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performancememoryx86x86-64memory-alignment
Jason Yu
Jason Yu

Reputation: 2038

Does modern x86-64 cpu still benefit from memory data alignment?

As titled,

I've searched a lot of old articles on the Internet regarding memory data alignment, but I am not sure whether they are still useful nowadays. So, the question is regarding modern x86-64 CPUs, whether the memory data alignment is still beneficial for efficient data access? Or just an old convention adopted by all compilers for backward compatibility?

Upvotes: 2

Views: 280

Answers (1)

Peter Cordes
Peter Cordes

Reputation: 364947

Yes for arrays because it means you'll avoid cache-line and page splits. See How can I accurately benchmark unaligned access speed on x86_64? for details on various penalties and how to measure them.

This is especially significant for vectorization with AVX-512, where looping over a misaligned array with 512-bit vectors means every load is a cache-line split. The penalty can be ~20%, vs. a few % with AVX 256-bit vectors on the same CPU even for data that's coming from L3 or DRAM, not L2 or L1d hits.

No for misalignment within a single cache line, for both integer and SIMD loads/stores on modern AMD and Intel microarchitectures. (Except for legacy-SSE where only aligned loads can be folded into a memory source operand like addps xmm0, [rdi] instead of separate movups. Unlike AVX where vaddps xmm0, xmm0, [rdi] doesn't require alignment.)

And yes potentially indirectly in terms of keeping all a struct's members in the same cache line, improving spatial locality.

Upvotes: 2

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