared.
>
> This will work for applications or a tool to test if the library is build
> against the same ABI.
> However, it will not help a library to detect if an application was build
> with the matching MPI header. :/
>
> Best,
> Christoph
>
> ----- Original Message -----
> From: "Holmes, Daniel John" <daniel.john.holmes at intel.com>
> To: "Jeff Hammond" <jeff.science at gmail.com>
> Cc: mpiwg-abi at lists.mpi-forum.org
> Sent: Tuesday, 24 January, 2023 16:45:36
> Subject: Re: [mpiwg-abi] MPI-5 ABI proposal
>
> Hi Jeff,
>
> Thanks - I looked through quickly - there's nothing outlandish it there,
> IMHO - seems sensible/modest.
>
> Slide 9 question "both mpi.h and mpi_abi.h in same program?"
>
> Do you mean "in the same translation unit" or "in different libraries
> linked together into a single binary" or something else?
>
> I'm thinking that the World model encourages the user to pass a
> communicator from the place where MPI_INIT was called into each
> sub-program/library - there must be agreement between them on the
> type/value.
>
> There are a bunch of what-if scenarios here: passing an MPI_Status from
> one place to another using a parameter/return value suggests your option 1
> is the correct answer there (language copy would work).
>
> Best wishes,
> Dan.
>
> From: mpiwg-abi <mpiwg-abi-bounces at lists.mpi-forum.org> On Behalf Of Jeff
> Hammond
> Sent: 24 January 2023 15:01
> To: mpiwg-abi at lists.mpi-forum.org
> Subject: [mpiwg-abi] MPI-5 ABI proposal
>
> As requested, I have made a concrete proposal for the MPI-5 ABI.
>
>
> https://docs.google.com/presentation/d/1cU9ewMUa7w3eaRVtfzov8NinyjzNSp5nF_peT92SE8I/edit?usp=sharing
>
> Request edit access if you want to paint the bike shed with me.
>
> Jeff
>
> --
> Jeff Hammond
> jeff.science at gmail.com<mailto:jeff.science at gmail.com>
> http://jeffhammond.github.io/
>
> --
> mpiwg-abi mailing list
> mpiwg-abi at lists.mpi-forum.org
> https://lists.mpi-forum.org/mailman/listinfo/mpiwg-abi
>


-- 
Jeff Hammond
jeff.science at gmail.com
http://jeffhammond.github.io/
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