[RFC] Offloading Support in libgomp

[Michael V. Zolotukhin]

Hi Nathan,

> This is an interesting design.  It appears similar to how we'd
> envisioned implementing openacc support -- namely leverage the LTO
> machinery to communicate from the host compiler to the device
> compiler.  Your design looks more detailed, which is good.
Thanks, do you have a similar description of your design? It would be pretty
interesting to take a look at it.

> Are you envisioning the device compilers to be stand alone
> compilers, built separately.  Or are you envisioning extending the
> configuration machinery by adding something like
> --enable-acclerator=<list> so that:
>   .../configure --target=x86_64-linux --enable-accelerator=foo,baz
> causes
> * a build of an x86_64 compiler aware of the foo and baz accelerators
> * build of an appropriate runtime support library
> * a build of a foo lto accelerator backend, assembler (and linker?)
> * (if needed) build of a foo support library
> * a build of a baz lto accelerator backend
> * (if needed) build of a baz support library, assembler (and linker?)
> 
> or are you expecting something more like 3 separate configures & build?
>   .../configure --target=x86_64-linux --enable-accelerator=foo,baz
>   .../configure --target=foo --enable-languages=lto-accelerator
>   .../configure --target=baz --enable-languages=lto-accelerator
> 
> I'd been imagining the former scheme, as it provides for a more
> integrated build, fwiw.
That's an open question, and we'd like to clarify it too.  We'd appreciate any
inputs on this.

Personally, I see actually one more option.  Similar of how libgomp figures out
which runtimes are available (by looking for the corresponding plugins), we
could look for available target compilers at compile-time and produce as many
target images as number of compilers we have.  Thus, we won't need to rebuild
host compiler to support more targets - we'd just need to place the
corresponding target compiler somewhere.  That looks more like your second
option, but differs a bit from it in that we don't need to specify enabled
accelerators.

Michael
> nathan