3

I want to define a assembly to read register from arm64. Code is like the following:

asm volatile(
        "str <reg> [%0]\n"
        :
        : "r"(value)
        : "memory"
    );

how to use a macro to define such code. <reg> is in a string literal and the behaviour of macros make me confused when I use # and ##

I tried to define even in the following way, it still causes an error.

#define STR_x0 "str x0 [%0]\n"
#define STR_x1 "str x1 [%0]\n"
...
#define STR_REG(reg) STR_##reg

#define ASM_READ_REG(reg) \
    asm volatile( \
        STR_REG(reg) \
        : \
        : "r"(value) \
        : "memory" \
    )
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3
  • 2
    The operand for value needs to be a pointer not a value. Confusing name. Also, you're asking the compiler for a pointer in a register, so it's going to have to generate code that sets a register. It's likely to pick x0 or x1, overwriting whatever you had there. You could maybe put reg in the clobber list as well, or use { register unsigned long regval asm("x0"); asm volatile("" : "=r"(regval)); value = regval;} to capture a register value as an output operand for an asm statement. Commented Apr 8 at 3:03
  • It is also usually kind of pointless to know the value of a certain (general purpose) register when you are inside compiled code, since the compiler may have chosen to store anything in that register. Some strongly related questions: stackoverflow.com/questions/67860150/… stackoverflow.com/questions/67864098/… stackoverflow.com/questions/2114163/… Commented Apr 8 at 11:02
  • hi it still causes an error. what error? Commented Apr 12 at 10:53
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