Re: undefined reference to... when using intel inline asm

[Lennyk <lennyk430@xxxxxxxxx>]

Ian Lance Taylor wrote:
> Lennyk <lennyk430@xxxxxxxxx> writes:
>
>   
> > Code (Window-style):
> > int bswap(int n)
> > {
> >    __asm
> >    {
> >       mov eax, n
> >       bswap eax
> >       mov n, eax
> >    }
> >    return n;
> > }
> >
> > After looking at some guides/tutorials I've translated this code to Linux:
> > int bswap(int n)
> > {
> >    asm(".intel_syntax noprefix");
> >    asm("mov eax, n");
> >    asm("bswap eax");
> >    asm("mov n, eax");
> >    asm(".att_syntax noprefix");
> >    return n;
> > }
> >
> > Compilation passes - but the linker shouts: "undefined reference to `n'"
> >
> > What am I doing wrong? Shouldn't it be straight forward to translate
> > these simple commands to Linux?
> >     
>
> gcc inline assembler does not work like that.  You can't simply refer to
> local variables in the assembler code.  I recommend the friendly manual:
>
> http://gcc.gnu.org/onlinedocs/gcc-4.4.0/gcc/Extended-Asm.html
>
> In this case, though, you shouldn't use inline assembler at all, just
> use __builtin_bswap32.
>
> http://gcc.gnu.org/onlinedocs/gcc-4.4.0/gcc/Other-Builtins.html
>
> Ian
>
>   
Thanks Ian!

After, reading further the extended asm guide I managed to implement the 
above function - but I still have difficulties with dereferenced pointers.

I would like to translate the following function (Windows-style):

void bswap(int* n)
{
   __asm
   {
      mov eax, [n]
      bswap eax
      mov [n], eax
   }
}

According to the guide - the corresponding GCC translation - should look 
like this:

void bswap(int* n)
{
   asm("movl       %0, %%eax" : : "g" (*n));
   asm("bswap    %eax");
   asm("movl      %%eax, %0" : "=g" (*n));
}

But, for some reason, the result is not a "bswapped" n.
Why?

Thanks!

BTW, __builtin_bswap32 is not available in the GCC version I'm currently 
using - but thanks for the tip!