The first declaration of a function shall specify the carries_dependency attribute for its declarator-id if any declaration of the function specifies the carries_dependency attribute. Furthermore, the first declaration of a function shall specify the carries_dependency attribute for a parameter if any declaration of that function specifies the carries_dependency attribute for that parameter. If a function or one of its parameters is declared with the carries_dependency attribute in its first declaration in one translation unit and the same function or one of its parameters is declared without the carries_dependency attribute in its first declaration in another translation unit, the program is ill-formed, no diagnostic required.
[ Note: The carries_dependency attribute does not change the meaning of the program, but may result in generation of more efficient code. — end note ]
[ Example:
/* Translation unit A. */
struct foo { int* a; int* b; };
std::atomic<struct foo *> foo_head[10];
int foo_array[10][10];
[[carries_dependency]] struct foo* f(int i) {
return foo_head[i].load(memory_order_consume);
}
int g(int* x, int* y [[carries_dependency]]) {
return kill_dependency(foo_array[*x][*y]);
}
/* Translation unit B. */
[[carries_dependency]] struct foo* f(int i);
int g(int* x, int* y [[carries_dependency]]);
int c = 3;
void h(int i) {
struct foo* p;
p = f(i);
do_something_with(g(&c, p->a));
do_something_with(g(p->a, &c));
}
The carries_dependency attribute on function f means that the return value carries a dependency out of f, so that the implementation need not constrain ordering upon return from f. Implementations of f and its caller may choose to preserve dependencies instead of emitting hardware memory ordering instructions (a.k.a. fences).
Function g's second parameter has a carries_dependency attribute, but its first parameter does not. Therefore, function h's first call to g carries a dependency into g, but its second call does not. The implementation might need to insert a fence prior to the second call to g.
— end example ]
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