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The [[carries_dependency]] attribute is a C++11 optimization hint used in concurrent programming to propagate data dependency chains across function boundaries. It informs the compiler that a value passed into or returned from a function carries a dependency originating from an atomic load operation with std::memory_order_consume, thereby suppressing the generation of unnecessary hardware memory barriers on weakly-ordered architectures.

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Syntax and Placement

The attribute appertains to two specific entities within a function signature:
  1. Function Declaration: Indicates that the function’s return value carries a dependency chain originating from within the function. It appertains to the function itself, not the return type.
  2. Function Parameter: Indicates that the argument passed to the function carries a dependency chain originating from the caller. To apply correctly to the parameter (and not the pointer/reference type), it must be placed before the declaration specifiers or after the parameter name.
// 1. Applied to the function declaration (indicates the return value carries a dependency)
[[carries_dependency]] int* process_node(int* node);

// 2. Applied to a function parameter 
// (Placed before the declaration specifiers)
void evaluate_node([[carries_dependency]] int* node);

// (Alternatively, placed after the parameter name)
void evaluate_node_alt(int* node [[carries_dependency]]);

// 3. Applied to both the function declaration and a parameter
[[carries_dependency]] int* transform_node([[carries_dependency]] int* node);

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Technical Semantics

To understand [[carries_dependency]], it must be contextualized with std::memory_order_consume. When a thread loads an atomic variable using memory_order_consume, the compiler establishes a dependency chain. Any subsequent operations that rely on the loaded value (e.g., dereferencing a loaded pointer) are guaranteed to be ordered after the load, without requiring a heavy memory fence. However, compilers typically treat function calls as opaque boundaries. Without the attribute: If a value carrying a dependency is passed into a function, or returned from one, the compiler must assume the dependency chain is broken. To maintain the memory ordering guarantees required by the C++ memory model, the compiler is forced to emit a hardware memory barrier (fence) instruction at the function boundary. With the attribute: [[carries_dependency]] bridges this opaque boundary.
  • When applied to a parameter, the callee assumes the caller has provided a value that already carries a dependency. The compiler can safely omit the memory barrier inside the callee.
  • When applied to a function declaration, the caller assumes the callee is returning a value that carries a dependency. The compiler can safely omit the memory barrier at the call site.

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Compilation and ABI Handling

The attribute affects the Application Binary Interface (ABI) at the compiler level. The compiler must manage the dependency state between the caller and the callee:
  • Matching Contexts: If the caller provides a dependency and the callee expects one ([[carries_dependency]] parameter), the code executes optimally without injected fences.
  • Mismatch Handling: If a function expects a dependency but is called with a standard, non-dependent value, the compiler automatically injects a memory barrier at the call site to satisfy the callee’s internal memory ordering requirements.

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Current Compiler Implementation Reality

While mechanically defined in the C++ standard, [[carries_dependency]] is effectively a historical artifact in modern C++ development. Because tracking dependency chains through arbitrary code paths proved exceptionally difficult for compiler authors, all major compilers (GCC, Clang, MSVC) currently map std::memory_order_consume directly to the stronger std::memory_order_acquire. Because memory_order_acquire enforces ordering via memory barriers rather than data dependency chains, the [[carries_dependency]] attribute is currently ignored by these compilers and acts as a semantic no-op.
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