github.com/afumu/libc@v0.0.6/musl/src/math/__expo2.c (about)

     1  #include "libm.h"
     2  
     3  /* k is such that k*ln2 has minimal relative error and x - kln2 > log(DBL_MIN) */
     4  static const int k = 2043;
     5  static const double kln2 = 0x1.62066151add8bp+10;
     6  
     7  /* exp(x)/2 for x >= log(DBL_MAX), slightly better than 0.5*exp(x/2)*exp(x/2) */
     8  double __expo2(double x, double sign)
     9  {
    10  	double scale;
    11  
    12  	/* note that k is odd and scale*scale overflows */
    13  	INSERT_WORDS(scale, (uint32_t)(0x3ff + k/2) << 20, 0);
    14  	/* exp(x - k ln2) * 2**(k-1) */
    15  	/* in directed rounding correct sign before rounding or overflow is important */
    16  	return exp(x - kln2) * (sign * scale) * scale;
    17  }