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by BeeOnRope 2383 days ago
I'm not sure if you were implying it or just using it as example of another type of unhelpful claim, but this test does not involve AVX-512.

I agree using Westmere isn't necessarily the best approach, but there is no difference in this case with either -march=native or -march=znver1.

The loop is small and simple, with only 9 instructions and compiles more or less the same regardless of march setting (I observed some basically no-op changes such as a mov and blsr swapping places). Here's the assembly (for the second test, with the bigger IPC gap):

    top:
    tzcnt  r8,rcx
    add    r8d,edx
    mov    DWORD PTR [rdi+rax*4],r8d
    mov    eax,DWORD PTR [rsi]
    inc    eax
    blsr   rcx,rcx
    mov    DWORD PTR [rsi],eax
    jne    .top
1 comments
endorphone 2382 days ago
"I'm not sure if you were implying it or just using it as example of another type of unhelpful claim, but this test does not involve AVX-512."

Even worse! Is this a defense, because it's remarkably unhelpful as one.

The blog post was clearly a cry for attention for some project -- let's just use some clickbait IPC claims to gain it -- and continually alluded to a whole project -- an extreme niche project that still wouldn't have any relevance. But instead it's a meaningless, completely misrepresentative micro-loop.

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BeeOnRope 2382 days ago
My read is different than yours.

I think Daniel uses those examples because they are actual examples from projects that he is or has been working on, and he's familiar with them and actually cares about them, and because it's at least a notch more realistic than something totally synthetic.

It seems like a very roundabout thing to use as a cry for attention for SIMDjson (the project I assume you are talking about), and I don't believe that's the purpose. I see no problem in linking the project.

Picking two random benchmarks and trying to extract any kind of more general IPC claim is not on solid ground, but I'm pretty sure Daniel will say he's not doing that: he's only sharing these two specific results. That's a style that reoccurs across several entries in that blog, however, so if it triggers (as it has me on occasion) you might want to look elsewhere.

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