Modify atomic_add to use built in GCC function where available. Correct comments.

server/core/atomic.c

@@ -29,23 +29,32 @@
  */
 
 /**
- * Implementation of an atomic add operation for the X86 processor.
+ * Implementation of an atomic add operation for the GCC environment, or the 
+ * X86 processor.  If we are working within GNU C then we can use the GCC
+ * atomic add built in function, which is portable across platforms that
+ * implement GCC.  Otherwise, this function currently supports only X86
+ * architecture (without further development).
+ * 
  * Adds a value to the contents of a location pointed to by the first parameter.
- * The add operation is atomic and the return value is the value stored in the location
- * prior to the operation. The number that is added may be signed, therefore atomic_subtract
- * is merely an atomic add with a negative value.
+ * The add operation is atomic and the return value is the value stored in the 
+ * location prior to the operation. The number that is added may be signed, 
+ * therefore atomic_subtract is merely an atomic add with a negative value.
  *
  * @param variable	Pointer the the variable to add to
  * @param value		Value to be added
- * @return		Pointer to the value of variable before the add occured
+ * @return		The value of variable before the add occurred
  */
 int
 atomic_add(int *variable, int value)
 {
+#ifdef __GNUC__
+        return (int) __sync_fetch_and_add (variable, value);
+#else    
 	asm volatile(
 		"lock; xaddl %%eax, %2;"
 		:"=a" (value)
 		: "a" (value), "m" (*variable)
 		: "memory" );
 	return value;
+#endif
 }