NAME

std::kill_dependency - std::kill_dependency

Synopsis

Defined in header <atomic>
template< class T > (since C++11)
T kill_dependency( T y ) noexcept;

Informs the compiler that the dependency tree started by an
std::memory_order_consume atomic load operation does not extend past the return
value of std::kill_dependency; that is, the argument does not carry a dependency
into the return value.

This may be used to avoid unnecessary std::memory_order_acquire fences when the
dependency chain leaves function scope (and the function does not have the
[[carries_dependency]] attribute)

Parameters

y - the expression whose return value is to be removed from a dependency tree

Return value

Returns y, no longer a part of a dependency tree.

Examples

file1.cpp:

struct foo { int* a; int* b; };
std::atomic<struct foo*> foo_head[10];
int foo_array[10][10];

// consume operation starts a dependency chain, which escapes this function
[[carries_dependency]] struct foo* f(int i) {
return foo_head[i].load(memory_order_consume);
}

// the dependency chain enters this function through the right parameter
// and is killed before the function ends (so no extra acquire operation takes place)
int g(int* x, int* y [[carries_dependency]]) {
return std::kill_dependency(foo_array[*x][*y]);
}

file2.cpp:

[[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); // dependency chain started inside f continues into p without undue acquire
do_something_with(g(&c, p->a)); // p->b is not brought in from the cache
do_something_with(g(p->a, &c)); // left argument does not have the carries_dependency
// attribute: memory acquire fence may be issued
// p->b becomes visible before g() is entered
}

See also

memory_order defines memory ordering constraints for the given atomic operation
(C++11) (enum)