| #include "libm.h" |
| |
| /* k is such that k*ln2 has minimal relative error and x - kln2 > log(FLT_MIN) */ |
| static const int k = 235; |
| static const float kln2 = 0x1.45c778p+7f; |
| |
| /* expf(x)/2 for x >= log(FLT_MAX), slightly better than 0.5f*expf(x/2)*expf(x/2) */ |
| float __expo2f(float x, float sign) |
| { |
| float scale; |
| |
| /* note that k is odd and scale*scale overflows */ |
| SET_FLOAT_WORD(scale, (uint32_t)(0x7f + k/2) << 23); |
| /* exp(x - k ln2) * 2**(k-1) */ |
| /* in directed rounding correct sign before rounding or overflow is important */ |
| return expf(x - kln2) * (sign * scale) * scale; |
| } |
