What is the difference between _mm256_lddqu_si256 and _mm256_loadu_si256

Question

I used _mm256_lddqu_si256the example that I found on the Internet. I later discovered _mm256_loadu_si256. The Intel Intrinsics manual only states that a version lddqumay work better when crossing the cache line boundary. What could be the benefits loadu? In general, how do these functions differ?

Answer 1

It makes no sense to use_mm256_lddqu_si256 , considering it a synonym _mm256_loadu_si256. lddquexists only for historical reasons, since x86 has evolved to improve carrier support without binding, and processors that support the AVX version run them the same way. No version of AVX512.

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asm :

  # SSE packed-single instructions are shorter than SSE2 integer / packed-double
  4000e3:       0f 10 07                movups xmm0, [rdi]   

  4000e6:       f2 0f f0 07             lddqu  xmm0, [rdi]
  4000ea:       f3 0f 6f 07             movdqu xmm0, [rdi]

  4000ee:       c5 fb f0 07             vlddqu xmm0, [rdi]
  4000f2:       c5 fa 6f 07             vmovdqu xmm0, [rdi]
  # AVX-256 is the same as AVX-128, but with one more bit set in the VEX prefix

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Core2 lddqu, movdqu. Intel lddqu Core2, .

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. Dark Shikaris 2009 Diary Of x264 : Cacheline , .

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