This option treats the specified register names as fixed registers, and prevents the compiler from using them in the code that is generated.
Try to avoid using this option, because it restricts the compiler in terms of register allocation and can potentially give a negative effect on code generation and performance.
the AAPCS name of the register, denoted by an integer value in the
sequentially onto registers
unspecified, the compiler faults use of
ARM Software Development Toolkit Reference Guide.