__expo2f.c raw

   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, float sign)
   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  	/* in directed rounding correct sign before rounding or overflow is important */
  16  	return expf(x - kln2) * (sign * scale) * scale;
  17  }
  18