[cfe-dev] [RFC] Unified offloading option for CUDA/HIP/OpenMP

[Liu, Yaxun (Sam) via cfe-dev]

[AMD Public Use]

Sorry for the delay.

Both Johannes' and Artem's proposals should satisfy the needs of users:

Option 1:

`-offload=<offload-pattern> optA optB optC`.

Option 2:

`-offload=<offload-pattern>,optA,optB,optC`.

Compared to the old options, they are more concise and more readable.

The main difference is the delimiter. To me option 2 is more attractive since it does not need quotations for most cases.

Can we reach an agreement on option 2?

Thanks.

Sam


From: Artem Belevich <tra at google.com>
Sent: Tuesday, December 15, 2020 2:13 PM
To: Ben Boeckel <ben.boeckel at kitware.com>
Cc: Doerfert, Johannes <jdoerfert at anl.gov>; Liu, Yaxun (Sam) <Yaxun.Liu at amd.com>; Lieberman, Ron <Ron.Lieberman at amd.com>; a.bataev at hotmail.com; Chan, SiuChi <siuchi.chan at amd.com>; Searles, Mark <Mark.Searles at amd.com>; cfe-dev (cfe-dev at lists.llvm.org) <cfe-dev at lists.llvm.org>
Subject: Re: [cfe-dev] [RFC] Unified offloading option for CUDA/HIP/OpenMP

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On Tue, Dec 15, 2020 at 10:23 AM Ben Boeckel <ben.boeckel at kitware.com<mailto:ben.boeckel at kitware.com>> wrote:
On Mon, Dec 14, 2020 at 14:04:43 -0800, Artem Belevich via cfe-dev wrote:
> It all may be an utter overkill, too. WDYT?

Note that tools such as ccache and sccache generally need to be able to
understand what's going on (I believe distcc and other distributed
compilation tools also generally need to know too), so making it
sensible enough for interpretation based on just the flags to be
possible should be considered.

I think this is somewhat orthogonal to how we specify per-target options. Such a tool almost never knows about all possible compiler options and has to pass through the unknown options as-is.  However, any form of 'nested' options specified on the command line will have a chance to confuse such tool. E.g. if I want to pass '-E' to some sub-tool for a particular offload-target, ccache, not being aware that it's not a top-level compilation option, may interpret it as an attempt to preprocess the TU.

I wonder if it would make sense to just move all this per-target option complexity into an external response file. As far as existing tools are concerned, it would look like `--offload-options=target-opts.file` without affecting tool's general idea what this compilation is about to do, and the external file would allow us to be as flexible as we need to be to specify per-target options. It could be just a flat list of pairs `-Xarch_... optA`.  Or we could use YAML.

That approach, however, has its own issues and would still need to be optional. If it's the only way to specify offload options, that will complicate other use cases as now they would have to deal with temporary files.

Maybe a slightly modified variant of jdoefert@'s idea would work better:
> >>    -offload="amd -march=gfx906 -fno-vectorize" -fopenmp

Implement it in a way similar to -Wl,optA,optB,optC and extend it to match an offload scope glob/regex.
E.g. `-offload=<offload-pattern>,optA,optB,optC`.
As far as the external tools are concerned, it's just one option to pass though. At the same time it should be flexible enough to apply the options to subset of offload targets in a human-manageable way.

--
--Artem Belevich
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