[llvm-dev] [RFC] Lanai backend

Pete Cooper via llvm-dev llvm-dev at lists.llvm.org
Tue Feb 9 20:59:58 PST 2016

Hi Sean

I think you’ve summed it up really well here.

Personally I don’t think we should accept backends for which there is no way to run the code.  The burden (however small) on the community to having an in-tree backend they can’t use is too high IMO.

As you point out ‘no way to run the code’ may mean not having access to HW, or having HW but no API.

NVPTX is a good example.  Now you can take the output from LLVM and run it on HW.  It may or may not be how Nvidia do it in their code, but that doesn’t matter, you can do it.  Same for AMDGPU.

So -1 from me to having backends we can’t make use of.

Finally, one option is to have perpetually experimental backends.  Then all the code is in tree but no-one in tree should ever be expected to update it.  That does have the big advantage that all of the code is there to discuss and the maintainers can make contributions to common code and gain/provide help in the community.  They can also be involved in discussions which impact them such as changes to common code.

> On Feb 9, 2016, at 4:18 PM, Sean Silva via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> One data point (IIRC) is that the NVPTX backend sat in tree for a long time without a way to actually use them. But lately this has been opening up (e.g. http://llvm.org/docs/CompileCudaWithLLVM.html <http://llvm.org/docs/CompileCudaWithLLVM.html>). However, the obstacle for NVPTX was mostly a software proprietary-ness (no way to plug it into the driver stack really, except via nvidia's own proprietary software), whereas the actual hardware was available. For the Lanai stuff, it seems like the hardware is fundamentally not available for purchase.
> The reverse situation is with e.g. Apple's GPU backends, where the devices are readily available, but (AFAIK) even if the backend were open-source you couldn't run the code produced by the open-source compiler.
> Or to put it in matrix form (this is all heavily prefixed by "AFAIK"; corrections welcome):
> AMDGPU:      InTree:Yes DevicesAvailable:Yes CanIRunTheCode:Yes
> NVPTX:       InTree:Yes DevicesAvailable:Yes CanIRunTheCode:Yes
> Lanai:       InTree:?   DevicesAvailable:No  CanIRunTheCode:No
> Apple GPU's: InTree:No  DevicesAvailable:Yes CanIRunTheCode:No
> I couldn't come up with a good name for "Can I Run The Code" column. Basically it means: "assuming the backend were in open source, could I actually run the code produced by the open source backend somehow?".
> I had a quick look at lib/Target and it seems like every backend we have has "CanIRunTheCode:Yes" in theory.
> IIRC, the NVPTX stuff used to actually be "No" though?
> Anyway, just a random thought. Not sure what the conclusion is.

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