1 files changed, 24 insertions, 0 deletions
diff --git a/lib/std/special/compiler_rt/floatundidf.zig b/lib/std/special/compiler_rt/floatundidf.zig
new file mode 100644
index 0000000000..68759a2acd
--- /dev/null
+++ b/lib/std/special/compiler_rt/floatundidf.zig
@@ -0,0 +1,24 @@
+const builtin = @import("builtin");
+const std = @import("std");
+
+const twop52: f64 = 0x1.0p52;
+const twop84: f64 = 0x1.0p84;
+const twop84_plus_twop52: f64 = 0x1.00000001p84;
+
+pub extern fn __floatundidf(a: u64) f64 {
+    @setRuntimeSafety(builtin.is_test);
+
+    if (a == 0) return 0;
+
+    var high = @bitCast(u64, twop84);
+    var low = @bitCast(u64, twop52);
+
+    high |= a >> 32;
+    low |= a & 0xFFFFFFFF;
+
+    return (@bitCast(f64, high) - twop84_plus_twop52) + @bitCast(f64, low);
+}
+
+test "import floatundidf" {
+    _ = @import("floatundidf_test.zig");
+}