How to Handle Alignment Issues When Using AVX Load/Store Operations?

How to Address Alignment Issues with AVX Load/Store Operations

Problem:

When using YMM registers with AVX intrinsics, developers may encounter alignment issues, leading to a program crash when trying to store to a memory address that is not properly aligned to 32-byte boundaries. This alignment issue is caused by the fact that YMM registers require 32-byte alignment for optimal performance.

Workaround:

To resolve this issue, developers can utilize AVX unaligned load/store intrinsics _mm256_loadu_ps / storeu. These intrinsics allow data to be loaded or stored even if it is not properly aligned. While using unaligned memory access may lead to a slight performance penalty, it ensures that the program can run without crashing.

Best Practices:

For optimal performance, it is generally recommended to align data to 32-byte boundaries whenever possible. This can be achieved using alignas(32) when declaring arrays or structures. By default, new and malloc allocate memory with an alignment of max_align_t, which may be insufficient for AVX operations.

Alternatives:

• new(std::align_val_t(32)): In C 17 and above, this syntax can be used to explicitly allocate memory with 32-byte alignment.
• std::aligned_alloc(32, size): This function attempts to allocate memory with 32-byte alignment. However, it is important to note that it requires the size to be a multiple of 32.
• posix_memalign: This POSIX function can allocate memory with arbitrary alignment. However, it is not standardized and may not be available on all platforms.
• _mm_malloc: This Intel function allocates memory with 32-byte alignment. However, it is only compatible with Intel's MKL (_mm_whatever_ps) functions and not with standard C or C memory management functions.
• mmap / VirtualAlloc: System-level functions can be used to allocate memory with specific alignment and page permissions. This approach is typically recommended for large memory allocations.

Additional Considerations:

• Alignas on Arrays/Structs: In C 11 and later, alignas(32) can be used on arrays or struct members to enforce 32-byte alignment.
• Alignment in C 17: C 17 introduces automatic alignment for certain types like __m256, ensuring that they are allocated with the correct alignment.
• Trade-Off: It is important to balance alignment requirements with performance considerations. Unaligned memory access can lead to performance penalties, so it should only be used when necessary.

The above is the detailed content of How to Handle Alignment Issues When Using AVX Load/Store Operations?. For more information, please follow other related articles on the PHP Chinese website!