Re: Help with Hand-Optimized Assembly

[sfuerst]

On Jan 19, 1:08 pm, Bill Woessner <woess...@nospicedham.gmail.com>
wrote:
> Success!  Thanks to everyone who helped me out.  This was a great
> little project to learn about inline assembly in GCC.  For those of
> you who are interested, here's the final product:
>
> inline double DiffAngle(double theta1, double theta2)
> {
>   double TWO_PI __attribute__ ((aligned (16)))(2 * M_PI),
>     NEG_TWO_PI __attribute__ ((aligned (16)))(-TWO_PI);
>   double delta(theta1 - theta2);
>
>   asm("cmpnlesd %0, %1\n\t"
>       "cmpltsd %0, %2\n\t"
>       "andpd %3, %1\n\t"
>       "andpd %4, %2\n\t"
>       "addsd %1, %0\n\t"
>       "addsd %2, %0\n\t"
>       : "+x" (delta)
>       : "x" (M_PI),
>         "x" (-M_PI),
>         "m" (TWO_PI),
>         "m" (NEG_TWO_PI)
>       :
>       );
>
>   return delta;
>
> }
>
> One final question - is it possible to declare TWO_PI and NEG_TWO_PI
> as const?  And, if so, what constraint would I use?  Right now, if I
> declare them const, I get the error "memory input 3 is not directly
> addressable".
>
> Thanks,
> Bill

This works for me:

#include <math.h>
double DiffAngle(double theta1, double theta2)
{
        static const double TWO_PI __attribute__ ((aligned (16))) = 2 * M_PI;
        double delta = theta1 - theta2;

        asm("cmpnlesd %0, %1\n"
                "cmpltsd %0, %2\n"
                "andpd %3, %1\n"
                "andpd %3, %2\n"
                "addsd %1, %0\n"
                "subsd %2, %0\n"
                : "+x" (delta)
                : "x" (M_PI), "x" (-M_PI), "m" (TWO_PI)
                :
        );

        return delta;
}


(Using another trick, where the variable containing -2pi can be
replaced by a subtraction of +2pi.)

Steven