Hassan_Fathi's blog

AC turns into TLE with a weird change
By Hassan_Fathi, history, 6 months ago, In English

Hello guys!

I was solving some problems on dp on subsets and I found this strange one

this got AC, but that gets TLE, and the only difference between them is swaping the inner 2 loops (mask and j).

I'm really curious to know why :D

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6 months ago, # |
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I guess it's a compiler optimization

In your code if $$$g[i][j]$$$ is false, it won't do anything in the loop.

I guess GCC adds if (!g[i][j]) break; to the end of line 19

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    6 months ago, # ^ |
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    Great, but this condition will never happen if the entire grid = 1

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      6 months ago, # ^ |
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      You're right, it might be because of random-access and you are defining vectors inside the function (i.e. heap/stack thing).

      But I don't think so because I resubmitted with defining in global and no difference.

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        6 months ago, # ^ |
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        Thanks for your time, I think now you are curious too :D

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6 months ago, # |
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Second code gets TLE'd because of some cache shenanigans.

My thoughts that CPU in first submit prefetches full cache line, so access to $$$newdp[mask]$$$ is almost free, but in the other submit, it can be washed out by accessing $$$g[i][j]$$$.

And AFAIK, arrays with len that are powers of two are quite bad for performance reasons on some CPUs, see this link for reference.

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    6 months ago, # ^ |
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    I thought that cache stuff don't affect complexity that much.

    Now I got it thanks :D

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      6 months ago, # ^ |
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      Technically you're right. It doesn't affect the complexity at all. It does affect the constant factor though, which is what's causing the 2.5x speedup here.

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        6 months ago, # ^ |
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        No words come after yours sir.

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6 months ago, # |
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similar thing has happened with me once on cf. i think it is due to more cache miss rate in second one but can't guess why it's happening

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