[Mlir-commits] [mlir] [MLIR][TOSA] Add --tosa-remove-redundant-transposes pass (PR #108260)

[Jacques Pienaar]

================
@@ -0,0 +1,731 @@
+//===- TosaRemoveRedundantTransposes.cpp
+//------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// ----------
+// Motivation:
+// ----------
+
+// Some legalization pathways introduce redundant tosa.TRANSPOSE
+// operations that result in avoidable data movement. For example,
+// PyTorch -> TOSA contains a lot of unnecessary transposes due
+// to conversions between NCHW and NHWC.
+
+// We wish to remove all the ones that we can, since in general
+// it is possible to remove the overwhelming majority.
+
+// -------------------
+// High-Level Overview:
+// -------------------
+
+// The pass begins at a downstream transpose with some perms tensor.
+// It traverses the dependencies upward, accepting only TosaElementwise
+// operators. Dependencies must terminate in nullifying transposes (when
+// composed, they form the identity), reshapes we can fold the transpose into,
+// or consts.
+
+// Conceptually, we then "bubble up" the downstream transpose until
+// we hit the sources. For constants, we generate a new constants, composed
+// with the downstream transpose. For nullifying transposes, we "cancel"
+// them. For reshapes, we fold the transpose into them.
+
+// We then ensure that we do not cause any duplication by "converting"
+// this chain we bubbled-up into its transposed form. We do this by analyzing
+// the dependency fan-ins across all transposes with the same perms tensor
+// in order to ensure that they do not have uses outside this group, which
+// would cause the old code section to remain "live", and not removed by
+// DCE.
+
+// We then perform a simple one-pass DCE, so no canonicalization is necessary.
+
+// --------------
----------------
jpienaar wrote:

This seems more like a PR description comment to motivate it rather than pass comment.

https://github.com/llvm/llvm-project/pull/108260