Can you somehow grab a value from a register and put it into a C variable in C with the asm keyword for arm64 macOS assembly?

Can you somehow grab a value from a register and put it into a C variable in C with the asm keyword for arm64 macOS assembly?

I have seen code from other stack overflows that have already answered this question for another CPU's assembly so I was wondering if I could somehow do that on arm64 macOS Assembly

Here's an example in WSL/Linux:

#include <inttypes.h>
#include <stdio.h>

void test(uint64_t u64)
{
    uint64_t reg;

    // Read the first argument into a local variable. 
    // Constraints make sure we get the correct value even if the compiler moves registers around.
    asm volatile("" : "=r"(reg) : "D"(u64));

    (void)printf("reg: 0x%" PRIX64 "\n", reg);
    (void)printf("u64: 0x%" PRIX64 "\n", u64);
}

int main(void)
{
    test(0xDEADBEEFCAFEBABELL);
    return 0;
}

$ gcc main.c -o main; ./main
reg: 0xDEADBEEFCAFEBABE
u64: 0xDEADBEEFCAFEBABE

This may work for you. I was able to test with QEMU:

#include <inttypes.h>
#include <stdio.h>

void test(uint64_t u64)
{
    uint64_t reg;

    // Read the first argument into a local variable. 
    // Constraints make sure we get the correct value even if the compiler moves registers around.
    asm volatile("mov %0, %1" : "=r"(reg) : "r"(u64));

    (void)printf("reg: 0x%" PRIX64 "\n", reg);
    (void)printf("u64: 0x%" PRIX64 "\n", u64);
}

int main(void)
{
    test(0xDEADBEEFCAFEBABELL);
    return 0;
}

$ aarch64-linux-gnu-gcc -static main.c -o main.arm64; qemu-aarch64 ./main.arm64;
reg: 0xDEADBEEFCAFEBABE
u64: 0xDEADBEEFCAFEBABE

I can create a text dump of the disassembly, for your system, like this:

$ aarch64-linux-gnu-gcc -S -O0 main.c -o main.s; aarch64-linux-gnu-objdump -d main.arm64 > disassembly.txt;

Let's look at the disassembly text dump for asm volatile("" : "=r"(reg) : "r"(u64));:

4007bc: f9400fe0  ldr x0, [sp, #24]
4007c0: aa0003e0  mov x0, x0
4007c4: f90017e0  str x0, [sp, #40]

1. ldr x0, [sp, #24] loads value in u64 from the stack into register x0
2. mov x0, x0 acts like a no-op
3. str x0, [sp, #40] stores value from register x0 into reg on the stack

Note: The compiler decides which registers to use for each variable, so there's no way to know exactly which physical register holds a value like u64.