Re: Clang as default compiler November 4th

On 15-09-2012 17:39, Mehmet Erol Sanliturk wrote:
> On Sat, Sep 15, 2012 at 7:30 AM, Tijl Coosemans <tijl_at_coosemans.org> wrote: 
>> On 15-09-2012 16:09, Roman Divacky wrote:
>>> Is this correct?
>>>
>>> lev ~$ ./cos 1.23456789e20
>>> 6.031937e-01
>>> -9.629173e-02
>>> 2.814722e-01
>>
>> Yes, that's what the libm call returns. 
> 
> Linux z 3.5.3-1.fc17.x86_64 #1 SMP Wed Aug 29 18:46:34 UTC 2012 x86_64
> x86_64 x86_64 GNU/Linux
> 
> clang version 3.0 (tags/RELEASE_30/final)
> Target: x86_64-redhat-linux-gnu
> Thread model: posix
> 
> 
> Output of the initial program is the following :
> 
> #include <math.h>
> #include <stdio.h>
> #include <stdlib.h>
> 
> int
> main( int argc, char **argv ) {
>         double d = strtod( argv[ 1 ], NULL );
> 
>         printf( " cos : %e\n", ( double ) cos( d ));
>         printf( "cosf : %e\n", ( double ) cosf( d ));
>         printf( "cosl : %e\n", ( double ) cosl( d ));
>         return( 0 );
> }
> 
> 
> cos : 2.814722e-01
> cosf : -9.629173e-02
> cosl : 7.738403e-01

This is probably because SSE instructions are used on amd64.

> Output of the following program is different :

The reason is that...

> #include <math.h>
> #include <stdio.h>
> #include <stdlib.h>
> 
> int
> main( int argc, char **argv ) {
>         double d ;
>         double two_pi ;
>         double f ;
>         double v ;
> 
>     two_pi = 2 * 3.14159265358979323846 ;
>         d = strtod( argv[ 1 ], NULL );
> 
>     f = floor ( d / two_pi ) ;
>         v = d - f * two_pi ;

...this is a poor way to compute a remainder. Try to use fmod() or
remainder() instead.

>         printf( "  given : %e\n", ( double ) d );
>         printf( "  multiplier : %e\n", ( double ) f );
>         printf( "reduced : %e\n", ( double ) v );
> 
> 
>         printf( " cos ( %e ) : %e\n", d , ( double ) cos( d ));
>         printf( "cosf ( %e ) : %e\n", d , ( double ) cosf( d ));
>         printf( "cosl ( %e ) : %e\n", d , ( double ) cosl( d ));
> 
> 
>         printf( " cos ( %e ) : %e\n", v , ( double ) cos( v ));
>         printf( "cosf ( %e ) : %e\n", v , ( double ) cosf( v ));
>         printf( "cosl ( %e ) : %e\n", v , ( double ) cosl( v ));
> 
> 
>     return( 0 );
> }
> 
> 
>   given : 1.234568e+20
>   multiplier : 1.964876e+19
> reduced : 1.638400e+04
> 
> 
>  cos ( 1.234568e+20 ) : 2.814722e-01
> cosf ( 1.234568e+20 ) : -9.629173e-02
> cosl ( 1.234568e+20 ) : 7.738403e-01
> 
>  cos ( 1.638400e+04 ) : -8.285342e-01
> cosf ( 1.638400e+04 ) : -8.285342e-01
> cosl ( 1.638400e+04 ) : -8.285342e-01