1570af302Sopenharmony_ci#include "libm.h" 2570af302Sopenharmony_ci 3570af302Sopenharmony_ci/* k is such that k*ln2 has minimal relative error and x - kln2 > log(FLT_MIN) */ 4570af302Sopenharmony_cistatic const int k = 235; 5570af302Sopenharmony_cistatic const float kln2 = 0x1.45c778p+7f; 6570af302Sopenharmony_ci 7570af302Sopenharmony_ci/* expf(x)/2 for x >= log(FLT_MAX), slightly better than 0.5f*expf(x/2)*expf(x/2) */ 8570af302Sopenharmony_cifloat __expo2f(float x, float sign) 9570af302Sopenharmony_ci{ 10570af302Sopenharmony_ci float scale; 11570af302Sopenharmony_ci 12570af302Sopenharmony_ci /* note that k is odd and scale*scale overflows */ 13570af302Sopenharmony_ci SET_FLOAT_WORD(scale, (uint32_t)(0x7f + k/2) << 23); 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 expf(x - kln2) * (sign * scale) * scale; 17570af302Sopenharmony_ci} 18