johnchen902's blog

TIL NaNs and infinities slow down programs
By johnchen902, history, 8 years ago, In English

Today I learned that NaNs and infinites may slow down programs significantly. So instead of

// some computation that produce NaN when arg == 0
if(arg == 0) // special case
    result = 0;

one should write

if(arg == 0)
    result = 0; // special case
else
    // some computation that produce NaN when arg == 0

Compare submission 19257027 and 19257693 and you'll see.

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8 years ago, # |
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Instructions involving denormalized(very small) values are also very slow.

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    8 years ago, # ^ |
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    I've heard of a trick to optimize the DP (sometimes up to tens times!):

    d[i][j] = ... (compute d[i][j] here)
if (d[i][j] < 1e-18) d[i][j] = 0; // this is the trick

    After this optimization no computations are made with very small numbers, so denormalized numbers are not involved too, so it causes a speedup.

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    8 years ago, # ^ |
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    I've never experimented with denormals, but supposedly you can turn them off by calling _controlfp_s with the _DN_FLUSH flag set.

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8 years ago, # |
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After reading this blog, I realized...

Thanks for the heads-up :) I guess I got really lucky today. (After fixing issue you described the time became 140ms)

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