std::kill_dependency
From cppreference.com
| Defined in header <atomic>
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| template< class T > T kill_dependency( T y ) noexcept; |
(since C++11) | |
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).
Contents |
[edit] Parameters
| y | - | the expression whose return value is to be removed from a dependency tree |
[edit] Return value
Returns y, no longer a part of a dependency tree.
[edit] Examples
[edit] file1.cpp:
struct Foo { int* a; int* b; }; std::atomic<Foo*> foo_head[10]; int foo_array[10][10]; // consume operation starts a dependency chain, which escapes this function [[carries_dependency]] 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]); }
[edit] file2.cpp:
[[carries_dependency]] struct Foo* f(int i); int g(int* x, int* y [[carries_dependency]]); int c = 3; void h(int i) { 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 }
[edit] See also
| (C++11) |
defines memory ordering constraints for the given atomic operation (enum) |
| C documentation for kill_dependency
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