[SYCL] Implement accessors implicit conversions (#8369)

- Implement implicit conversions between readonly accessors with const /
non const types.
[4.7.6.9.3. Read only buffer command accessors and implicit
conversions](https://registry.khronos.org/SYCL/specs/sycl-2020/html/sycl-2020.html#sec:accessor.command.buffer.conversions)
- Implement implicit conversion from read_write T accessor to read only
T (const) accessor
- Add test for conversion.

Author: mmoadeli

sycl/include/sycl/accessor.hpp

@@ -1257,6 +1257,37 @@ class __SYCL_EBO __SYCL_SPECIAL_CLASS __SYCL_TYPE(accessor) accessor :
   // -------+---------+-------+----+-----+--------------
 
 public:
+  // implicit conversion between const / non-const types for read only accessors
+  template <typename DataT_,
+            typename = detail::enable_if_t<
+                IsAccessReadOnly && !std::is_same_v<DataT_, DataT> &&
+                std::is_same_v<std::remove_const_t<DataT_>,
+                               std::remove_const_t<DataT>>>>
+  accessor(const accessor<DataT_, Dimensions, AccessMode, AccessTarget,
+                          IsPlaceholder, PropertyListT> &other)
+#ifdef __SYCL_DEVICE_ONLY__
+      : impl(other.impl) {
+#else
+      : accessor(other.impl) {
+#endif // __SYCL_DEVICE_ONLY__
+  }
+
+  // implicit conversion from read_write T accessor to read only T (const)
+  // accessor
+  template <typename DataT_, access::mode AccessMode_,
+            typename = detail::enable_if_t<
+                (AccessMode_ == access_mode::read_write) && IsAccessReadOnly &&
+                std::is_same_v<std::remove_const_t<DataT_>,
+                               std::remove_const_t<DataT>>>>
+  accessor(const accessor<DataT_, Dimensions, AccessMode_, AccessTarget,
+                          IsPlaceholder, PropertyListT> &other)
+#ifdef __SYCL_DEVICE_ONLY__
+      : impl(other.impl) {
+#else
+      : accessor(other.impl) {
+#endif // __SYCL_DEVICE_ONLY__
+  }
+
   template <typename T = DataT, int Dims = Dimensions, typename AllocatorT,
             typename detail::enable_if_t<
                 detail::IsRunTimePropertyListT<PropertyListT>::value &&

sycl/unittests/accessor/AccessorImplicitConversion.cpp

@@ -0,0 +1,54 @@
+#include <gtest/gtest.h>
+
+#include <sycl/sycl.hpp>
+
+TEST(AccessorImplicitConversionTest, NonConstToConstTypeInReadAccessor) {
+  using accessor_non_const_type =
+      sycl::accessor<int, 1, sycl::access::mode::read, sycl::target::device>;
+  using acc_const_type = sycl::accessor<const int, 1, sycl::access::mode::read,
+                                        sycl::target::device>;
+  accessor_non_const_type acc_a;
+  auto acc_b = acc_const_type(acc_a);
+  acc_const_type acc_c = acc_a;
+  EXPECT_EQ(acc_a, acc_b);
+  EXPECT_EQ(acc_a, acc_c);
+}
+
+TEST(AccessorImplicitConversionTest, ConstToNonConstTypeInReadAccessor) {
+  using accessor_non_const_type =
+      sycl::accessor<int, 1, sycl::access::mode::read, sycl::target::device>;
+  using acc_const_type =
+      sycl::accessor<int, 1, sycl::access::mode::read, sycl::target::device>;
+  acc_const_type acc_a;
+  auto acc_b = accessor_non_const_type(acc_a);
+  accessor_non_const_type acc_c = acc_a;
+  EXPECT_EQ(acc_a, acc_b);
+  EXPECT_EQ(acc_a, acc_c);
+}
+
+TEST(AccessorImplicitConversionTest, ReadWriteAccessorToReadAccessorNonConst) {
+  using read_write_accessor_non_const_type =
+      sycl::accessor<int, 1, sycl::access::mode::read_write,
+                     sycl::target::device>;
+  using read_accessor_non_const_type =
+      sycl::accessor<int, 1, sycl::access::mode::read, sycl::target::device>;
+  read_write_accessor_non_const_type acc_a;
+  auto acc_b = read_accessor_non_const_type(acc_a);
+  read_accessor_non_const_type acc_c = acc_a;
+  EXPECT_EQ(acc_a.impl, acc_b.impl);
+  EXPECT_EQ(acc_a.impl, acc_c.impl);
+}
+
+TEST(AccessorImplicitConversionTest, ReadWriteAccessorToReadAccessorConst) {
+  using read_write_accessor_non_const_type =
+      sycl::accessor<int, 1, sycl::access::mode::read_write,
+                     sycl::target::device>;
+  using read_accessor_non_const_type =
+      sycl::accessor<const int, 1, sycl::access::mode::read,
+                     sycl::target::device>;
+  read_write_accessor_non_const_type acc_a;
+  auto acc_b = read_accessor_non_const_type(acc_a);
+  read_accessor_non_const_type acc_c = acc_a;
+  EXPECT_EQ(acc_a.impl, acc_b.impl);
+  EXPECT_EQ(acc_a.impl, acc_c.impl);
+}

sycl/unittests/accessor/CMakeLists.txt

@@ -1,5 +1,6 @@
 add_sycl_unittest(AccessorTests OBJECT
   AccessorHostTask.cpp
+  AccessorImplicitConversion.cpp
   AccessorIterator.cpp
   AccessorPlaceholder.cpp
   AccessorReverseIterator.cpp