How Does a Single MOV Instruction Achieve Acquire-Release Semantics on x86?

C Memory Ordering: Achieve Release-and-Acquire Semantics with MOV on x86

Question: How does the MOV instruction provide acquire-release semantics on x86 without the use of additional memory barriers or synchronization primitives?

Answer:

Unlike conventional processor architectures where multiple instructions are required to enforce release-and-acquire memory ordering, x86's MOV instruction achieves this functionality elegantly. This is possible due to the cache-coherent nature of x86 processors, which ensures a coherent shared view of memory across all cores.

The x86 memory model adheres to the following principles:

• Loads are never reordered with other loads.
• Writes are not reordered with older reads.
• Writes to memory are not reordered with other writes, with the exception of store buffering.

Store buffering introduces a level of local reordering within each CPU core. However, once a store becomes globally visible, it becomes visible to all cores simultaneously and without any reordering.

Therefore, the MOV instruction performs atomic stores with acquire-release semantics by leveraging the following properties:

• Store Ordering: The memory write performed by MOV immediately becomes visible to other cores without any intermediate reordering.
• Cache Coherence: All cores share a coherent view of shared memory, ensuring that writes become visible to all other cores at the same time.

As a result, a single MOV instruction on x86 both releases the updated value in shared memory for other threads to acquire and acquires the value from shared memory for the current thread. This behavior effectively implements the acquire-release semantics required for synchronization in high-performance computing.

Additional Considerations:

• Other ISAs may have weaker memory models that allow more aggressive reordering, in which case additional synchronization primitives may be necessary to achieve acquire-release semantics.
• For stronger memory models, such as those commonly used in C , the MOV instruction alone is sufficient to implement acquire-release semantics because these models forbid certain reordering scenarios that would compromise memory consistency.

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