diff --git a/std/special/compiler_rt/floatuntisf.zig b/std/special/compiler_rt/floatuntisf.zig
new file mode 100644
index 000000000..e83affd87
--- /dev/null
+++ b/std/special/compiler_rt/floatuntisf.zig
@@ -0,0 +1,59 @@
+const builtin = @import("builtin");
+const is_test = builtin.is_test;
+
+const FLT_MANT_DIG = 24;
+
+pub extern fn __floatuntisf(arg: u128) f32 {
+    @setRuntimeSafety(is_test);
+
+    if (arg == 0)
+        return 0.0;
+
+    var a = arg;
+    const N: u32 = @sizeOf(u128) * 8;
+    const sd = @bitCast(i32, N -% @clz(a)); // number of significant digits
+    var e: i32 = sd -% 1; // exponent
+    if (sd > FLT_MANT_DIG) {
+        //  start:  0000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQxxxxxxxxxxxxxxxxxx
+        //  finish: 000000000000000000000000000000000000001xxxxxxxxxxxxxxxxxxxxxxPQR
+        //                                                12345678901234567890123456
+        //  1 = msb 1 bit
+        //  P = bit FLT_MANT_DIG-1 bits to the right of 1
+        //  Q = bit FLT_MANT_DIG bits to the right of 1
+        //  R = "or" of all bits to the right of Q
+        switch (sd) {
+            FLT_MANT_DIG + 1 => {
+                a <<= 1;
+            },
+            FLT_MANT_DIG + 2 => {},
+            else => {
+                const shift_amt = @bitCast(i32, N +% (FLT_MANT_DIG + 2)) -% sd;
+                const shift_amt_u7 = @intCast(u7, shift_amt);
+                a = (a >> @intCast(u7, sd -% (FLT_MANT_DIG + 2))) |
+                    @boolToInt((a & (u128(@maxValue(u128)) >> shift_amt_u7)) != 0);
+            },
+        }
+        // finish
+        a |= @boolToInt((a & 4) != 0); // Or P into R
+        a +%= 1; // round - this step may add a significant bit
+        a >>= 2; // dump Q and R
+        // a is now rounded to FLT_MANT_DIG or FLT_MANT_DIG+1 bits
+        if ((a & (u128(1) << FLT_MANT_DIG)) != 0) {
+            a >>= 1;
+            e +%= 1;
+        }
+        // a is now rounded to FLT_MANT_DIG bits
+    } else {
+        a <<= @intCast(u7, FLT_MANT_DIG -% sd);
+        // a is now rounded to FLT_MANT_DIG bits
+    }
+
+    const high = @bitCast(u32, (e +% 127) << 23); // exponent
+    const low = @truncate(u32, a) & 0x007fffff; // mantissa
+
+    return @bitCast(f32, high | low);
+}
+
+test "import floatuntisf" {
+    _ = @import("floatuntisf_test.zig");
+}
diff --git a/std/special/compiler_rt/floatuntisf_test.zig b/std/special/compiler_rt/floatuntisf_test.zig
new file mode 100644
index 000000000..7f84c1f96
--- /dev/null
+++ b/std/special/compiler_rt/floatuntisf_test.zig
@@ -0,0 +1,72 @@
+const __floatuntisf = @import("floatuntisf.zig").__floatuntisf;
+const assert = @import("std").debug.assert;
+
+fn test__floatuntisf(a: u128, expected: f32) void {
+    const x = __floatuntisf(a);
+    assert(x == expected);
+}
+
+test "floatuntisf" {
+    test__floatuntisf(0, 0.0);
+
+    test__floatuntisf(1, 1.0);
+    test__floatuntisf(2, 2.0);
+    test__floatuntisf(20, 20.0);
+
+    test__floatuntisf(0x7FFFFF8000000000, 0x1.FFFFFEp+62);
+    test__floatuntisf(0x7FFFFF0000000000, 0x1.FFFFFCp+62);
+
+    test__floatuntisf(make_ti(0x8000008000000000, 0), 0x1.000001p+127);
+    test__floatuntisf(make_ti(0x8000000000000800, 0), 0x1.0p+127);
+    test__floatuntisf(make_ti(0x8000010000000000, 0), 0x1.000002p+127);
+
+    test__floatuntisf(make_ti(0x8000000000000000, 0), 0x1.000000p+127);
+
+    test__floatuntisf(0x0007FB72E8000000, 0x1.FEDCBAp+50);
+
+    test__floatuntisf(0x0007FB72EA000000, 0x1.FEDCBA8p+50);
+    test__floatuntisf(0x0007FB72EB000000, 0x1.FEDCBACp+50);
+
+    test__floatuntisf(0x0007FB72EC000000, 0x1.FEDCBBp+50);
+
+    test__floatuntisf(0x0007FB72E6000000, 0x1.FEDCB98p+50);
+    test__floatuntisf(0x0007FB72E7000000, 0x1.FEDCB9Cp+50);
+    test__floatuntisf(0x0007FB72E4000000, 0x1.FEDCB9p+50);
+
+    test__floatuntisf(0xFFFFFFFFFFFFFFFE, 0x1p+64);
+    test__floatuntisf(0xFFFFFFFFFFFFFFFF, 0x1p+64);
+
+    test__floatuntisf(0x0007FB72E8000000, 0x1.FEDCBAp+50);
+
+    test__floatuntisf(0x0007FB72EA000000, 0x1.FEDCBAp+50);
+    test__floatuntisf(0x0007FB72EB000000, 0x1.FEDCBAp+50);
+    test__floatuntisf(0x0007FB72EBFFFFFF, 0x1.FEDCBAp+50);
+    test__floatuntisf(0x0007FB72EC000000, 0x1.FEDCBCp+50);
+    test__floatuntisf(0x0007FB72E8000001, 0x1.FEDCBAp+50);
+
+    test__floatuntisf(0x0007FB72E6000000, 0x1.FEDCBAp+50);
+    test__floatuntisf(0x0007FB72E7000000, 0x1.FEDCBAp+50);
+    test__floatuntisf(0x0007FB72E7FFFFFF, 0x1.FEDCBAp+50);
+    test__floatuntisf(0x0007FB72E4000001, 0x1.FEDCBAp+50);
+    test__floatuntisf(0x0007FB72E4000000, 0x1.FEDCB8p+50);
+
+    test__floatuntisf(make_ti(0x0000000000001FED, 0xCB90000000000001), 0x1.FEDCBAp+76);
+    test__floatuntisf(make_ti(0x0000000000001FED, 0xCBA0000000000000), 0x1.FEDCBAp+76);
+    test__floatuntisf(make_ti(0x0000000000001FED, 0xCBAFFFFFFFFFFFFF), 0x1.FEDCBAp+76);
+    test__floatuntisf(make_ti(0x0000000000001FED, 0xCBB0000000000000), 0x1.FEDCBCp+76);
+    test__floatuntisf(make_ti(0x0000000000001FED, 0xCBB0000000000001), 0x1.FEDCBCp+76);
+    test__floatuntisf(make_ti(0x0000000000001FED, 0xCBBFFFFFFFFFFFFF), 0x1.FEDCBCp+76);
+    test__floatuntisf(make_ti(0x0000000000001FED, 0xCBC0000000000000), 0x1.FEDCBCp+76);
+    test__floatuntisf(make_ti(0x0000000000001FED, 0xCBC0000000000001), 0x1.FEDCBCp+76);
+    test__floatuntisf(make_ti(0x0000000000001FED, 0xCBD0000000000000), 0x1.FEDCBCp+76);
+    test__floatuntisf(make_ti(0x0000000000001FED, 0xCBD0000000000001), 0x1.FEDCBEp+76);
+    test__floatuntisf(make_ti(0x0000000000001FED, 0xCBDFFFFFFFFFFFFF), 0x1.FEDCBEp+76);
+    test__floatuntisf(make_ti(0x0000000000001FED, 0xCBE0000000000000), 0x1.FEDCBEp+76);
+}
+
+fn make_ti(high: u64, low: u64) u128 {
+    var result: u128 = high;
+    result <<= 64;
+    result |= low;
+    return result;
+}