Calculating cosinus/sinus

[David Brown]

On 11/05/13 17:20, jacob navia wrote:
> Le 11/05/13 16:01, Ondřej Bílka a écrit :
>> As 1) only way is measure that. Compile following an we will see who is
>> rigth.
>>
>> cat "
>> #include <math.h>
>>
>> int main(){ int i;
>>    double x=0;
>>
>>    double ret=0;
>>    double f;
>>    for(i=0;i<10000000;i++){
>>       ret+=sin(x);
>>      x+=0.3;
>>    }
>>    return ret;
>> }
>> " > sin.c
> OK I did a similar thing. I just compiled sin(argc) in main.
> The results prove that you were right. The single fsin instruction
> takes longer than several HUNDRED instructions (calls, jumps
> table lookup what have you)
>
> Gone are the times when an fsin would take 30 cycles or so.
> Intel has destroyed the FPU.
>

What makes you so sure that it takes more than 30 cycles to execute 
hundreds of instructions in the library?  Modern cpus often do several 
instructions per cycle (I am not considering multiple cores here).  They 
can issue several instructions per cycle, and predicted jumps can often 
be eliminated entirely in the decode stages.

The moral here is that /you/ need to benchmark /your/ code on /your/ 
processor - don't jump to conclusions, or accept other benchmarks as 
giving the complete picture.