The vendor-defined XTHeadBa (predating the standard Zba extension)
extension adds an address-generation instruction (th.addsl) with
similar semantics as shadd from Zba. It is supported by the C9xx
cores (e.g., found in the wild in the Allwinner D1) by Alibaba T-Head.
The current (as of this commit) public documentation for XTHeadBa is
Support for these instructions has already landed in GNU Binutils: