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Allowing memref of index elements

In the context of lowering shape computations, we have come across the problem that we now allow tensor<index> but not memref<index>.

As another example, the HLO dialect uses tensor<index> to specify shapes in the case of dynamic operations. When we want to lower these to buffer form, we would expect to have memref<index> that represents those shapes. Even if we would defer lowering the shape vectors to buffer form, we have to do so before we get to LLVM, as they might need a memory representation if their size is not known.

The same holds true for lowering shape computations in the shape dialect.

The rationale document says

While it may be useful to have memref<?xindex> to express indirect accesses, e.g. sparse matrix manipulations or lookup tables, it creates problems MLIR is not ready to address yet. MLIR needs to internally store constants of aggregate types and emit code operating on values of those types, which are subject to target-specific size and alignment constraints. Since MLIR does not have a target description mechanism at the moment, it cannot reliably emit such code. Moreover, some platforms may not support vectors of type equivalent to index .

I think this is no longer true. We have a mechanism to describe the size of index when lowering to LLVM and we need to be able to represent constants due to the tensor type already.

I would like to lift this restriction and allow memref of index type and convert them, in the LLVM case, equivalently to a memref of integers of the bitwidth that was configured.This would be consistent with how we treat index in the conversion otherwise.

@ftynse @nicolasvasilache FYI

SGTM.

I wrote that rationale. The actual issue was storing constants of index type, or tensors thereof. This is fine today because the relevant attribute storage is now based on APInt. The lowering of the constant operation could complain if APInt overflows the target type. In any case, memref was actually there by extension to of tensor and vector, and allowing memref<... index> does not affect the storage problem in any way. We know how to allocate such memrefs now.

Thanks for working on this! Lifting this restriction sounds good to me as well. We also came across the same need for memrefs and vectors for some future use cases.

Sounds great to remove this restriction. In fact, I thought we had already got rid of this at the time tensor<..x index> was introduced. Thanks!

Created https://reviews.llvm.org/D84934 for this.

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