When I recently installed Windows 11 on my desktop, my photo editor Darktable suddenly got much slower than it used to be. When I looked into its preferences, I noticed OpenCL was no longer available.
As it turns out, some versions of the AMD graphics driver apparently no longer ship with OpenCL support on Windows. However, they do ship with the necessary libraries, it's just that these libraries are not registered any longer.
To register them, open the Registry Editor (aka regedit.exe), navigate to the key
HKEY_LOCAL_MACHINE\SOFTWARE\Khronos\OpenCL\Vendors (if the key does not exist, create it), and create a new DWORD of Value
0. Now rename the DWORD to be the path to your amdocl64.dll. Mine is
C:\Windows\System32\DriverStore\FileRepository\u0344717.inf_amd64_d38cec78c83eee99\B343886\amdocl64.dll. Search C:\Windows\System32\ for amdocl64.dll to find the correct path on your computer.
With that, OpenCL was once again recognized by Darktable and the rest of my photo editing programs.