std::kill_dependency (3) - Linux Manuals

std::kill_dependency: std::kill_dependency

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
  }