1 files changed, 53 insertions, 0 deletions
diff --git a/library/compiler-builtins/libm/src/math/tanh.rs b/library/compiler-builtins/libm/src/math/tanh.rs
new file mode 100644
index 00000000000..cc0abe4fcb2
--- /dev/null
+++ b/library/compiler-builtins/libm/src/math/tanh.rs
@@ -0,0 +1,53 @@
+use super::expm1;
+
+/* tanh(x) = (exp(x) - exp(-x))/(exp(x) + exp(-x))
+ *         = (exp(2*x) - 1)/(exp(2*x) - 1 + 2)
+ *         = (1 - exp(-2*x))/(exp(-2*x) - 1 + 2)
+ */
+
+/// The hyperbolic tangent of `x` (f64).
+///
+/// `x` is specified in radians.
+#[cfg_attr(all(test, assert_no_panic), no_panic::no_panic)]
+pub fn tanh(mut x: f64) -> f64 {
+    let mut uf: f64 = x;
+    let mut ui: u64 = f64::to_bits(uf);
+
+    let w: u32;
+    let sign: bool;
+    let mut t: f64;
+
+    /* x = |x| */
+    sign = ui >> 63 != 0;
+    ui &= !1 / 2;
+    uf = f64::from_bits(ui);
+    x = uf;
+    w = (ui >> 32) as u32;
+
+    if w > 0x3fe193ea {
+        /* |x| > log(3)/2 ~= 0.5493 or nan */
+        if w > 0x40340000 {
+            /* |x| > 20 or nan */
+            /* note: this branch avoids raising overflow */
+            t = 1.0 - 0.0 / x;
+        } else {
+            t = expm1(2.0 * x);
+            t = 1.0 - 2.0 / (t + 2.0);
+        }
+    } else if w > 0x3fd058ae {
+        /* |x| > log(5/3)/2 ~= 0.2554 */
+        t = expm1(2.0 * x);
+        t = t / (t + 2.0);
+    } else if w >= 0x00100000 {
+        /* |x| >= 0x1p-1022, up to 2ulp error in [0.1,0.2554] */
+        t = expm1(-2.0 * x);
+        t = -t / (t + 2.0);
+    } else {
+        /* |x| is subnormal */
+        /* note: the branch above would not raise underflow in [0x1p-1023,0x1p-1022) */
+        force_eval!(x as f32);
+        t = x;
+    }
+
+    if sign { -t } else { t }
+}