After preprocessing, __asm functions can only contain assembly code, with the exception of the following embedded assembler builtins:

__cpp(expr)
__offsetof_base(D, B)
__mcall_is_virtual(D, f)
__mcall_is_in_vbase(D, f)
__mcall_offsetof_base(D, f)
__mcall_this_offset(D, f)
__vcall_offsetof_vfunc(D, f)

No return instructions are generated by the compiler for an __asm function. If you want to return from an __asm function, you must include the return instructions, in assembly code, in the body of the function.

__asm functions do not change the ARM Architecture Procedure Call Standard (AAPCS) rules that apply. This means that all calls between an __asm function and a normal C or C++ function must adhere to the AAPCS, even though there are no restrictions on the assembly code that an __asm function can use (for example, change state).