atomic_add_and_test

atomic_add_and_test

[Mark Lobo]

Guys
Im trying to implement this function which I absolutely need. The function 
is as follows:
int atomic_add_and_test(atomic_t *value, int addval)
This adds "addval" to "value" and returns the old value of "value" back.
Im not an expert at assembly syntax, Im not even sure how to write this 
function. I had done some 886 assembly and some microcontroller assembly 
stuff, but that was ages back. I would really really appreciate if someone 
can correct the source I have below. I did try to read up and I will 
definitely learn it back:) But I need this right now, so I would really 
appreciate it if  someone can correct this for me:) This might be assembly 
101:) but Im unfortunately its been ages since I did it.

static __inline__ int32_t atomic_add_and_test(atomic_t * p_plValue, int32_t 
p_lAdd)
{
    int32_t l_lResult;
    __asm__ __volatile__ (
	    "push %%EBX;"
	    "movl %1, %%EBX;"
	    "movl %2, %%EAX;"
	    "lock; xadd %%EAX, (%%EBX);"
	    "inc %%EAX;"
	    "mov %%EAX, %0;"
	    "pop %%EBX"
	    : "=g"(l_lResult) : "g"(p_plValue), "g"(p_lAdd):"%eax", "ebx" );
    return l_lResult;
}

Thanks!
Mark

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Re: atomic_add_and_test

[Maciej Hrebien]

Mark Lobo wrote:
> 
> static __inline__ int32_t atomic_add_and_test(atomic_t * p_plValue, int32_t
> p_lAdd)
> {
>     int32_t l_lResult;
>     __asm__ __volatile__ (
>             "push %%EBX;"
>             "movl %1, %%EBX;"
>             "movl %2, %%EAX;"
>             "lock; xadd %%EAX, (%%EBX);"
>             "inc %%EAX;"
>             "mov %%EAX, %0;"
>             "pop %%EBX"
>             : "=g"(l_lResult) : "g"(p_plValue), "g"(p_lAdd):"%eax", "ebx" );
>     return l_lResult;
> }

Can't you just:

inline int add_and_test(int* value, int to_add)
{
   int old_result;

   asm volatile("
      lock
      xadd %%eax,(%%ebx)
   " : "=eax" (old_result) : "a" (to_add), "b" (value) );

   return old_result;
}

? Do you have to push %ebx on stack? Why do you inc %eax after xadd?

Regards,

-- 
Maciej Hrebien