C汇编中的变量用法，如何在arm aarch64中选择32位操作数

Question

I am trying to use C variables in assembly. The purpose is to read a 32bit memory and assign it to a C variable.

uint32_t ASMRegRd32(uint32_t addr) {

uint32_t data;

    asm volatile (

        "ldr %0, [%1]"          "\n"

        : "=r" (data)

        : "r"(addr)

          );

return data;

}

Sadly being on AARCH64, above ldr instruction is returning a 64bit value, as compiler is choosing a 64bit operand Xt.

How can I restrict it to use a 32bit operand?
Should this work?

ldr w0, [%1];

mov %0, w0;

Answer 1

You use the constraint template modifier "w".

asm (
    "ldr %w[DST],[%[SRC]]" "\n"
    : [DST] "=r" (data) // 32 bit variable
    : [SRC] "X" (addr) // source address variable (expands into a x register)
);

Answer 2

Maybe you should change your API like below if you don't want to recompile the code in A32 compiler.

uint32_t ASMRegRd32(uint64_t addr) {

uint32_t data;

asm volatile (
    "ldrh w0, [%1]\n"
    : "=&r" (data)
    : "r"(addr)
    : "cc"
      );

return data;

}

because the default register is 64-bit, so the var addr conversion from 64-bit to 32-bit will make the program core dump.