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