Inline atan2 function (asm)

my_atan2.c

#include <stdio.h>
#include <stdlib.h>
#include <math.h>

/*
  Custom atan2 function calling the FPU directly, for use when the
  math library isn't available.

  Tobin Fricke <tobin.fricke@ligo.org> 2012-03-13
  
  Here's a reference for the GCC inline assembly syntax:

  http://www.ibiblio.org/gferg/ldp/GCC-Inline-Assembly-HOWTO.html

*/

inline double my_atan2 (double y, double x) { 
  double result; 
  asm (
     "fpatan\n\t" 
     : "=t" (result)       // outputs; t = top of fpu stack
     : "0" (x),            // inputs; 0 = same as result
       "u" (y)             //         u = 2nd floating point register
     );   
  return result; 
}

/* Test program prints results of the custom function along side
   the results of the C library function.

   Example usage:

   tobin@thinktop:~$ gcc -lm -Wall inlineatan2.c -o inlineatan2

   tobin@thinktop:~$ echo -1 0 | ./inlineatan2 
   -1.000000 0.000000 3.141593 3.141593

*/

int main(int argc, char **argv) {
  double a, b, x, y;
  while (2==scanf("%lf %lf", &x, &y)) {
    a = my_atan2(y, x);
    b =    atan2(y, x);
    printf("%lf %lf %lf %lf\n", x, y, a, b);
  }
  return 0;
}