[Lang] MatrixField refactor 9/n: Allow dynamic index of matrix field …

…when real_matrix=True (#6194)

Issue: #5959

### Brief Summary

This PR makes the rightmost path of #5959 running end-to-end.

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

taichi/transforms/lower_matrix_ptr.cpp

@@ -13,14 +13,34 @@ class LowerMatrixPtr : public BasicStmtVisitor {
 
   void visit(MatrixPtrStmt *stmt) override {
     if (stmt->origin->is<MatrixOfGlobalPtrStmt>()) {
-      TI_ASSERT(stmt->offset->is<ConstStmt>());
       auto origin = stmt->origin->as<MatrixOfGlobalPtrStmt>();
-      auto offset = stmt->offset->as<ConstStmt>();
-      auto lowered = std::make_unique<GlobalPtrStmt>(
-          origin->snodes[offset->val.val_int()], origin->indices);
-      stmt->replace_usages_with(lowered.get());
-      modifier.insert_before(stmt, std::move(lowered));
-      modifier.erase(stmt);
+      if (stmt->offset->is<ConstStmt>()) {
+        auto offset = stmt->offset->as<ConstStmt>();
+        auto lowered = std::make_unique<GlobalPtrStmt>(
+            origin->snodes[offset->val.val_int()], origin->indices);
+        stmt->replace_usages_with(lowered.get());
+        modifier.insert_before(stmt, std::move(lowered));
+        modifier.erase(stmt);
+      } else {
+        TI_ASSERT_INFO(
+            origin->dynamic_indexable,
+            "Element of the MatrixField is not dynamic indexable.\n{}",
+            stmt->tb);
+        auto stride = std::make_unique<ConstStmt>(
+            TypedConstant(origin->dynamic_index_stride));
+        auto offset = std::make_unique<BinaryOpStmt>(
+            BinaryOpType::mul, stmt->offset, stride.get());
+        auto ptr_base =
+            std::make_unique<GlobalPtrStmt>(origin->snodes[0], origin->indices);
+        auto lowered =
+            std::make_unique<MatrixPtrStmt>(ptr_base.get(), offset.get());
+        stmt->replace_usages_with(lowered.get());
+        modifier.insert_before(stmt, std::move(stride));
+        modifier.insert_before(stmt, std::move(offset));
+        modifier.insert_before(stmt, std::move(ptr_base));
+        modifier.insert_before(stmt, std::move(lowered));
+        modifier.erase(stmt);
+      }
     }
   }
 

tests/python/test_matrix.py

@@ -193,10 +193,7 @@ def func2(b: ti.types.ndarray(element_dim=1)):
     assert v[3][9] == 9
 
 
-@test_utils.test(require=ti.extension.dynamic_index,
-                 dynamic_index=True,
-                 debug=True)
-def test_matrix_non_constant_index():
+def _test_matrix_non_constant_index():
     m = ti.Matrix.field(2, 2, ti.i32, 5)
     v = ti.Vector.field(10, ti.i32, 5)
 
@@ -248,6 +245,20 @@ def func4(k: ti.i32):
     func4(10)
 
 
+@test_utils.test(require=ti.extension.dynamic_index,
+                 dynamic_index=True,
+                 debug=True)
+def test_matrix_non_constant_index():
+    _test_matrix_non_constant_index()
+
+
+@test_utils.test(require=ti.extension.dynamic_index,
+                 real_matrix=True,
+                 debug=True)
+def test_matrix_non_constant_index_real_matrix():
+    _test_matrix_non_constant_index()
+
+
 def _test_matrix_constant_index():
     m = ti.Matrix.field(2, 2, ti.i32, 5)