1 files changed, 33 insertions, 0 deletions
diff --git a/lib/std/special/compiler_rt/floatunsidf.zig b/lib/std/special/compiler_rt/floatunsidf.zig
new file mode 100644
index 0000000000..efeb4c2672
--- /dev/null
+++ b/lib/std/special/compiler_rt/floatunsidf.zig
@@ -0,0 +1,33 @@
+const builtin = @import("builtin");
+const std = @import("std");
+const maxInt = std.math.maxInt;
+
+const implicitBit = u64(1) << 52;
+
+pub extern fn __floatunsidf(arg: u32) f64 {
+    @setRuntimeSafety(builtin.is_test);
+
+    if (arg == 0) return 0.0;
+
+    // The exponent is the width of abs(a)
+    const exp = u64(31) - @clz(u32, arg);
+    // Shift a into the significand field and clear the implicit bit
+    const shift = @intCast(u6, 52 - exp);
+    const mant = u64(arg) << shift ^ implicitBit;
+
+    return @bitCast(f64, mant | (exp + 1023) << 52);
+}
+
+fn test_one_floatunsidf(a: u32, expected: u64) void {
+    const r = __floatunsidf(a);
+    std.testing.expect(@bitCast(u64, r) == expected);
+}
+
+test "floatsidf" {
+    // Test the produced bit pattern
+    test_one_floatunsidf(0, 0x0000000000000000);
+    test_one_floatunsidf(1, 0x3ff0000000000000);
+    test_one_floatunsidf(0x7FFFFFFF, 0x41dfffffffc00000);
+    test_one_floatunsidf(@intCast(u32, 0x80000000), 0x41e0000000000000);
+    test_one_floatunsidf(@intCast(u32, 0xFFFFFFFF), 0x41efffffffe00000);
+}