src/math/__expo2f.c

   1 #include "libm.h"
   2
   3 /* k is such that k*ln2 has minimal relative error and x - kln2 > log(FLT_MIN) */
   4 static const int k = 235;
   5 static const float kln2 = 0x1.45c778p+7f;
   6
   7 /* expf(x)/2 for x >= log(FLT_MAX), slightly better than 0.5f*expf(x/2)*expf(x/2) */
   8 float __expo2f(float x)
   9 {
  10         float scale;
  11
  12         /* note that k is odd and scale*scale overflows */
  13         SET_FLOAT_WORD(scale, (uint32_t)(0x7f + k/2) << 23);
  14         /* exp(x - k ln2) * 2**(k-1) */
  15         return expf(x - kln2) * scale * scale;
  16 }