Hi Hans-Peter, > While the choice of static-chain register does not affect the > ABI, it's the other way round: the choice of static-chain > register matters, specifically it's call-clobberedness.
Agreed. > It looks like the current aarch64 static-chain register R18 is > call-saved but without special provisions to save and restore > the static chain register, i.e. the port is broken wrt. > trampolines but may appear to work (likely as-if you got the > call-clobberedness wrong for a special case; I haven't > investigated). I understand the i386 port gets this right. > The CRIS port does not, but attempts and adds another bug (you > can't use the trampoline as a register-save area on return). > So, changing from R18 to R11 for aarch64 seems right, as the > latter is call-clobbered and the former is call-saved IIUC. The AArch64 ABI defines x18 as platform specific: http://infocenter.arm.com/help/topic/com.arm.doc.ihi0055b/IHI0055B_aapcs64.pdf On Linux it is call-clobbered, but it could be a fixed register on other platforms (eg. a thread-local pointer). I don't think it's possible to make it a callee-save. Still it is the wrong register to use since it already has different uses. Using x9 would make its use as an extra argument clearer. Wilco
