X18 on AArch64

[André Hentschel]

Hi all,
Note: I'm new to that mailinglist...

On AArch64 X18 is used as a platform register for some platforms, so to generate portable executables it should not be used by the compiler.
The use case i have for this is Wine. Windows arm64 programs use X18 as TLS register, thus it shouldn't be changed, otherwise it leads to a crash.
I worked on a patch [1], but realized that X17 is a bad choice (IP registers should be avoided on arm64 for the chain register it seems...),
so I wonder which register would be better. Something from X9-X15?

See also the bug report containing the patch:
[1] https://bugs.winehq.org/show_bug.cgi?id=38780