How to Ensure 32-Byte Alignment for Optimal AVX Load/Store Performance?

How to Handle 32-Byte Alignment for AVX Load/Store Operations

In this context, you encounter an alignment issue with AVX load/store operations due to unaligned memory access. Here's how to address this issue:

Unaligned Load/Store Operations with _mm256_loadu_ps / _mm256_storeu_ps

For unaligned memory access, you can use _mm256_loadu_ps and _mm256_storeu_ps instead. These intrinsics perform load and store operations without requiring alignment. In most cases, using these intrinsics for aligned data is just as efficient as using the alignment-required load/store operations.

Considerations for Alignment

Alignment is particularly crucial for 512-bit AVX-512 vectors, where proper alignment can improve performance by up to 20%. For AVX2 CPUs, alignment is still significant, especially if the data is stored in L2 or L1d cache.

Dynamic Allocation of Aligned Memory

In C 17, you can use aligned_new operator to allocate aligned memory. This operator ensures that the allocated memory is aligned according to the alignment specified for the type being allocated.

For example, to allocate an array of aligned floats:

float *arr = new (std::align_val_t(32)) float[size];  // C++17

Workarounds for Plain-Delete Compatible Allocation

If you cannot use aligned_new due to incompatibility with plain delete, you can use the following workarounds:

• Structure Wrapping:

  struct alignas(32) s { float v; };
  new s[numSteps];

• Placement Parameters:

  new (std::align_val_t(32)) float[numSteps];

Other Dynamic Allocation Options

Other dynamic allocation options include std::aligned_alloc, posix_memalign, and _mm_malloc. However, these options have limitations and may not be compatible with free.

Alignas() with Arrays and Structures

In C 11 and later, you can use alignas(32) to enforce alignment for struct/class members, ensuring that static and automatic storage objects of that type have 32B alignment. However, dynamic allocation of such types requires C 17 compatibility.

Beware of Unnecessary Padding

Finally, avoid unnecessary padding by allocating a larger buffer and manually aligning it. This approach is inefficient and impractical.

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