This option prevents the compiler from using the x18 register. It is only supported on aarch64.

From the ABI spec:

X18 is the platform register and is reserved for the use of platform ABIs. This is an additional temporary register on platforms that don't assign a special meaning to it.

This flag only has an effect when the x18 register would otherwise be considered a temporary register. When the flag is applied, x18 is always a reserved register.

This flag is intended for use with the shadow call stack sanitizer. Generally, when that sanitizer is enabled, the x18 register is used to store a pointer to the shadow stack. Enabling this flag prevents the compiler from overwriting the shadow stack pointer with temporary data, which is necessary for the sanitizer to work correctly.

Currently, the -Zsanitizer=shadow-call-stack flag is only supported on platforms that always treat x18 as a reserved register, and the -Zfixed-x18 flag is not required to use the sanitizer on such platforms. However, the sanitizer may be supported on targets where this is not the case in the future.

It is undefined behavior for -Zsanitizer=shadow-call-stack code to call into code where x18 is a temporary register. On the other hand, when you are not using the shadow call stack sanitizer, compilation units compiled with and without the -Zfixed-x18 flag are compatible with each other.