[mlir][vector] Handle corner cases in DropUnitDimsFromTransposeOp. (#102518)

da8778e499
breaks the lowering of vector.transpose that all the dimensions are unit
dimensions. The revision fixes the issue and adds a test.

---------

Signed-off-by: hanhanW <hanhan0912@gmail.com>

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

@@ -1771,6 +1771,13 @@ struct DropUnitDimsFromTransposeOp final
       newPerm.push_back(idx - droppedDimsBefore[idx]);
     }
 
+    // Fixup for `newPerm`. The `sourceTypeWithoutUnitDims` could be vector<1xT>
+    // type when the dimensions are unit dimensions. In this case, the newPerm
+    // should be [0].
+    if (newPerm.empty()) {
+      newPerm.push_back(0);
+    }
+
     Location loc = op.getLoc();
     // Drop the unit dims via shape_cast.
     auto dropDimsShapeCast = rewriter.create<vector::ShapeCastOp>(

mlir/test/Dialect/Vector/vector-transfer-flatten.mlir

@@ -737,6 +737,18 @@ func.func @transpose_with_scalable_unit_dims(%vec: vector<[1]x1x2x4x1xf32>) -> v
 
 // -----
 
+func.func @transpose_with_all_unit_dims(%vec: vector<1x1x1xf32>) -> vector<1x1x1xf32> {
+  %res = vector.transpose %vec, [0, 2, 1] : vector<1x1x1xf32> to vector<1x1x1xf32>
+  return %res : vector<1x1x1xf32>
+}
+// The `vec` is returned because there are other flattening patterns that fold
+// vector.shape_cast ops away.
+// CHECK-LABEL: func.func @transpose_with_all_unit_dims
+// CHECK-SAME:      %[[VEC:.[a-zA-Z0-9]+]]
+// CHECK-NEXT:    return %[[VEC]]
+
+// -----
+
 func.func @negative_transpose_with_no_unit_dims(%vec: vector<4x2x3xf32>) -> vector<4x3x2xf32> {
   %res = vector.transpose %vec, [0, 2, 1] : vector<4x2x3xf32> to vector<4x3x2xf32>
   return %res : vector<4x3x2xf32>