1 files changed, 14 insertions, 0 deletions
diff --git a/library/compiler-builtins/libm/src/math/expo2.rs b/library/compiler-builtins/libm/src/math/expo2.rs
new file mode 100644
index 00000000000..82e9b360a76
--- /dev/null
+++ b/library/compiler-builtins/libm/src/math/expo2.rs
@@ -0,0 +1,14 @@
+use super::{combine_words, exp};
+
+/* exp(x)/2 for x >= log(DBL_MAX), slightly better than 0.5*exp(x/2)*exp(x/2) */
+#[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
+}