[SYCL] Clarify why global address space is used instead of constant

Signed-off-by: Mariya Podchishchaeva <[email protected]>

Author: Fznamznon

sycl/include/CL/sycl/access/access.hpp

@@ -127,6 +127,14 @@ struct PtrValueType<ElementType, access::address_space::global_space> {
 
 template <typename ElementType>
 struct PtrValueType<ElementType, access::address_space::constant_space> {
+// Current implementation of address spaces handling leads to possibility
+// of emitting incorrect (in terms of OpenCL) address space casts from constant
+// to generic (and vise-versa). So, global address space is used here instead of
+// constant to avoid incorrect address space casts in the produced device code.
+// "const" qualifier is not used here because multi_ptr interface contains
+// methods which return pure ElementType without qualifiers and adding const
+// qualifier here will require adding const casts to multi_ptr methods to remove
+// const qualifiers from underlying pointer type.
   using type = SYCL_GLOBAL_AS ElementType;
 };