C++ Passing arguments to inline assembler function

Question

I have problem with inline asm in C++. I'm trying to implement fast strlen, but it is not working - when I use __declspec(naked) keyword debugger shows address of input as 0x000000, when I don't use that keyword, eax is pointing for some trash, and function returns various values.

Here's code:

   int fastStrlen(char *input) // I know that function does not calculate strlen
{                              // properly, but I just want to know why it crashes
    _asm                       // access violation when I try to write to variable x
    {
        mov ecx, dword ptr input
            xor eax, eax
        start:
            mov bx, [ecx]
            cmp bl, '\0'
            je Sxend
            inc eax
            cmp bh, '\0'
            je Sxend
            inc eax
            add ecx, 2
            jmp start
        Sxend:
            ret
    }
}

int _tmain(int argc, _TCHAR* argv[])
{
    char* test = "test";
    int x = fastStrlen(test);
    cout << x;
    return 0;
}

can anybody point me out what am I doing wrong?

Answer 1

Don't use __declspec(naked) since in that case the complier doesn't generate epilogue and prologue instructions and you need to generate a prologue just like compiler expects you to if you want to access the argument fastStrlen . Since you don't know what the compiler expects you should just let it generate the prologue.

This means you can't just use ret to return to the caller because this means you're supplying your own epilogue. Since you don't know what prologue the compiler used, you don't know what epilogue you need implement to reverse it. Instead assign the return value to a C variable you declare inside the function before the inline assembly statement and return that variable in a normal C return statement. For example:

int fastStrlen(char *input)
{
    int retval;
    _asm
    {
        mov ecx, dword ptr input
        ...
    Sxend:
        mov retval,eax
    }
    return retval;
}

As noted in your comments your code will not be able to improve on the strlen implementation in your compiler's runtime library. It also reads past the end of strings of even lengths, which will cause a memory fault if the byte past the end of a string isn't mapped into memory.