1570af302Sopenharmony_ci#include "libm.h" 2570af302Sopenharmony_ci 3570af302Sopenharmony_ci/* k is such that k*ln2 has minimal relative error and x - kln2 > log(DBL_MIN) */ 4570af302Sopenharmony_cistatic const int k = 2043; 5570af302Sopenharmony_cistatic const double kln2 = 0x1.62066151add8bp+10; 6570af302Sopenharmony_ci 7570af302Sopenharmony_ci/* exp(x)/2 for x >= log(DBL_MAX), slightly better than 0.5*exp(x/2)*exp(x/2) */ 8570af302Sopenharmony_cidouble __expo2(double x, double sign) 9570af302Sopenharmony_ci{ 10570af302Sopenharmony_ci double scale; 11570af302Sopenharmony_ci 12570af302Sopenharmony_ci /* note that k is odd and scale*scale overflows */ 13570af302Sopenharmony_ci INSERT_WORDS(scale, (uint32_t)(0x3ff + k/2) << 20, 0); 14570af302Sopenharmony_ci /* exp(x - k ln2) * 2**(k-1) */ 15570af302Sopenharmony_ci /* in directed rounding correct sign before rounding or overflow is important */ 16570af302Sopenharmony_ci return exp(x - kln2) * (sign * scale) * scale; 17570af302Sopenharmony_ci} 18