Bug in std::floor?

[Marc Glisse]

On Fri, 10 Nov 2017, Nil Geisweiller wrote:

> On 11/09/2017 05:53 PM, Vincent Lefevre wrote:> For instance:
>> 
>> #include <stdio.h>
>> #include <math.h>
>> 
>> int main (void)
>> {
>>    double x = 1.7;
>>
>>    printf ("%.30g\n", x);
>>    printf ("%.30g\n", 100 * x);
>>    printf ("%.30g\n", floor (100 * x));
>>    return 0;
>> }
>> 
>> outputs:
>> 
>> 1.69999999999999995559107901499
>> 170
>> 170
>
> Indeed! I would really like to have a look at the floor implementation, but 
> it has proven difficult. It calls __builtin_floor, that is no where to be 
> found. It seems to be implemented in gcc/builtins.c but it's too obfuscated 
> for me to understand.

Look at the generated asm? Here, it is a single instruction: roundsd.
For constant folding, you would want to look at real.c.
builtins.c essentially delegates the work to each target to specify how 
floor should be expanded, and generates a call to the system's C library 
when that's not specified.

-- 
Marc Glisse