Indicates that dependency chain in release-consume std::memory_order propagates in and out of the function, which allows the compiler to skip unnecessary memory fence instructions.
[edit] Syntax [edit] ExplanationThis attribute may appear in two situations:
1) it may apply to the parameter declarations of a function or lambda-expressions, in which case it indicates that initialization of the parameter carries dependency into lvalue-to-rvalue conversion of that object.
2) It may apply to the function declaration as a whole, in which case it indicates that the return value carries dependency to the evaluation of the function call expression.
This attribute must appear on the first declaration of a function or one of its parameters in any translation unit. If it is not used on the first declaration of a function or one of its parameters in another translation unit, the program is ill-formed; no diagnostic required.
[edit] ExampleAdapted almost without change from SO.
#include <atomic>
#include <iostream>
void print(int* val)
{
std::cout << *val << std::endl;
}
void print2(int* val [[carries_dependency]])
{
std::cout << *val << std::endl;
}
int main()
{
int x{42};
std::atomic<int*> p = &x;
int* local = p.load(std::memory_order_consume);
if (local)
{
// The dependency is explicit, so the compiler knows that local is
// dereferenced, and that it must ensure that the dependency chain
// is preserved in order to avoid a fence (on some architectures).
std::cout << *local << std::endl;
}
if (local)
{
// The definition of print is opaque (assuming it is not inlined),
// so the compiler must issue a fence in order to ensure that
// reading *p in print returns the correct value.
print(local);
}
if (local)
{
// The compiler can assume that although print2 is also opaque then
// the dependency from the parameter to the dereferenced value is
// preserved in the instruction stream, and no fence is necessary (on
// some architectures). Obviously, the definition of print2 must actually
// preserve this dependency, so the attribute will also impact the
// generated code for print2.
print2(local);
}
}
Possible output:
[edit] ReferencesRetroSearch is an open source project built by @garambo | Open a GitHub Issue
Search and Browse the WWW like it's 1997 | Search results from DuckDuckGo
HTML:
3.2
| Encoding:
UTF-8
| Version:
0.7.4