torch-mlir/frontends/pytorch

NPComp - PyTorch frontend integration

This directory contains optional components for interfacing PyTorch to NPComp. Integration is targeted at multiple levels:

• Via program capture with a ATen pseudo-device.
• Via IR-level integration with PyTorch (via tracing or scripting interfaces).
• Interfaces to facilitate checking against reference implementations and verification.

In all situations, the target dialects are maintained in the outer project, along with their lowerings to common intermediate dialects and backends. This directory should be purely about interfacing with the PyTorch/LibTorch components for extracting and executing programs.

The code in this directory is intended to integrate tightly with pytorch, and follows the code style for pytorch. See the overall documentation for frontends for further details about code layout and integration philosophy. In particular, this directory exists to provide a working frontend to an MLIR based pytorch compilation flow and is not intended to be contributed to the LLVM monorepo. If the project is successful, it makes more sense to either break it out as an independent project that depends on LLVM/MLIR/npcomp or contribute it upstream to PyTorch. However, as it will be quite some time before the components are in a state to support such a dependency, it is being carried in-tree in the interim.

Program capture with a ATen pseudo-device.

Integration with a pseudo-device is typified by code like the following:

import npcomp.frontends.pytorch as torch_mlir

dev = torch_mlir.mlir_device()
t0 = torch.randn((4,4), device=dev)
t1 = torch.randn((4,4)).to(dev)
t2 = t0 + t1
t2_mlir = torch_mlir.get_mlir( t2 )
t2_cpu = t2.to('cpu')

In this case t2_cpu contains the result of the computation, and t2_mlir contains the mlir description of the computation. Tensors are allocated directly on the virtual device using the device= argument, or computed on the host and then moved to the virtual device using the to(dev) call. Subsequent calls on those tensors construct a graph of computation, but do not perform compute in most cases. This computation graph is returned in MLIR format by the get_mlir call, or lazily evaluated to return a regular pytorch tensor by the to(cpu) call.

This technique has several advantages and disadvantages. For training use cases, this technique generates a backward path automatically using the same method that pytorch natively uses. The resulting graph also tends to be simpler, since it will not reflect conditionals in the original python code. Lastly, it is natural if MLIR is being used as a frontend target for an actual device of some sort. In this case, the MLIR could go through a device-specific lowering path and the resulting code run on a device. The implementation of this technique is largely modeled after pytorch_xla.