Can a Simple MOV Instruction Achieve Acquire-Release Semantics on x86 Architecture?

Acquire-Release Semantics in x86 Architecture: How MOV Can Achieve It

In the realm of multithreaded programming, ensuring proper data consistency is crucial. Memory ordering mechanisms like acquire-release (acq_rel) provide a means to enforce ordering constraints on memory operations. A common misconception is that acq_rel semantics on x86 require complex instructions like LOCK, fences, or xchg. However, the MOV instruction alone can suffice under certain circumstances.

Intel's documentation suggests that x86 enforces ordering principles within a single core. More specifically:

• Reads are not reordered with other reads.
• Writes are not reordered with older reads.
• Writes to memory are not reordered with other writes (with some exceptions on the same core).

However, in a multi-core system, the picture becomes more intricate. The ordering principles still apply to individual processors, but writes from different processors can be reordered with respect to each other.

Understanding the x86 Memory Model

The key to understanding how MOV alone can achieve acq_rel on x86 lies in the underlying memory model. Despite the potential for reordering within a core, the model assumes that access to shared memory is cache-coherent. This means that when a processor stores a value to shared memory, all other processors will eventually see the updated value.

With this assumption, the following phenomenon occurs:

• Once a store becomes globally visible to all cores, it did so instantaneously and consistently.
• No other cores could have observed the store before it became globally visible.

This behavior mimics acq_rel semantics, which requires that a release (store) operation makes the modified data visible to all other threads before any subsequent acquire (load) operation.

MOV Instruction and Acq_rel

In the case of MOV, it performs both a load and store operation in one instruction. However, due to the memory model outlined earlier, the store component of MOV acts as a release operation, while the load component acts as an acquire operation.

This means that when a thread writes a value to memory using MOV, any subsequent load operations by other threads will be guaranteed to see the updated value. Moreover, no other load operations from other threads can be reordered to happen before the MOV load operation by the releasing thread.

Implications for Multithreaded Programming

This understanding of how MOV achieves acq_rel on x86 has significant implications for multithreaded programming. Developers can use MOV to implement atomic variables and other synchronization primitives, ensuring proper data consistency without the overhead of complex instructions like fences or locks.

However, it's important to note that MOV alone cannot enforce sequential consistency. For that, a full memory barrier is required to prevent all reordering across processor cores.

The above is the detailed content of Can a Simple MOV Instruction Achieve Acquire-Release Semantics on x86 Architecture?. For more information, please follow other related articles on the PHP Chinese website!