1 files changed, 28 insertions, 0 deletions
diff --git a/lib/std/special/compiler_rt/floatunditf.zig b/lib/std/special/compiler_rt/floatunditf.zig
new file mode 100644
index 0000000000..9af83a2b5d
--- /dev/null
+++ b/lib/std/special/compiler_rt/floatunditf.zig
@@ -0,0 +1,28 @@
+const builtin = @import("builtin");
+const is_test = builtin.is_test;
+const std = @import("std");
+
+pub extern fn __floatunditf(a: u128) f128 {
+    @setRuntimeSafety(is_test);
+
+    if (a == 0) {
+        return 0;
+    }
+
+    const mantissa_bits = std.math.floatMantissaBits(f128);
+    const exponent_bits = std.math.floatExponentBits(f128);
+    const exponent_bias = (1 << (exponent_bits - 1)) - 1;
+    const implicit_bit = 1 << mantissa_bits;
+
+    const exp = (u128.bit_count - 1) - @clz(u128, a);
+    const shift = mantissa_bits - @intCast(u7, exp);
+
+    var result: u128 align(16) = (a << shift) ^ implicit_bit;
+    result += (@intCast(u128, exp) + exponent_bias) << mantissa_bits;
+
+    return @bitCast(f128, result);
+}
+
+test "import floatunditf" {
+    _ = @import("floatunditf_test.zig");
+}