Forcing the compiler to use a certain register for a certain variable

Question

Is it possible to force a compiler (gcc or clang in my case) to use a certain register, lets say R15 on x86_64 for a certain variable and also prohibit it from using R15 for any other purpose besides that variable.

Answer 1

gcc can do it.

The already cited gcc docs answers your question of how to force a variable into a certain register.

To stop the compiler from generating code using this register for other purposes, use the gcc switch -ffixed-reg (see gcc doc for details).