| use super::{combine_words, exp}; |
| |
| /* exp(x)/2 for x >= log(DBL_MAX), slightly better than 0.5*exp(x/2)*exp(x/2) */ |
| #[inline] |
| #[cfg_attr(all(test, assert_no_panic), no_panic::no_panic)] |
| pub(crate) fn expo2(x: f64) -> f64 { |
| /* k is such that k*ln2 has minimal relative error and x - kln2 > log(DBL_MIN) */ |
| const K: i32 = 2043; |
| let kln2 = f64::from_bits(0x40962066151add8b); |
| |
| /* note that k is odd and scale*scale overflows */ |
| let scale = combine_words(((0x3ff + K / 2) as u32) << 20, 0); |
| /* exp(x - k ln2) * 2**(k-1) */ |
| exp(x - kln2) * scale * scale |
| } |
