diff --git a/std/special/compiler_rt/floatunditf.zig b/std/special/compiler_rt/floatunditf.zig
new file mode 100644
index 0000000000..b8b097581f
--- /dev/null
+++ b/std/special/compiler_rt/floatunditf.zig
@@ -0,0 +1,28 @@
+const builtin = @import("builtin");
+const is_test = builtin.is_test;
+const std = @import("../../index.zig");
+
+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(a);
+    const shift = mantissa_bits - @intCast(u7, exp);
+
+    var result: u128 = (a << shift) ^ implicit_bit;
+    result += (@intCast(u128, exp) + exponent_bias) << mantissa_bits;
+
+    return @bitCast(f128, result);
+}
+
+test "import floatunditf" {
+    _ = @import("floatunditf_test.zig");
+}
diff --git a/std/special/compiler_rt/floatunditf_test.zig b/std/special/compiler_rt/floatunditf_test.zig
new file mode 100644
index 0000000000..8533c75070
--- /dev/null
+++ b/std/special/compiler_rt/floatunditf_test.zig
@@ -0,0 +1,33 @@
+const __floatunditf = @import("floatunditf.zig").__floatunditf;
+const assert = @import("std").debug.assert;
+
+fn test__floatunditf(a: u128, expected_hi: u64, expected_lo: u64) void {
+    const x = __floatunditf(a);
+
+    const x_repr = @bitCast(u128, x);
+    const x_hi = @intCast(u64, x_repr >> 64);
+    const x_lo = @truncate(u64, x_repr);
+
+    if (x_hi == expected_hi and x_lo == expected_lo) {
+        return;
+    }
+    // nan repr
+    else if (expected_hi == 0x7fff800000000000 and expected_lo == 0x0) {
+        if ((x_hi & 0x7fff000000000000) == 0x7fff000000000000 and ((x_hi & 0xffffffffffff) > 0 or x_lo > 0)) {
+            return;
+        }
+    }
+
+    @panic("__floatunditf test failure");
+}
+
+test "floatunditf" {
+    test__floatunditf(0xffffffffffffffff, 0x403effffffffffff, 0xfffe000000000000);
+    test__floatunditf(0xfffffffffffffffe, 0x403effffffffffff, 0xfffc000000000000);
+    test__floatunditf(0x8000000000000000, 0x403e000000000000, 0x0);
+    test__floatunditf(0x7fffffffffffffff, 0x403dffffffffffff, 0xfffc000000000000);
+    test__floatunditf(0x123456789abcdef1, 0x403b23456789abcd, 0xef10000000000000);
+    test__floatunditf(0x2, 0x4000000000000000, 0x0);
+    test__floatunditf(0x1, 0x3fff000000000000, 0x0);
+    test__floatunditf(0x0, 0x0, 0x0);
+}
diff --git a/std/special/compiler_rt/floatunsitf.zig b/std/special/compiler_rt/floatunsitf.zig
new file mode 100644
index 0000000000..dfee826b32
--- /dev/null
+++ b/std/special/compiler_rt/floatunsitf.zig
@@ -0,0 +1,29 @@
+const builtin = @import("builtin");
+const is_test = builtin.is_test;
+const std = @import("../../index.zig");
+
+pub extern fn __floatunsitf(a: u64) 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 = (u64.bit_count - 1) - @clz(a);
+    const shift = mantissa_bits - @intCast(u7, exp);
+
+    // TODO: @bitCast alignment error
+    var result align(16) = (@intCast(u128, a) << shift) ^ implicit_bit;
+    result += (@intCast(u128, exp) + exponent_bias) << mantissa_bits;
+
+    return @bitCast(f128, result);
+}
+
+test "import floatunsitf" {
+    _ = @import("floatunsitf_test.zig");
+}
diff --git a/std/special/compiler_rt/floatunsitf_test.zig b/std/special/compiler_rt/floatunsitf_test.zig
new file mode 100644
index 0000000000..06f54cde03
--- /dev/null
+++ b/std/special/compiler_rt/floatunsitf_test.zig
@@ -0,0 +1,29 @@
+const __floatunsitf = @import("floatunsitf.zig").__floatunsitf;
+const assert = @import("std").debug.assert;
+
+fn test__floatunsitf(a: u64, expected_hi: u64, expected_lo: u64) void {
+    const x = __floatunsitf(a);
+
+    const x_repr = @bitCast(u128, x);
+    const x_hi = @intCast(u64, x_repr >> 64);
+    const x_lo = @truncate(u64, x_repr);
+
+    if (x_hi == expected_hi and x_lo == expected_lo) {
+        return;
+    }
+    // nan repr
+    else if (expected_hi == 0x7fff800000000000 and expected_lo == 0x0) {
+        if ((x_hi & 0x7fff000000000000) == 0x7fff000000000000 and ((x_hi & 0xffffffffffff) > 0 or x_lo > 0)) {
+            return;
+        }
+    }
+
+    @panic("__floatunsitf test failure");
+}
+
+test "floatunsitf" {
+    test__floatunsitf(0x7fffffff, 0x401dfffffffc0000, 0x0);
+    test__floatunsitf(0, 0x0, 0x0);
+    test__floatunsitf(0xffffffff, 0x401efffffffe0000, 0x0);
+    test__floatunsitf(0x12345678, 0x401b234567800000, 0x0);
+}
diff --git a/std/special/compiler_rt/truncXfYf2.zig b/std/special/compiler_rt/truncXfYf2.zig
index 04b815e86b..5cb2f61568 100644
--- a/std/special/compiler_rt/truncXfYf2.zig
+++ b/std/special/compiler_rt/truncXfYf2.zig
@@ -85,8 +85,8 @@ inline fn truncXfYf2(comptime dst_t: type, comptime src_t: type, a: src_t) dst_t
         // a underflows on conversion to the destination type or is an exact
         // zero.  The result may be a denormal or zero.  Extract the exponent
         // to get the shift amount for the denormalization.
-        const aExp = aAbs >> srcSigBits;
-        const shift = srcExpBias - dstExpBias - aExp + 1;
+        const aExp = @intCast(u32, aAbs >> srcSigBits);
+        const shift = @intCast(u32, srcExpBias - dstExpBias - aExp + 1);
 
         const significand: src_rep_t = (aRep & srcSignificandMask) | srcMinNormal;