[Base] Add aliasing-safe xe::memory::Reinterpret

Accessing the same memory as different types (other than char) using
reinterpret_cast or a union is undefined behavior that has already caused
issues like #1971.

Also adds a XE_RESTRICT_VAR definition for declaring non-aliasing pointers
in performance-critical areas in the future.

src/xenia/base/memory.h

@@ -16,6 +16,7 @@
 #include <functional>
 #include <string>
 #include <string_view>
+#include <type_traits>
 
 #include "xenia/base/assert.h"
 #include "xenia/base/byte_order.h"
@@ -24,6 +25,30 @@
 namespace xe {
 namespace memory {
 
+// For variable declarations (not return values or `this` pointer).
+// Not propagated.
+#define XE_RESTRICT_VAR __restrict
+
+// Aliasing-safe bit reinterpretation.
+// For more complex cases such as non-trivially-copyable types, write copying
+// code respecting the requirements for them externally instead of using these
+// functions.
+
+template <typename Dst, typename Src>
+void Reinterpret(Dst& XE_RESTRICT_VAR dst, const Src& XE_RESTRICT_VAR src) {
+  static_assert(sizeof(Dst) == sizeof(Src));
+  static_assert(std::is_trivially_copyable_v<Dst>);
+  static_assert(std::is_trivially_copyable_v<Src>);
+  std::memcpy(&dst, &src, sizeof(Dst));
+}
+
+template <typename Dst, typename Src>
+Dst Reinterpret(const Src& XE_RESTRICT_VAR src) {
+  Dst dst;
+  Reinterpret(dst, src);
+  return dst;
+}
+
 #if XE_PLATFORM_ANDROID
 void AndroidInitialize();
 void AndroidShutdown();