https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106338
--- Comment #6 from Kito Cheng <kito at gcc dot gnu.org> --- My understanding is static chain is sort of compiler internal implementation, any register could be picked if that is not used for passing argument, so I would also prefer keep that out psABI spec for now. And just record info for myself: x86-64 ABI has document function's static chain pointer in their ABI https://gitlab.com/x86-psABIs/x86-64-ABI/-/blob/master/x86-64-ABI/low-level-sys-info.tex#L701
