Replacing inline assembly code

I have to write inline assembly code that executes a custom instruction that I integrated into my hardware.

Depending on what hardware is to find on the actual chip, the instruction behaves differently. My assembly looks as follows:

    asm volatile (
    " instr_generic %1, %2, %0          \n\t"
    : "=r" (c)            
    : "r" (a), "r" (b)       
    : "%g0"                                                   
    );

This instr_generic could now execute either an addition or subtraction for example, depending on what is on the hardware.

Now, instead of instr_generic I wanna write cust_add or cust_sub and this should then be replaced wi开发者_开发技巧th instr_generic. In other words, it should look like this here

    #define cust_add instr_generic

    ...

    asm volatile (
    " cust_add %1, %2, %0          \n\t"
    : "=r" (c)            
    : "r" (a), "r" (b)       
    : "%g0"                                                   
    );

But I guess I can't use the pre-processor in this context to replace inline assemly is that right? Is there another way to do that easily?

...
#define cust(arg) \
asm volatile (
" " #arg " %1, %2, %0          \n\t" \
: "=r" (c) \           
: "r" (a), "r" (b) \      
: "%g0" \                                                   
)

...
cust(cust_add);

I would either do an if-then-else with the different solutions based on runtime detection of the processor, or to squeeze a little speed, use a function pointer to functions containing the different solution, if detected a then funptr = a_solution, else if detected b then funptr = b_solution, etc. Do that one time then use funptr for the duration of the program.

As already mentioned the custom instruction needs to be compiled at compile time not runtime. if you want to change the instruction runtime that is a third option to do self-modifying code to insert the proper instruction at runtime.

Can't you just use string concatenation? Or is there some reason you can't do it that way?

#define cust_add "instr_generic"

...

asm volatile (
cust_add " %1, %2, %0          \n\t"
: "=r" (c)            
: "r" (a), "r" (b)       
: "%g0"                                                   
);