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

This commit is contained in:
counterpoint
2014-10-17 14:10:16 +01:00
parent e8758d3678
commit bdff223607

View File

@ -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. * 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 * The add operation is atomic and the return value is the value stored in the
* prior to the operation. The number that is added may be signed, therefore atomic_subtract * location prior to the operation. The number that is added may be signed,
* is merely an atomic add with a negative value. * therefore atomic_subtract is merely an atomic add with a negative value.
* *
* @param variable Pointer the the variable to add to * @param variable Pointer the the variable to add to
* @param value Value to be added * @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 int
atomic_add(int *variable, int value) atomic_add(int *variable, int value)
{ {
#ifdef __GNUC__
return (int) __sync_fetch_and_add (variable, value);
#else
asm volatile( asm volatile(
"lock; xaddl %%eax, %2;" "lock; xaddl %%eax, %2;"
:"=a" (value) :"=a" (value)
: "a" (value), "m" (*variable) : "a" (value), "m" (*variable)
: "memory" ); : "memory" );
return value; return value;
#endif
} }