[mlir][vector] Add a new TD Op for patterns leveraging ShapeCastOp (llvm#110525)

Adds a new Transform Dialect Op that collects patters for dropping unit
dims from various Ops:
  * `transform.apply_patterns.vector.drop_unit_dims_with_shape_cast`.

It excludes patterns for vector.transfer Ops - these are collected
under:
  * `apply_patterns.vector.rank_reducing_subview_patterns`,

and use ShapeCastOp _and_ SubviewOp to reduce the rank (and to eliminate
unit dims).

This new TD Ops allows us to test the "ShapeCast folder" pattern in
isolation. I've extracted the only test that I could find for that
folder from "vector-transforms.mlir" and moved it to a dedicated file:
"shape-cast-folder.mlir". I also added a test case with scalable
vectors.

Changes in VectorTransforms.cpp are not needed (added a comment with
a TODO + ordered the patterns alphabetically). I am Including them here
to avoid a separate PR.

Author: banach-space

mlir/include/mlir/Dialect/Vector/TransformOps/VectorTransformOps.td

@@ -68,6 +68,22 @@ def ApplyRankReducingSubviewPatternsOp : Op<Transform_Dialect,
   let assemblyFormat = "attr-dict";
 }
 
+def ApplyDropUnitDimWithShapeCastPatternsOp : Op<Transform_Dialect,
+    "apply_patterns.vector.drop_unit_dims_with_shape_cast",
+    [DeclareOpInterfaceMethods<PatternDescriptorOpInterface>]> {
+  let description = [{
+     Apply vector patterns to fold unit dims with vector.shape_cast Ops:
+      - DropUnitDimFromElementwiseOps
+      - DropUnitDimsFromScfForOp
+      - DropUnitDimsFromTransposeOp
+
+    Excludes patterns for vector.transfer Ops. This is complemented by
+    shape_cast folding patterns.
+  }];
+
+  let assemblyFormat = "attr-dict";
+}
+
 def ApplyTransferPermutationPatternsOp : Op<Transform_Dialect,
     "apply_patterns.vector.transfer_permutation_patterns",
     [DeclareOpInterfaceMethods<PatternDescriptorOpInterface>]> {

mlir/lib/Dialect/Vector/TransformOps/VectorTransformOps.cpp

@@ -85,6 +85,11 @@ void transform::ApplyTransferPermutationPatternsOp::populatePatterns(
   vector::populateVectorTransferPermutationMapLoweringPatterns(patterns);
 }
 
+void transform::ApplyDropUnitDimWithShapeCastPatternsOp::populatePatterns(
+    RewritePatternSet &patterns) {
+  vector::populateDropUnitDimWithShapeCastPatterns(patterns);
+}
+
 void transform::ApplyLowerBitCastPatternsOp::populatePatterns(
     RewritePatternSet &patterns) {
   vector::populateVectorBitCastLoweringPatterns(patterns);

mlir/lib/Dialect/Vector/Transforms/VectorTransforms.cpp

@@ -2056,8 +2056,13 @@ void mlir::vector::populateShapeCastFoldingPatterns(RewritePatternSet &patterns,
 
 void mlir::vector::populateDropUnitDimWithShapeCastPatterns(
     RewritePatternSet &patterns, PatternBenefit benefit) {
-  patterns.add<DropUnitDimFromElementwiseOps, DropUnitDimsFromTransposeOp,
-               ShapeCastOpFolder, DropUnitDimsFromScfForOp>(
+  // TODO: Consider either:
+  //  * including DropInnerMostUnitDimsTransferRead and
+  //    DropInnerMostUnitDimsTransferWrite, or
+  //  * better naming to distinguish this and
+  //    populateVectorTransferCollapseInnerMostContiguousDimsPatterns.
+  patterns.add<DropUnitDimFromElementwiseOps, DropUnitDimsFromScfForOp,
+               DropUnitDimsFromTransposeOp, ShapeCastOpFolder>(
       patterns.getContext(), benefit);
 }
 

mlir/test/Dialect/Vector/shape-cast-folder.mlir

@@ -0,0 +1,38 @@
+// RUN: mlir-opt %s --transform-interpreter --split-input-file | FileCheck %s
+
+///----------------------------------------------------------------------------------------
+/// [Pattern: ShapeCastOpFolder]
+///----------------------------------------------------------------------------------------
+
+// CHECK-LABEL: func @fixed_width
+//  CHECK-SAME: %[[A0:.*0]]: vector<2x4xf32>
+//   CHECK-NOT: vector.shape_cast
+//       CHECK: return %[[A0]] : vector<2x4xf32>
+func.func @fixed_width(%arg0 : vector<2x4xf32>) -> vector<2x4xf32> {
+  %0 = vector.shape_cast %arg0 : vector<2x4xf32> to vector<8xf32>
+  %1 = vector.shape_cast %0 : vector<8xf32> to vector<2x4xf32>
+  return %1 : vector<2x4xf32>
+}
+
+// CHECK-LABEL: func @scalable
+//  CHECK-SAME: %[[A0:.*0]]: vector<2x[4]xf32>
+//   CHECK-NOT: vector.shape_cast
+//       CHECK: return %[[A0]] : vector<2x[4]xf32>
+func.func @scalable(%arg0 : vector<2x[4]xf32>) -> vector<2x[4]xf32> {
+  %0 = vector.shape_cast %arg0 : vector<2x[4]xf32> to vector<[8]xf32>
+  %1 = vector.shape_cast %0 : vector<[8]xf32> to vector<2x[4]xf32>
+  return %1 : vector<2x[4]xf32>
+}
+
+// ============================================================================
+//  TD sequence
+// ============================================================================
+module attributes {transform.with_named_sequence} {
+  transform.named_sequence @__transform_main(%root : !transform.any_op {transform.readonly}) {
+    %func_op = transform.structured.match ops{["func.func"]} in %root : (!transform.any_op) -> !transform.op<"func.func">
+    transform.apply_patterns to %func_op {
+      transform.apply_patterns.vector.drop_unit_dims_with_shape_cast
+    } : !transform.op<"func.func">
+    transform.yield
+  }
+}

mlir/test/Dialect/Vector/vector-transforms.mlir

@@ -184,15 +184,6 @@ func.func @vector_transfers(%arg0: index, %arg1: index) {
   return
 }
 
-// CHECK-LABEL: func @cancelling_shape_cast_ops
-//  CHECK-SAME: %[[A0:.*0]]: vector<2x4xf32>
-//       CHECK: return %[[A0]] : vector<2x4xf32>
-func.func @cancelling_shape_cast_ops(%arg0 : vector<2x4xf32>) -> vector<2x4xf32> {
-  %0 = vector.shape_cast %arg0 : vector<2x4xf32> to vector<8xf32>
-  %1 = vector.shape_cast %0 : vector<8xf32> to vector<2x4xf32>
-  return %1 : vector<2x4xf32>
-}
-
 // CHECK-LABEL: func @elementwise_unroll
 //  CHECK-SAME: (%[[ARG0:.*]]: memref<4x4xf32>, %[[ARG1:.*]]: memref<4x4xf32>)
 //       CHECK-DAG:   %[[C2:.*]] = arith.constant 2 : index