[mlir] Rank reducing subview conversion to LLVM

[limo1996]

Summary

This commit adjusts SubViewOp lowering to take rank reduction into account.

Depends On D88879

[nicolasvasilache]

Thanks @limo1996 , let's also add a conversion to LLVM test somewhere around here: https://github.com/llvm/llvm-project/blob/master/mlir/test/Conversion/StandardToLLVM/convert-to-llvmir.mlir#L1222

mlir/lib/Conversion/StandardToLLVM/StandardToLLVM.cpp

3005

Let's please extract this into a helper function.
If I am not mistaken this could also be reused in Ops.cpp for isRankReduced?

mlir/lib/Dialect/StandardOps/IR/Ops.cpp

2819

This should not be exposed to Ops.h and ideally deprecated.
The problem is at this time the result types of map_range and ArrayAttr::getAsValueRange are unfathomble.

This is a rare case where I'd say it's ok to copy the function where you need it (and keep it static) for now.

[limo1996]

Comments of Nicolas incorporated

[nicolasvasilache]

mlir/include/mlir/Dialect/StandardOps/IR/Ops.h

307

The name of the function and the doc comment are quite misleading.
From this, I wouldn't know how to find this functionality in 1 month once this gets out of my short-term memory...

I'd suggest :

Given an `originalShape` and a `reducedShape` assumed to be a subset of `originalShape` with some `1` entries erased, return the vector of booleans that specifies which of the entries of `originalShape` are keep to obtain `reducedShape`.
The returned mask can be applied as a projection to `originalShape` to obtain the `reducedShape`.
This mask is useful to track which dimensions must be kept when e.g. compute MemRef strides under rank-reducing operations.
Return None if reducedShape cannot be obtained by dropping only `1` entries in `originalShape`.

llvm::Optional<SmallVector<bool, 4>> computeRankReductionMask(ArrayRef<int64_t> originalShape, ArrayRef<int64_t> reducedShape);

Feel free to further improve amd/or find a better name if one comes to mind.

mlir/lib/Dialect/StandardOps/IR/Ops.cpp

2828

reducedIdx is misleading, it should be called numReducedDims.

Also, it seems that you only use this in the verification.
So the other use of the API is less intuitive.

If you implement this with returning an llvm::Optional<SmallVector<bool, 4>>, you can test on whether a value is actually returned.

2836

reads somewhat hacky with the hardcoded -2.

Let's please rewrite as:

keepMask[originalIdx] = (reducedIdx < reducedRank) && 
  (originalShape[originalIdx] == reducedShape[reducedIdx]);
if (keepMask[originalIdx])
  reducedIdx++;

2886–2889

if (reducedRank != std::count_if(keepMask.begin(), keepMask.end(), [](bool b){ return b; }))
  return ...;

2890

if (!keepMask.hasValue())

Seems like, in the limit, this could be defined on empty shapes too.
Returning llvm::None is a clear error failure that reducedShape cannot be obtained from originalShape by dropping only 1's.

[limo1996]

Next wave of comments addressed

[nicolasvasilache]

Thanks @limo1996 !