[mlir][ArmSME] Fold extracts from 3D create_masks of SME-like masks (#80148)
When unrolling the reduction dimension of something like a matmul for
SME, it is possible to get 3D masks, which are vectors of SME-like
masks. The 2D masks for individual operations are then extracted from
the 3D masks.
i.e.:
```mlir
%mask = vector.create_mask %nonConstantDim, %a, %b : vector<4x[4]x[4]xi1>
%subMask = vector.extract %mask[2]
: vector<[4]x[4]xi1> from vector<4x[4]x[4]xi1>
```
ArmSME only supports lowering 2D create_masks, so we must fold the
extract into the create_mask. This can be done by checking if the
extraction index is within the true region, then using that select the
first dimension of the 2D mask. This is shown below.
```mlir
%extractionInTrueRegion = arith.cmpi slt, %c2, %nonConstantDim : index
%newMaskFrontDim = arith.select %extractionInTrueRegion, %a, %c0 : index
%subMask = vector.create_mask %newMaskFrontDim, %b : vector<[4]x[4]xi1>
```
Welcome to the LLVM project!
This repository contains the source code for LLVM, a toolkit for the construction of highly optimized compilers, optimizers, and run-time environments.
The LLVM project has multiple components. The core of the project is itself called “LLVM”. This contains all of the tools, libraries, and header files needed to process intermediate representations and convert them into object files. Tools include an assembler, disassembler, bitcode analyzer, and bitcode optimizer.
C-like languages use the Clang frontend. This component compiles C, C++, Objective-C, and Objective-C++ code into LLVM bitcode -- and from there into object files, using LLVM.
Other components include: the libc++ C++ standard library, the LLD linker, and more.
Consult the Getting Started with LLVM page for information on building and running LLVM.
For information on how to contribute to the LLVM project, please take a look at the Contributing to LLVM guide.
Join the LLVM Discourse forums, Discord chat, LLVM Office Hours or Regular sync-ups.
The LLVM project has adopted a code of conduct for participants to all modes of communication within the project.