如何在ASM内联语句中请求通用寄存器？

Question

Is there a way to ask gcc to allocate a register for asm inline internal use only? Here is an example (in pseudo-asm) where r5 is directly used in the asm, but it could be any general purpose register, it is only for internal use, so neither an input nor an output :

asm("load_immediate_value %%r5,0;"
    /* ... */
    "add_immediate_value %%r5,%%r5,42"
    /* ... */
    :
    :
    : "r5");

In this example, I choose register r5 and tell gcc that I am using it through the clobber list. But what if r5 is an ABI register?! I need a way to simply ask for a register without having to name it myself.

Answer 1

The way this is usually done is:

static __inline__ void set_archctrl0(unsigned long val) {
    register unsigned long archctrl0 __asm__("archctrl0") = val;

    __asm__ __volatile__("" : "+r"(archctrl0) : : "cc", "memory");
}

The empty asm statement is there only to force the compiler not to optimize the whole thing out.

Or, a more practical example, reading the stackpointer register can be done like:

static __inline__ void * getSP(void) {
    register void * sp asm("sp");
    asm ("" : "=r"(sp));
    return sp;
}

The method to bind variables to specific registers is documented in the Explicit Register Variables section of the GCC manual.

Of course, if writing the arch-specific register requires a custom instruction (ie cannot be done using whatever type of "move" or "load" the CPU normally uses to initialize register values), you'll rather require something like:

static __inline__ void set_archctrl0(unsigned long val) {
    __asm__ __volatile__("setarchctrl0 %0" : : "r"(val) : "cc", "memory");
}

All of those, obviously, only work for registers that are not being used by "normal" code (ie outside the platform ABI / explicitly reserved by the platform ABI as globals / as regs not to be touched by compiled code).

You cannot instruct the compiler to "keep its hands off a general-purpose register" this way. To my knowledge, that isn't possible in gcc.