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diff --git a/library/compiler-builtins/libm/src/math/tan.rs b/library/compiler-builtins/libm/src/math/tan.rs
new file mode 100644
index 00000000000..a072bdec56e
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+++ b/library/compiler-builtins/libm/src/math/tan.rs
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+// origin: FreeBSD /usr/src/lib/msun/src/s_tan.c */
+//
+// ====================================================
+// Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
+//
+// Developed at SunPro, a Sun Microsystems, Inc. business.
+// Permission to use, copy, modify, and distribute this
+// software is freely granted, provided that this notice
+// is preserved.
+// ====================================================
+
+use super::{k_tan, rem_pio2};
+
+// tan(x)
+// Return tangent function of x.
+//
+// kernel function:
+//      k_tan           ... tangent function on [-pi/4,pi/4]
+//      rem_pio2        ... argument reduction routine
+//
+// Method.
+//      Let S,C and T denote the sin, cos and tan respectively on
+//      [-PI/4, +PI/4]. Reduce the argument x to y1+y2 = x-k*pi/2
+//      in [-pi/4 , +pi/4], and let n = k mod 4.
+//      We have
+//
+//          n        sin(x)      cos(x)        tan(x)
+//     ----------------------------------------------------------
+//          0          S           C             T
+//          1          C          -S            -1/T
+//          2         -S          -C             T
+//          3         -C           S            -1/T
+//     ----------------------------------------------------------
+//
+// Special cases:
+//      Let trig be any of sin, cos, or tan.
+//      trig(+-INF)  is NaN, with signals;
+//      trig(NaN)    is that NaN;
+//
+// Accuracy:
+//      TRIG(x) returns trig(x) nearly rounded
+
+/// The tangent of `x` (f64).
+///
+/// `x` is specified in radians.
+#[cfg_attr(all(test, assert_no_panic), no_panic::no_panic)]
+pub fn tan(x: f64) -> f64 {
+    let x1p120 = f32::from_bits(0x7b800000); // 0x1p120f === 2 ^ 120
+
+    let ix = (f64::to_bits(x) >> 32) as u32 & 0x7fffffff;
+    /* |x| ~< pi/4 */
+    if ix <= 0x3fe921fb {
+        if ix < 0x3e400000 {
+            /* |x| < 2**-27 */
+            /* raise inexact if x!=0 and underflow if subnormal */
+            force_eval!(if ix < 0x00100000 {
+                x / x1p120 as f64
+            } else {
+                x + x1p120 as f64
+            });
+            return x;
+        }
+        return k_tan(x, 0.0, 0);
+    }
+
+    /* tan(Inf or NaN) is NaN */
+    if ix >= 0x7ff00000 {
+        return x - x;
+    }
+
+    /* argument reduction */
+    let (n, y0, y1) = rem_pio2(x);
+    k_tan(y0, y1, n & 1)
+}