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Date: Mon, 06 Jul 2009 07:40:00 -0000
From: Lennyk <lennyk430@gmail.com>
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To: Ian Lance Taylor <iant@google.com>
CC: gcc-help@gcc.gnu.org
Subject: Re: undefined reference to... when using intel inline asm
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Ian Lance Taylor wrote:
> Lennyk <lennyk430@gmail.com> 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!