[cfe-dev] [RFC][OpenMP][CUDA] Unified Offloading Support in Clang Driver

[Justin Lebar via cfe-dev]

> I agree with Justin when he says that supporting all possible combinations of host-device bundling, each with a custom format adds a lot of complexity and does not necessarily helps the user.

Maybe it would be helpful to speak concretely about this.  What is a
specific case you're concerned about?

Frankly if we try to build a thing that does everything -- both embeds
and bundles -- I'm concerned that will be much more complex than
either approach by itself.  But maybe there's something about bundling
code for many different devices that's qualitatively different than
bundling code for one or just a few devices?  For example, are there
extant formats that we would like to be compatible with that are
incompatible with each other?  Again, I think it would be helpful to
be concrete.

> Using a header compatible with tar is not something hard to do, having the bundler tool as part of clang ensures that it works across operating systems too.

This is a minor point, but -- without getting myself back into the
minefield of discussing whether using the tar format is or is not good
-- if we do use said format, given what I understand at the moment,
I'm not in favor of building our own user-facing tool, if that's
possible.  If you're on Windows and want to inspect a tar file you
can, download 7z.  It's no different than downloading objdump.  But we
can postpone this discussion until later; I suspect it won't be
necessary, given the pushback to using tar in the first place.

> this leaves room to have custom-format bundling activated with options for the cases (would have to be evaluated one by one) that would greatly benefit of interoperability.

This is, as I read it, exactly what everyone else has been arguing so
strenuously against (and what I conceded to).

On Mon, Mar 14, 2016 at 7:25 PM, Samuel F Antao <sfantao at us.ibm.com> wrote:
> Hi all,
>
> I decided a take shot on a possible implementation for the part of this
> proposal that I think is more consensual (i.e. the part that does not relate
> with the bundling). I posted three patches (http://reviews.llvm.org/D18170,
> http://reviews.llvm.org/D18171, http://reviews.llvm.org/D18172) with a
> possible implementation, so that we have something more concrete to discuss.
> Let me know your thoughts.
>
> Going back to the bundling discussion:
>
> I agree with Justin when he says that supporting all possible combinations
> of host-device bundling, each with a custom format adds a lot of complexity
> and does not necessarily helps the user. Therefore, I think reasonable to
> have intermediate files bundled in some already existent format (say tar)
> that is agnostic of the programming model. Actually, that was my motivation
> when I proposed the custom format in the bundler.
>
> When I look at all the different concerns I think that a possible solution
> is to have a bundler with three operation modes:
>  i) "Embed": it generates a host object that contains the device image and
> properly defines the symbols a programming model requires. Therefore it can
> be linked with host objects successfully. This file is never exposed to the
> user unless save-temps is used.
> ii) "Bundle": Combines host and device object using, by default, a format
> easy to interact with that is agnostic of the programming model.
> iii) "Unbundle": The inverse of ii), it assumes the input uses that default
> format.
>
> Using a header compatible with tar is not something hard to do, having the
> bundler tool as part of clang ensures that it works across operating systems
> too. At the same time, this leaves room to have custom-format bundling
> activated with options for the cases (would have to be evaluated one by one)
> that would greatly benefit of interoperability.
>
> Does this sound reasonable?
>
> Thanks!
> Samuel
>
> 2016-03-10 13:59 GMT-05:00 Justin Lebar via cfe-dev
> <cfe-dev at lists.llvm.org>:
>>
>> > Justin, is this convincing enough?
>>
>> Okay, okay.  Uncle.
>>
>> There are two things here that I find convincing.
>>
>> 1) Although we're not going to be compatible with the details of some,
>> if not all, other compilers' formats, we can at least be compatible
>> with the spirit by using object files as opposed to tar.
>>
>> 2) The postscript in Andrey's e-mail:
>>
>> > Re: objdump doesn't understand ELF format with code for multiple
>> > targets. The same is true for fat executable files as well, isn't? So if we
>> > need to teach objdump how to recognize fat files, we already have this
>> > problem.
>>
>> It's probably much more important that objdump work on executables
>> than on object files, since if you have object files, you can probably
>> recompile with -save-temps, but if you only have an executable, you
>> don't necessarily have access to intermediate files, or even a
>> compiler for the relevant architecture, much less the specific
>> compiler which generated the executable.
>>
>> Shoving device code into the host *executable* seems unavoidable.  I'm
>> still not thrilled with doing the same for object files -- it still
>> feels like we're using ELF when we actually want an archive format --
>> but (1) makes it less bad.
>>
>> -Justin
>>
>> On Wed, Mar 9, 2016 at 5:59 AM, Andrey Bokhanko
>> <andreybokhanko at gmail.com> wrote:
>> > All,
>> >
>> > I asked Intel GCC guys who implemented OpenMP offloading support in GCC,
>> > and
>> > as they told me, GCC also employs option #4 from Hal's list -- it puts
>> > both
>> > host and target code in a single ELF file. "Code" in GCC case is always
>> > GCC's IR (Gimple), though -- they require GCC invocation from linker in
>> > order to produce a multi-target executable. This makes GCC
>> > non-interoperable
>> > with any other offloading compiler and effectively produces its own
>> > standard.
>> >
>> > Thus, prior art from:
>> > * nvcc
>> > * Pathscale
>> > * GCC
>> > * ICC
>> >
>> > indicates only one direction -- compiler driver produces a single object
>> > file with target code embedded in data section.
>> >
>> > Justin, is this convincing enough? I don't see any good reasons why
>> > clang
>> > should go against what every other compiler on the planet does.
>> >
>> > Re: objdump doesn't understand ELF format with code for multiple
>> > targets.
>> > The same is true for fat executable files as well, isn't? So if we need
>> > to
>> > teach objdump how to recognize fat files, we already have this problem.
>> >
>> > Yours,
>> > Andrey
>> > =====
>> > Software Engineer
>> > Intel Compiler Team
>> >
>> >
>> _______________________________________________
>> cfe-dev mailing list
>> cfe-dev at lists.llvm.org
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>
>