ARM 程序集 - 访问参数与返回值？

Question

I have a function prototype int Palindrome(const char *c_style_string);

In ARM v8 assembly, I believe that the parameter is stored in register w0. However, isn't this also the register that ret outputs the value of?

If so, what do I need to do so that values do not get overwritten? I was thinking something like mov w0, w1 at the beginning of my code so that I refer to c_style_string as w1 whenever I parse through it, and then edit w0 to store an int...would this be right?

Thank you!

Answer 1

You may want to write your assembly code in compliance with the ABI for ARM 64-bit Architecture .
In the example above, you could keep the address for c_style_string in a 'Callee-saved' register (X19-X29)', and copy it to x0/w0 every time you are calling a Palindrome() - I am assuming here Palindrome() is a C function, and is therefore itself compliant with the ARCH 64-bit ABI.
A desirable side-effect would be that your C code could call always your assembly code, and vice-versa.

Answer 2

IMHO, your best solution is to write the C function, or minimal function, then tell the compiler to output the assembly language. This will show the calling interface for functions.

You could also look up the register passing convention in your compiler's documentation.

If you want to preserve register values, you should use the PUSH instruction (or it's equivalent, depending on ARM mode or Thumb mode). Also remember to POP the registers before the end of the function.