DLPack no longer works on Boolean tensors after 1.10+

[BarclayII]

🐛 Bug

torch.utils.dlpack.to_dlpack no longer works for Boolean Tensor.

To Reproduce

torch.utils.dlpack.to_dlpack(torch.BoolTensor([False, True]))   # Bool type is not supported by dlpack

Expected behavior

Should work.

Environment

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• PyTorch Version (e.g., 1.0): 1.10 RC and nightly
• OS (e.g., Linux): Linux
• How you installed PyTorch (conda, pip, source): pip
• Build command you used (if compiling from source):
• Python version: 3.8.3
• CUDA/cuDNN version:
• GPU models and configuration:
• Any other relevant information:

Additional context

According to dmlc/dgl#3406 the error is raised after #57110. I was wondering what the reason was behind removing boolean support?

This is blocking DGL's patch release with PyTorch 1.10 since we rely on DLPack to interact with PyTorch tensors in multiple places.

+@jermainewang @VoVAllen

cc @brianjo @mruberry @ezyang @gchanan @zou3519 @bdhirsh @jbschlosser

[mruberry]

cc @emcastillo @rgommers

[mruberry]

So I think DLPack doesn't support boolean tensors and the workaround for this is to first convert a tensor to uint8 before exporting it. @Barclayll would that work for you?

If I'm correct, the alternative would be that PyTorch does this bool -> uint8 conversion for the caller, but that may hide this unexpected behavior and lead to confusion when users reimported the same tensor and it was now in uint8.

[rgommers]

So I think DLPack doesn't support boolean tensors

Yes indeed, see dmlc/dlpack#75 for discussion.

torch.utils.dlpack.to_dlpack no longer works for Boolean Tensor.

This implies it did work before - what was it doing there exactly? Did it roundtrip correctly? What happened when you'd read in such an exported bool tensor with CuPy or JAX?

[mruberry]

Historically (as of PyTorch 1.9) PyTorch would write a uint8 tensor like this:

capsule = torch.utils.dlpack.to_dlpack(torch.tensor([False, True]))
torch.utils.dlpack.from_dlpack(capsule)
: tensor([0, 1], dtype=torch.uint8)

So no, it wouldn't round-trip. It was equivalent to the proposed workaround (first cast the tensor to uint8).

[rgommers]

Then I would say that raising an exception as in 1.10 is the desired behavior.

If I'm correct, the alternative would be that PyTorch does this bool -> uint8 conversion for the caller, but that may hide this unexpected behavior and lead to confusion when users reimported the same tensor and it was now in uint8.

This would also become an issue if DLPack resolves the issue I linked above and implements a bool dtype. That would likely force a backwards-incompatible change in the PyTorch implemetation.

[mruberry]

I agree with your thinking (as usual), @rgommers.

Unfortunately we did make an unexpected BC-breaking change by clarifying this DLPack behavior, and we're sorry that's so disruptive @BarclayII. It is probably the "right" change from a UX perspective, however.

What are your thoughts, @BarclayII?

[BarclayII]

I understand. As per DGL we have temporarily worked around it so this is no longer a major blocker.

One further question though. Since PyTorch is deprecating using ByteTensors to index into an array, I'm wondering if PyTorch has any plan on removing this support in the next release? If so, then I think it's better to enable boolean type support in DLPack.

[mruberry]

I understand. As per DGL we have temporarily worked around it so this is no longer a major blocker.

Glad to hear it!

One further question though. Since PyTorch is deprecating using ByteTensors to index into an array, I'm wondering if PyTorch has any plan on removing this support in the next release? If so, then I think it's better to enable boolean type support in DLPack.

Removing support for what, exactly?

DLPack has an issue for this already that you may want to comment on, too: dmlc/dlpack#75.