[all-commits] [llvm/llvm-project] 917d95: [mlir][Vector] Improve default lowering of vector ...

[Diego Caballero via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 917d95fc8adb2d68da442daee4e2508ada8c9bdc
      https://github.com/llvm/llvm-project/commit/917d95fc8adb2d68da442daee4e2508ada8c9bdc
  Author: Diego Caballero <diegocaballero at google.com>
  Date:   2022-03-07 (Mon, 07 Mar 2022)

  Changed paths:
    M mlir/lib/Dialect/Vector/Transforms/VectorTransforms.cpp
    M mlir/test/Dialect/Vector/vector-transpose-lowering.mlir

  Log Message:
  -----------
  [mlir][Vector] Improve default lowering of vector transpose operations

The default lowering of vector transpose operations generates a large sequence of
scalar extract/insert operations, one pair for each scalar element in the input tensor.
In other words, the vector transpose is scalarized. However, there are transpose
patterns where one or more adjacent high-order dimensions are not transposed (for
example, in the transpose pattern [1, 0, 2, 3], dimensions 2 and 3 are not transposed).
This patch improves the lowering of those cases by not scalarizing them and extracting/
inserting a full n-D vector, where 'n' is the number of adjacent high-order dimensions
not being transposed. By doing so, we prevent the scalarization of the code and generate a
more performant vector version.

Paradoxically, this patch shouldn't improve the performance of transpose operations if
we are using LLVM. The LLVM pipeline is able to optimize away some of the extract/insert
operations and the SLP vectorizer is converting the scalar operations back to its vector
form. However, scalarizing a vector version of the code in MLIR and relying on the SLP
vectorizer to reconstruct the vector code again is highly undesirable for several reasons.

Reviewed By: nicolasvasilache, ThomasRaoux

Differential Revision: https://reviews.llvm.org/D120601