Compiler does not recognize the reserve-r9 target feature

[Alvenix]

This issue is similar to this issue. On the armv7a-none-eabi target, specifying the reserve-r9 target feature using the flag -Ctarget-feature=+reserve-r9 results in the following warning:

warning: unknown and unstable feature specified for `-Ctarget-feature`: `reserve-r9`
  |
  = note: it is still passed through to the codegen backend, but use of this feature might be unsound and the behavior of this feature can change in the future
  = help: consider filing a feature request

The feature is currently working and is used by me and others too.

[Alvenix]

l think this should later be part of target modifiers.

[Darksonn]

Can you please clarify what this flag is used for?

[Alvenix]

Can you please clarify what this flag is used for?

I am using the flag with nightly Rust to perform FFI with u-boot for armv7-a (which builds with -ffixed-r9 with CC arm-linux-gnueabihf-gcc).

Quoting from u-boot repo:

The ARM backend can be instructed not to use the r9 and x18 registers using
-ffixed-r9 or -ffixed-x18 respectively. As global registers themselves are not
supported inline assembly is needed to get and set the r9 or x18 value. This
leads to larger code then strictly necessary, but at least works.

It seems that ffixed-x18 warning was removed but ffixed-r9 still have the warning.

Also quoting from this post in rust internals forum:

Hello, I'm developing for an embedded arm platform. Currently our code is in C and is compiled with clang. The code uses r9 as a "per thread" data and must remain untouched. To achieve that we use -ffixed-r9. We want to move to using rust on that platform and that requires as to do the same, keep r9 fixed.