[Relay][Pass] Simplify consecutive transpose/layout_transform

src/relay/op/make_op.h

@@ -75,6 +75,8 @@ Expr MakeSqueeze(Expr data, Array<Integer> axis);
 
 Expr MakeStack(Expr data, int axis);
 
+Expr MakeTranspose(Expr data, Array<Integer> axes);
+
 Expr MakeStridedSlice(Expr data, Array<Integer> begin, Array<Integer> end, Array<Integer> strides,
                       String slice_mode);
 

src/relay/transforms/simplify_expr.cc

@@ -82,6 +82,97 @@ class SimplifyReshape : public SimplifyPattern {
   DFPattern x_;
 };
 
+/*!
+ * \brief SimplifyTranspose matches the pattern of consecutive transpose op,
+ *   and merges or cancels them.
+ */
+class SimplifyTranspose : public SimplifyPattern {
+ public:
+  SimplifyTranspose() {
+    x_ = IsWildcard();
+    auto trans1 = IsOp("transpose") || IsOp("layout_transform");
+    auto trans2 = IsOp("transpose") || IsOp("layout_transform");
+    pattern_ = trans1({trans2({x_})});
+  }
+
+  Expr callback(const Expr& pre, const Expr& post,
+                const Map<DFPattern, Array<Expr>>& node_map) const override {
+    // Helper function to get the axes from call node attribute
+    auto get_axes_from_call = [](const Call trans_call, int ndim) {
+      std::vector<int> attr_axes;
+      if (auto attr = trans_call->attrs.as<TransposeAttrs>()) {
+        if (attr->axes.defined()) {
+          for (int i = 0; i < ndim; ++i) {
+            attr_axes.push_back(attr->axes[i]);
+          }
+        } else {
+          // Empty axes means reverse
+          for (int i = ndim - 1; i >= 0; --i) {
+            attr_axes.push_back(i);
+          }
+        }
+      } else if (auto attr = trans_call->attrs.as<LayoutTransformAttrs>()) {
+        Layout src_layout(attr->src_layout);
+        Layout dst_layout(attr->dst_layout);
+        for (int i = 0; i < ndim; ++i) {
+          attr_axes.push_back(src_layout.IndexOf(dst_layout[i]));
+        }
+      } else {
+        CHECK(false) << "Expected transpose or layout_transform, but got "
+                     << Downcast<Op>(trans_call->op)->name;
+      }
+      return std::move(attr_axes);
+    };
+
+    auto x = node_map[x_][0];
+
+    // Initialize axes
+    int ndim = Downcast<TensorType>(pre->checked_type())->shape.size();
+    Array<Integer> axes;
+    for (int i = 0; i < ndim; ++i) {
+      axes.push_back(i);
+    }
+
+    // Collect axes changes from the matched pattern, including two consecutive transposes.
+    std::vector<std::vector<int>> interm_axes;
+    Call trans_call = Downcast<Call>(post);
+    interm_axes.push_back(get_axes_from_call(trans_call, ndim));
+    trans_call = Downcast<Call>(trans_call->args[0]);
+    interm_axes.push_back(get_axes_from_call(trans_call, ndim));
+
+    // Calculate the final axes in reverse order (from root to output)
+    auto it = interm_axes.rbegin();
+    while (it != interm_axes.rend()) {
+      auto interm = *it;
+
+      Array<Integer> new_axes;
+      for (int i = 0; i < ndim; ++i) {
+        new_axes.push_back(axes[interm[i]]);
+      }
+      axes = new_axes;
+      it++;
+    }
+
+    // Check if the transpose is still required
+    bool need_transpose = false;
+    for (int i = 0; i < ndim; ++i) {
+      if (axes[i] != i) {
+        need_transpose = true;
+        break;
+      }
+    }
+
+    if (need_transpose) {
+      return MakeTranspose(x, axes);
+    }
+    return x;
+  }
+
+ private:
+  /*! \brief Pattern input */
+  DFPattern x_;
+};
+
 /*!
  * \brief FullArgwhere finds full followed by argwhere and turns it into an Arange op
  */
@@ -162,6 +253,7 @@ class ExprSimplifier {
  public:
   explicit ExprSimplifier(IRModule mod) : mod_(mod) {
     CreateCallback(SimplifyReshape());
+    CreateCallback(SimplifyTranspose());
     CreateCallback(FullElementwise());
   }
   template <typename T>

tests/python/relay/test_pass_simplify_expr.py

@@ -60,6 +60,54 @@ def symbolic():
     assert tvm.ir.structural_equal(zz, after)
 
 
+def test_simplify_transpose():
+    def before1():
+        x = relay.var("x", shape=(1, 3, 224, 224), dtype="float32")  # NCHW
+        y = relay.transpose(x, axes=[0, 2, 3, 1])  # To NHWC
+        y = relay.layout_transform(y, "NHWC", "HWCN")  # To HWCN
+        y = relay.transpose(y, axes=[3, 0, 1, 2])  # To NHWC
+        return relay.Function([x], y)
+
+    def expected1():
+        x = relay.var("x", shape=(1, 3, 224, 224), dtype="float32")  # NCHW
+        y = relay.transpose(x, axes=[0, 2, 3, 1])  # To NHWC
+        return relay.Function([x], y)
+
+    def before2():
+        x = relay.var("x", shape=(1, 3, 224, 224), dtype="float32")  # NCHW
+        y = relay.nn.relu(x)
+        y = relay.transpose(y, axes=[0, 2, 3, 1])  # To NHWC
+        y = relay.transpose(y, axes=[1, 2, 3, 0])  # To HWCN
+        y = relay.transpose(y, axes=[3, 2, 0, 1])  # To NCHW
+        return relay.Function([x], y)
+
+    def expected2():
+        x = relay.var("x", shape=(1, 3, 224, 224), dtype="float32")  # NCHW
+        y = relay.nn.relu(x)
+        return relay.Function([x], y)
+
+    def before3():
+        x = relay.var("x", shape=(1, 3, 224, 224), dtype="float32")  # NCHW
+        y = relay.nn.relu(x)
+        y = relay.transpose(y)  # Reverse
+        y = relay.transpose(y)  # Reverse
+        return relay.Function([x], y)
+
+    def expected3():
+        x = relay.var("x", shape=(1, 3, 224, 224), dtype="float32")  # NCHW
+        y = relay.nn.relu(x)
+        return relay.Function([x], y)
+
+    for before, expected in [
+        [before1(), expected1()],
+        [before2(), expected2()],
+        [before3(), expected3()],
+    ]:
+        after = run_opt_pass(before, transform.SimplifyExpr())
+        expected = run_opt_pass(expected, transform.InferType())
+        assert tvm.ir.structural_equal(after, expected)
+
+
 def test_simplify_full_elementwise():
     def validate(shape, value, dtype):
         def before_left(x, elem_op, full):
@@ -126,4 +174,5 @@ def after_right(x, elem_op, value):
 
 if __name__ == "__main__":
     test_simplify_reshape()
+    test_simplify_transpose()
     test_simplify_full_elementwise()