Bug in std::floor?

Jonathan Wakely jwakely@redhat.com
Wed Nov 8 22:25:00 GMT 2017


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On 08/11/17 23:13 +0200, Nil Geisweiller wrote:
>Hi,
>
>The following
>
>---------------------------------------------------------------------
>#include <iostream>
>#include <cmath>
>
>int main()
>{
>    double v = 4.6;
>    std::cout << "v = " << v << std::endl;
>    std::cout << "v*100 = " << v*100 << std::endl;
>    std::cout << "floor(v*100) = " << std::floor(v*100) << std::endl;
>}
>---------------------------------------------------------------------
>
>outputs
>
>------------------
>v = 4.6
>v*100 = 460
>floor(v*100) = 459
>------------------
>
>It that a bug?

No.

Try:

#include <iostream>
#include <iomanip>
#include <cmath>

int main()
{
    double v = 4.6;
	std::cout << "v = " << std::setprecision(17)  << v << std::endl;
	std::cout << "v*100 = " << std::setprecision(16) << v*100 << std::endl;
	std::cout << "floor(v*100) = " << std::floor(v*100) << std::endl;
}

>If so where is the implementation of floor (so I can study and fix)? I 
>know it is supposed to be in gcc/builtins.c but I don't understand 
>where.

It doesn't necessarily use __builtin_floor anyway (without optimzation
it just calls the C library's floor(double) function).

>You may run the program above from http://tpcg.io/QNfhYr
>
>Thanks,
>
>Nil



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