[llvm-dev] Clang option to provide list of target-subarchs.

Justin Lebar via llvm-dev llvm-dev at lists.llvm.org
Tue Feb 7 14:07:21 PST 2017

In principle this sounds fine to me.  Some questions/comments:

* You can specify --cuda-gpu-arch multiple times, and each time adds a
new CUDA arch.

How is this going to work with --target-subarchs?  Is there going to
be a --no-target-subarchs flag to disable subarchs?  What will the
semantics of this be, exactly?

The semantics of flags that mean "compile for this one subarch" and
"don't compile for this one subarch" seem a lot more straightforward
than flags that deal in lists.  What are your thoughts about making it
work that way instead?

* I don't think "target subarch" is a great name.  I don't think most
people think of GPU architectures as "sub-architectures"; "subarch"
implies to me that there are different GPU archs for each CPU arch,
which obviously isn't the case.

Similarly, what problem are we solving by putting "target" in the flag
name?  We already have e.g. -march; it's not -mtarget-arch.

"--offload-arch", maybe?

* As discussed offline, we'll need to continue supporting the existing
flags, e.g. --cuda-gpu-arch, probably forever.

* There are a bunch of other flags we may want to harmonize.  For
example, I believe CUDA and opencl both have separate
flush-denormals-to-zero flags.

On Tue, Feb 7, 2017 at 1:39 PM, Eric Christopher <echristo at gmail.com> wrote:
> Adding Justin as well.
> Overall this seems reasonable to me depending on the actual patch :)
> -eric
> On Mon, Feb 6, 2017 at 7:23 PM Rodgers, Gregory via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
>> There are at least four clang frontends for offloading to accelerators:
>> 1 Cuda clang  2 OpenMP  3 HCC and 4 OpenCL.    These frontends will
>> want to embed object code for multiple offload targets into a single
>> application binary to provide portability across different
>> subarchitectures
>> (e.g. sm_35, sm_50) and across different architectures (e.g
>> nvptx64,amdgcn).
>> Problem:  Different frontends are using different flags to provide a
>> list of subarchitectures.  For example, cuda clang repeats the flag
>> “--cuda-gpu-arch=sm_35  --cuda-gpu-arch=sm_50” and HCC uses
>> “--amdgpu-target=gfx701 --amdgpu-target=gfx802”.
>> We propose a common clang flag to provide a list of target
>> subarchitectures called “target-subarchs”. For example,
>> --target-subarchs=sm_35,sm_50,gfx701,gfx802
>> In discussions with HCC and OpenMP maintainers,  we believe a new
>> flag name would have these requirements:
>>    end in “s” because it is a list;
>>    not have vendor specific names like cuda and amd;
>>    not contain “gpu” because offloading may extend to non-gpu archs;
>>    avoid “arch” by itself so as not to be confused with first field of a
>> triple;
>>    and not collide with existing flags to allow both options.
>> "--target-subarchs" satisfies all the above.  Comments?
>> Greg Rodgers
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