CODEWITHSUNDEEP
c++c++11divisioncmath
alfC
alfC

Reputation: 16242

Why is std::remquo so slow?

In some inner loop I have:

    double x;
    ...
    int i = x/h_;
    double xx = x - i*h_;

Thinking that might be a better way to do this, I tried with std::remquo

    double x;
    ...
    int i;
    double xx = std::remquo(x, h_, &i);

Suddenly, timings went from 2.6 seconds to 40 seconds (for many executions of the loop).

The timing test is difficult to replicate here, but I did a online code to see if someone can help me to understand what is going on.

naive version: https://godbolt.org/z/PnsfR8

remquo version: https://godbolt.org/z/NSMwyW

It looks like the main difference is that remquo is not inlined and the naive code is. If that is the case, what is the purpose of remquo if it is going to be always slower than the manual code? Is it a matter of accuracy (e.g. for large argument) or not relying on (not well defined) casting conversion?

I just realized that the remquo version is not even doing something equivalent to the first code. So I am using it wrong. In any case, I am surprised that remquo is so slow.

Upvotes: 1

Views: 166

Answers (1)

M.M
M.M

Reputation: 141576

It's a rubbish function that was added to C99 to entice Fortran coders to switch to C. There's little reason to actually use it, so library vendors avoid wasting time optimizing it.

See also: What does the function remquo do and what can it be used for?.

BTW if you assumed that i gets the quotient stored in it, read the documentation more closely! (Or read the answers on the question linked in the previous paragraph).

Upvotes: 3

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