How Can I Fix AVX Load/Store Alignment Issues for Optimal Performance?

How Do I Resolve the 32-Byte Alignment Issue for AVX Load/Store Operations?

Using unaligned load and store operations for AVX intrinsic functions can introduce alignment issues and subsequent memory access errors. To resolve this, use the "_mm256_loadu_ps" and "_mm256_storeu_ps" functions for unaligned access instead of their counterparts "_mm256_load_ps" and "_mm256_store_ps."

Alignment becomes particularly crucial with 512-bit AVX-512 vectors, contributing a significant speed advantage (15-20% on SKX) even with large arrays. Ensuring data alignment is also key for efficient cache usage, preventing performance degradation due to cache line splits and associated delays.

Dynamic Memory Allocation Techniques

For dynamic memory allocation where alignment matters, consider these techniques:

• C 17 Aligned New: Use the "std::align_val_t" and "aligned new" to allocate memory with aligned addresses greater than the standard alignment. This is straightforward for arrays like "__m256 arr[N]__" in C 17.
• Aligned Alloc: Rely on the "std::aligned_alloc" function to allocate memory with a specified alignment. However, it requires the size to be a multiple of the requested alignment.
• POSIX Memalign: Use the "posix_memalign" function, which takes a pointer to the requested memory address, alignment, and size as arguments.
• _mm_malloc: Utilize "_mm_malloc" specifically for AVX-related memory allocation. Note that pointers obtained from "_mm_malloc" cannot be freed with standard "free," and compatibility with "_mm_free" is not guaranteed across platforms.

Other Considerations

• Alignas: Employ "alignas(32)" with arrays or struct members to enforce 32-byte alignment for static and automatic storage. This technique works with C 17 for dynamically allocated storage as well.
• Direct OS Control: Consider using system calls like "mmap" or "VirtualAlloc" for custom memory allocation, allowing for page-aligned memory and OS-level control over page size and memory management.

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