When to Use _mm_sfence, _mm_lfence, and _mm_mfence?

When to Use _mm_sfence, _mm_lfence, and _mm_mfence

Memory fences play a crucial role in multi-threaded programming to enforce memory ordering and prevent uncontrolled reordering of memory operations. Intel provides three types of memory fences: _mm_sfence, _mm_lfence, and _mm_mfence, each serving specific purposes.

_mm_sfence

_mm_sfence is primarily used when dealing with "NT stores," which are weakly-ordered memory operations. These stores are often used to improve performance by avoiding cache misses but require proper synchronization to ensure the correct order of memory operations. _mm_sfence acts as a "fence" that ensures all weakly-ordered operations preceding it are completed before any subsequent operations can proceed.

_mm_lfence

_mm_lfence is intended as a load fence, preventing the execution of any subsequent loads from bypassing the _mm_lfence instruction. However, this functionality is not typically practical as loads can only be weakly ordered in specific situations, such as when accessing Write-Combining (WC) memory regions. In most cases, the use of _mm_lfence to order loads is unnecessary.

_mm_mfence

_mm_mfence represents the strongest memory fence and ensures sequential consistency, forcing preceding writes to be globally visible before any subsequent operations. This guarantees that no later reads will observe a value until after all preceding stores become globally visible. While _mm_mfence provides the highest level of synchronization, it also incurs the highest performance overhead.

Alternatives to Memory Fences

For most scenarios, using C 11's std::atomic or C11's stdatomic is a more convenient and efficient approach for controlling memory ordering. These provide a comprehensive set of operations with built-in synchronization guarantees, eliminating the need for manual memory fence usage.

Conclusion

Understanding when to use _mm_sfence, _mm_lfence, and _mm_mfence is essential for ensuring correct behavior in multi-threaded code. While _mm_sfence is crucial for synchronizing weakly-ordered stores, _mm_lfence and _mm_mfence have more limited use cases. By leveraging these fences appropriately or using std::atomic, programmers can effectively manage memory ordering and prevent data races and other concurrency issues.

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