libc-top-half/musl/src/math/__expo2.c

   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 #ifdef __wasilibc_unmodified_upstream // Wasm doesn't have alternate rounding modes
   9 double __expo2(double x, double sign)
  10 #else
  11 double __expo2(double x)
  12 #endif
  13 {
  14         double scale;
  15
  16         /* note that k is odd and scale*scale overflows */
  17         INSERT_WORDS(scale, (uint32_t)(0x3ff + k/2) << 20, 0);
  18         /* exp(x - k ln2) * 2**(k-1) */
  19 #ifdef __wasilibc_unmodified_upstream // Wasm doesn't have alternate rounding modes
  20         /* in directed rounding correct sign before rounding or overflow is important */
  21         return exp(x - kln2) * (sign * scale) * scale;
  22 #else
  23         return exp(x - kln2) * scale * scale;
  24 #endif
  25 }