diff --git a/src/libcompiler_builtins/lib.rs b/src/libcompiler_builtins/lib.rs
index 4a703b3da68..a7345f169c5 100644
--- a/src/libcompiler_builtins/lib.rs
+++ b/src/libcompiler_builtins/lib.rs
@@ -17,3 +17,626 @@
 #![crate_name = "compiler_builtins"]
 #![crate_type = "rlib"]
 #![feature(staged_api)]
+#![feature(core_intrinsics)]
+#![feature(core_float)]
+#![feature(associated_consts)]
+#![cfg_attr(not(stage0), feature(i128_type))]
+
+#![allow(non_camel_case_types, unused_variables)]
+
+
+#[cfg(any(target_pointer_width="32", target_pointer_width="16"))]
+pub mod reimpls {
+    #![allow(unused_comparisons)]
+    // C API is expected to tolerate some amount of size mismatch in ABI. Hopefully the amount of
+    // handling is sufficient for bootstrapping.
+    #[cfg(stage0)]
+    type u128_ = u64;
+    #[cfg(stage0)]
+    type i128_ = i64;
+    #[cfg(not(stage0))]
+    type u128_ = u128;
+    #[cfg(not(stage0))]
+    type i128_ = i128;
+
+    fn unimplemented() -> ! {
+        unsafe { ::core::intrinsics::abort() }
+    }
+
+    macro_rules! ashl {
+        ($a:expr, $b:expr, $ty:ty) => {{
+            let (a, b) = ($a, $b);
+            let bits = (::core::mem::size_of::<$ty>() * 8) as $ty;
+            let half_bits = bits / 2;
+            if b & half_bits != 0 {
+                <$ty>::from_parts(0, a.low() << (b - half_bits))
+            } else if b == 0 {
+                a
+            } else {
+                <$ty>::from_parts(a.low() << b, (a.high() << b) | (a.low() >> (half_bits - b)))
+            }
+        }}
+    }
+
+    #[export_name="__ashlti3"]
+    pub extern fn shl(a: u128_, b: u128_) -> u128_ {
+        ashl!(a, b, u128_)
+    }
+
+    macro_rules! ashr {
+        ($a: expr, $b: expr, $ty:ty) => {{
+            let (a, b) = ($a, $b);
+            let bits = (::core::mem::size_of::<$ty>() * 8) as $ty;
+            let half_bits = bits / 2;
+            if b & half_bits != 0 {
+                <$ty>::from_parts((a.high() >> (b - half_bits)) as <$ty as LargeInt>::LowHalf,
+                                  a.high() >> (half_bits - 1))
+            } else if b == 0 {
+                a
+            } else {
+                let high_unsigned = a.high() as <$ty as LargeInt>::LowHalf;
+                <$ty>::from_parts((high_unsigned << (half_bits - b)) | (a.low() >> b),
+                                  a.high() >> b)
+            }
+        }}
+    }
+
+    #[export_name="__ashrti3"]
+    pub extern fn shr(a: i128_, b: i128_) -> i128_ {
+        ashr!(a, b, i128_)
+    }
+
+    macro_rules! lshr {
+        ($a: expr, $b: expr, $ty:ty) => {{
+            let (a, b) = ($a, $b);
+            let bits = (::core::mem::size_of::<$ty>() * 8) as $ty;
+            let half_bits = bits / 2;
+            if b & half_bits != 0 {
+                <$ty>::from_parts(a.high() >> (b - half_bits), 0)
+            } else if b == 0 {
+                a
+            } else {
+                <$ty>::from_parts((a.high() << (half_bits - b)) | (a.low() >> b), a.high() >> b)
+            }
+        }}
+    }
+
+
+    #[export_name="__lshrti3"]
+    pub extern fn lshr(a: u128_, b: u128_) -> u128_ {
+        lshr!(a, b, u128_)
+    }
+
+    #[cfg(stage0)]
+    #[export_name="__udivmodti4"]
+    pub extern fn u128_div_mod(n: u128_, d: u128_, rem: *mut u128_) -> u128_ {
+        unsafe {
+        if !rem.is_null() {
+            *rem = n % d;
+        }
+        n / d
+        }
+    }
+
+    #[cfg(not(stage0))]
+    #[export_name="__udivmodti4"]
+    pub extern fn u128_div_mod(n: u128_, d: u128_, rem: *mut u128_) -> u128_ {
+        unsafe {
+        // NOTE X is unknown, K != 0
+        if n.high() == 0 {
+            if d.high() == 0 {
+                // 0 X
+                // ---
+                // 0 X
+                if !rem.is_null() {
+                    *rem = u128::from(n.low() % d.low());
+                }
+                return u128::from(n.low() / d.low());
+            } else {
+                // 0 X
+                // ---
+                // K X
+                if !rem.is_null() {
+                    *rem = n;
+                }
+                return 0;
+            };
+        }
+
+        let mut sr;
+        let mut q;
+        let mut r;
+
+        if d.low() == 0 {
+            if d.high() == 0 {
+                // K X
+                // ---
+                // 0 0
+                unimplemented()
+            }
+
+            if n.low() == 0 {
+                // K 0
+                // ---
+                // K 0
+                if !rem.is_null() {
+                    *rem = u128::from_parts(0, n.high() % d.high());
+                }
+                return u128::from(n.high() / d.high());
+            }
+
+            // K K
+            // ---
+            // K 0
+
+            if d.high().is_power_of_two() {
+                if !rem.is_null() {
+                    *rem = u128::from_parts(n.low(), n.high() & (d.high() - 1));
+                }
+                return u128::from(n.high() >> d.high().trailing_zeros());
+            }
+
+            sr = d.high().leading_zeros().wrapping_sub(n.high().leading_zeros());
+
+            // D > N
+            if sr > 64 - 2 {
+                if !rem.is_null() {
+                    *rem = n;
+                }
+                return 0;
+            }
+
+            sr += 1;
+
+            // 1 <= sr <= u32::bits() - 1
+            q = n << (128 - sr);
+            r = n >> sr;
+        } else {
+            if d.high() == 0 {
+                // K X
+                // ---
+                // 0 K
+                if d.low().is_power_of_two() {
+                    if !rem.is_null() {
+                        *rem = u128::from(n.low() & (d.low() - 1));
+                    }
+
+                    if d.low() == 1 {
+                        return n;
+                    } else {
+                        let sr = d.low().trailing_zeros();
+                        return n >> sr;
+                    };
+                }
+
+                sr = 1 + 64 + d.low().leading_zeros() - n.high().leading_zeros();
+
+                // 2 <= sr <= u64::bits() - 1
+                q = n << (128 - sr);
+                r = n >> sr;
+            } else {
+                // K X
+                // ---
+                // K K
+                sr = d.high().leading_zeros().wrapping_sub(n.high().leading_zeros());
+
+                // D > N
+                if sr > 64 - 1 {
+                    if !rem.is_null() {
+                        *rem = n;
+                    }
+                    return 0;
+                }
+
+                sr += 1;
+
+                // 1 <= sr <= u32::bits()
+                q = n << (128 - sr);
+                r = n >> sr;
+            }
+        }
+
+        // Not a special case
+        // q and r are initialized with
+        // q = n << (u64::bits() - sr)
+        // r = n >> sr
+        // 1 <= sr <= u64::bits() - 1
+        let mut carry = 0;
+
+        for _ in 0..sr {
+            // r:q = ((r:q) << 1) | carry
+            r = (r << 1) | (q >> (128 - 1));
+            q = (q << 1) | carry as u128;
+
+            // carry = 0
+            // if r >= d {
+            //     r -= d;
+            //     carry = 1;
+            // }
+            let s = (d.wrapping_sub(r).wrapping_sub(1)) as i128 >> (128 - 1);
+            carry = (s & 1) as u64;
+            r -= d & s as u128;
+        }
+
+        if !rem.is_null() {
+            *rem = r;
+        }
+        (q << 1) | carry as u128
+        }
+    }
+
+    #[export_name="__umodti3"]
+    pub extern fn u128_mod(a: u128_, b: u128_) -> u128_ {
+        unsafe {
+            let mut r = ::core::mem::zeroed();
+            u128_div_mod(a, b, &mut r);
+            r
+        }
+    }
+
+    #[export_name="__modti3"]
+    pub extern fn i128_mod(a: i128_, b: i128_) -> i128_ {
+        let b = b.abs();
+        let sa = a.signum();
+        let a = a.abs();
+        unsafe {
+            let mut r = ::core::mem::zeroed();
+            u128_div_mod(a as u128_, b as u128_, &mut r);
+            if sa == -1 { -(r as i128_) } else { r as i128_ }
+        }
+    }
+
+    #[export_name="__divti3"]
+    pub extern fn i128_div(a: i128_, b: i128_) -> i128_ {
+        let sa = a.signum();
+        let sb = b.signum();
+        let a = a.abs();
+        let b = b.abs();
+        let sr = sa ^ sb;
+        unsafe {
+            let mut r = ::core::mem::zeroed();
+            if sa == -1 {
+                -(u128_div_mod(a as u128_, b as u128_, &mut r) as i128_)
+            } else {
+                u128_div_mod(a as u128_, b as u128_, &mut r) as i128_
+            }
+        }
+    }
+
+    #[export_name="__udivti3"]
+    pub extern fn u128_div(a: u128_, b: u128_) -> u128_ {
+        unsafe {
+            let mut r = ::core::mem::zeroed();
+            u128_div_mod(a, b, &mut r)
+        }
+    }
+
+    macro_rules! mulo {
+        ($a:expr, $b:expr, $o: expr, $ty: ty) => {{
+            let (a, b, overflow) = ($a, $b, $o);
+            *overflow = 0;
+            let result = a.wrapping_mul(b);
+            if a == <$ty>::min_value() {
+                if b != 0 && b != 1 {
+                    *overflow = 1;
+                }
+                return result;
+            }
+            if b == <$ty>::min_value() {
+                if a != 0 && a != 1 {
+                    *overflow = 1;
+                }
+                return result;
+            }
+
+            let bits = ::core::mem::size_of::<$ty>() * 8;
+            let sa = a >> (bits - 1);
+            let abs_a = (a ^ sa) - sa;
+            let sb = b >> (bits - 1);
+            let abs_b = (b ^ sb) - sb;
+            if abs_a < 2 || abs_b < 2 {
+                return result;
+            }
+            if sa == sb {
+                if abs_a > <$ty>::max_value() / abs_b {
+                    *overflow = 1;
+                }
+            } else {
+                if abs_a > <$ty>::min_value() / -abs_b {
+                    *overflow = 1;
+                }
+            }
+            result
+        }}
+    }
+
+    // FIXME: i32 here should be c_int.
+    #[export_name="__muloti4"]
+    pub extern fn i128_mul_oflow(a: i128_, b: i128_, o: &mut i32) -> i128_ {
+        if let Some(v) = (a as i64).checked_mul(b as i64) {
+            *o = 0;
+            v as i128_
+        } else {
+            *o = 1;
+            0
+        }
+    }
+
+    pub trait LargeInt {
+        type LowHalf;
+        type HighHalf;
+
+        fn low(self) -> Self::LowHalf;
+        fn high(self) -> Self::HighHalf;
+        fn from_parts(low: Self::LowHalf, high: Self::HighHalf) -> Self;
+    }
+    impl LargeInt for u64 {
+        type LowHalf = u32;
+        type HighHalf = u32;
+
+        fn low(self) -> u32 {
+            self as u32
+        }
+        fn high(self) -> u32 {
+            (self >> 32) as u32
+        }
+        fn from_parts(low: u32, high: u32) -> u64 {
+            low as u64 | ((high as u64) << 32)
+        }
+    }
+    impl LargeInt for i64 {
+        type LowHalf = u32;
+        type HighHalf = i32;
+
+        fn low(self) -> u32 {
+            self as u32
+        }
+        fn high(self) -> i32 {
+            (self >> 32) as i32
+        }
+        fn from_parts(low: u32, high: i32) -> i64 {
+            low as i64 | ((high as i64) << 32)
+        }
+    }
+    #[cfg(not(stage0))]
+    impl LargeInt for u128 {
+        type LowHalf = u64;
+        type HighHalf = u64;
+
+        fn low(self) -> u64 {
+            self as u64
+        }
+        fn high(self) -> u64 {
+            unsafe { *(&self as *const u128 as *const u64) }
+        }
+        fn from_parts(low: u64, high: u64) -> u128 {
+            #[repr(C, packed)] struct Parts(u64, u64);
+            unsafe { ::core::mem::transmute(Parts(low, high)) }
+        }
+    }
+    #[cfg(not(stage0))]
+    impl LargeInt for i128 {
+        type LowHalf = u64;
+        type HighHalf = i64;
+
+        fn low(self) -> u64 {
+            self as u64
+        }
+        fn high(self) -> i64 {
+            unsafe { *(&self as *const i128 as *const i64) }
+        }
+        fn from_parts(low: u64, high: i64) -> i128 {
+            u128::from_parts(low, high as u64) as i128
+        }
+    }
+
+    macro_rules! mul {
+        ($a:expr, $b:expr, $ty: ty) => {{
+            let (a, b) = ($a, $b);
+            let bits = ::core::mem::size_of::<$ty>() * 8;
+            let half_bits = bits / 4;
+            let lower_mask = !0 >> half_bits;
+            let mut low = (a.low() & lower_mask) * (b.low() & lower_mask);
+            let mut t = low >> half_bits;
+            low &= lower_mask;
+            t += (a.low() >> half_bits) * (b.low() & lower_mask);
+            low += (t & lower_mask) << half_bits;
+            let mut high = t >> half_bits;
+            t = low >> half_bits;
+            low &= lower_mask;
+            t += (b.low() >> half_bits) * (a.low() & lower_mask);
+            low += (t & lower_mask) << half_bits;
+            high += t >> half_bits;
+            high += (a.low() >> half_bits) * (b.low() >> half_bits);
+            high = high.wrapping_add(a.high().wrapping_mul(b.low()).wrapping_add(a.low().wrapping_mul(b.high())));
+            <$ty>::from_parts(low, high)
+        }}
+    }
+
+
+    #[export_name="__multi3"]
+    pub extern fn u128_mul(a: u128_, b: u128_) -> u128_ {
+        (a as u64 * b as u64) as u128_
+        // mul!(a, b, u128_)
+    }
+
+    trait FloatStuff: Sized {
+        type ToBytes;
+
+        const MANTISSA_BITS: u32;
+        const MAX_EXP: i32;
+        const EXP_MASK: Self::ToBytes;
+        const MANTISSA_MASK: Self::ToBytes;
+
+        fn to_bytes(self) -> Self::ToBytes;
+        fn get_exponent(self) -> i32;
+    }
+
+    impl FloatStuff for f32 {
+        type ToBytes = u32;
+        const MANTISSA_BITS: u32 = 23;
+        const MAX_EXP: i32 = 127;
+        const MANTISSA_MASK: u32 = 0x007F_FFFF;
+        const EXP_MASK: u32 = 0x7F80_0000;
+
+        fn to_bytes(self) -> u32 { unsafe { ::core::mem::transmute(self) } }
+        fn get_exponent(self) -> i32 {
+            (((self.to_bytes() & Self::EXP_MASK) >> Self::MANTISSA_BITS) as i32) - Self::MAX_EXP
+        }
+    }
+
+    impl FloatStuff for f64 {
+        type ToBytes = u64;
+        const MANTISSA_BITS: u32 = 52;
+        const MAX_EXP: i32 = 1023;
+        const EXP_MASK: u64 = 0x7FF0_0000_0000_0000;
+        const MANTISSA_MASK: u64 = 0x000F_FFFF_FFFF_FFFF;
+
+        fn to_bytes(self) -> u64 { unsafe { ::core::mem::transmute(self) } }
+        fn get_exponent(self) -> i32 {
+            (((self.to_bytes() & Self::EXP_MASK) >> Self::MANTISSA_BITS) as i32) - Self::MAX_EXP
+        }
+    }
+
+    macro_rules! float_as_unsigned {
+        ($from: expr, $fromty: ty, $outty: ty) => { {
+            use core::num::Float;
+            let repr = $from.to_bytes();
+            let sign = $from.signum();
+            let exponent = $from.get_exponent();
+            let mantissa = repr & <$fromty as FloatStuff>::MANTISSA_MASK;
+            if sign == -1.0 || exponent < 0 { return 0; }
+            if exponent > ::core::mem::size_of::<$outty>() as i32 * 8 {
+                return !0;
+            }
+            if exponent < (<$fromty as FloatStuff>::MANTISSA_BITS) as i32 {
+                mantissa as $outty >> (<$fromty as FloatStuff>::MANTISSA_BITS as i32 - exponent)
+            } else {
+                mantissa as $outty << (exponent - <$fromty as FloatStuff>::MANTISSA_BITS as i32)
+            }
+        } }
+    }
+
+    #[export_name="__fixunsdfti"]
+    pub extern fn f64_as_u128(a: f64) -> u128_ {
+        float_as_unsigned!(a, f64, u128_)
+    }
+
+    #[export_name="__fixunssfti"]
+    pub extern fn f32_as_u128(a: f32) -> u128_ {
+        float_as_unsigned!(a, f32, u128_)
+    }
+
+    macro_rules! float_as_signed {
+        ($from: expr, $fromty: ty, $outty: ty) => {{
+            use core::num::Float;
+            let repr = $from.to_bytes();
+            let sign = $from.signum();
+            let exponent = $from.get_exponent();
+            let mantissa = repr & <$fromty as FloatStuff>::MANTISSA_MASK;
+
+            if exponent < 0 { return 0; }
+            if exponent > ::core::mem::size_of::<$outty>() as i32 * 8 {
+                return if sign > 0.0 { <$outty>::max_value() } else { <$outty>::min_value() };
+            }
+            let r = if exponent < (<$fromty as FloatStuff>::MANTISSA_BITS) as i32 {
+                mantissa as $outty >> (<$fromty as FloatStuff>::MANTISSA_BITS as i32 - exponent)
+            } else {
+                mantissa as $outty << (exponent - <$fromty as FloatStuff>::MANTISSA_BITS as i32)
+            };
+            if sign >= 0.0 { r } else { -r }
+        }}
+    }
+
+    #[export_name="__fixdfti"]
+    pub extern fn f64_as_i128(a: f64) -> i128_ {
+        float_as_signed!(a, f64, i128_)
+    }
+
+    #[export_name="__fixsfti"]
+    pub extern fn f32_as_i128(a: f32) -> i128_ {
+        float_as_signed!(a, f32, i128_)
+    }
+
+    #[export_name="__floattidf"]
+    pub extern fn i128_as_f64(a: i128_) -> f64 {
+        match a.signum() {
+            1 => u128_as_f64(a.abs() as u128_),
+            0 => 0.0,
+            -1 => -u128_as_f64(a.abs() as u128_),
+            _ => unimplemented()
+        }
+    }
+
+    #[export_name="__floattisf"]
+    pub extern fn i128_as_f32(a: i128_) -> f32 {
+        match a.signum() {
+            1 => u128_as_f32(a.abs() as u128_),
+            0 => 0.0,
+            -1 => -u128_as_f32(a.abs() as u128_),
+            _ => unimplemented()
+        }
+    }
+
+    #[export_name="__floatuntidf"]
+    pub extern fn u128_as_f64(mut a: u128_) -> f64 {
+        use ::core::f64::MANTISSA_DIGITS;
+        if a == 0 { return 0.0; }
+        let sd = 128 - a.leading_zeros();
+        let mut e = sd - 1;
+        const MD1 : u32 = MANTISSA_DIGITS + 1;
+        const MD2 : u32 = MANTISSA_DIGITS + 2;
+
+        if sd > MANTISSA_DIGITS {
+            a = match sd {
+                MD1 => a << 1,
+                MD2 => a,
+                _ => (a >> (sd - (MANTISSA_DIGITS + 2))) |
+                     (if (a & (!0 >> (128 + MANTISSA_DIGITS + 2) - sd)) == 0 { 0 } else { 1 })
+            };
+            a |= if (a & 4) == 0 { 0 } else { 1 };
+            a += 1;
+            a >>= 2;
+            if a & (1 << MANTISSA_DIGITS) != 0 {
+                a >>= 1;
+                e += 1;
+            }
+        } else {
+            a <<= MANTISSA_DIGITS - sd;
+        }
+        unsafe {
+            ::core::mem::transmute(((e as u64 + 1023) << 52) | (a as u64 & 0x000f_ffff_ffff_ffff))
+        }
+    }
+
+    #[export_name="__floatuntisf"]
+    pub extern fn u128_as_f32(mut a: u128_) -> f32 {
+        use ::core::f32::MANTISSA_DIGITS;
+        if a == 0 { return 0.0; }
+        let sd = 128 - a.leading_zeros();
+        let mut e = sd - 1;
+        const MD1 : u32 = MANTISSA_DIGITS + 1;
+        const MD2 : u32 = MANTISSA_DIGITS + 2;
+
+        if sd > MANTISSA_DIGITS {
+            a = match sd {
+                MD1 => a << 1,
+                MD2 => a,
+                _ => (a >> (sd - (MANTISSA_DIGITS + 2))) |
+                     (if (a & (!0 >> (128 + MANTISSA_DIGITS + 2) - sd)) == 0 { 0 } else { 1 })
+            };
+            a |= if (a & 4) == 0 { 0 } else { 1 };
+            a += 1;
+            a >>= 2;
+            if a & (1 << MANTISSA_DIGITS) != 0 {
+                a >>= 1;
+                e += 1;
+            }
+        } else {
+            a <<= MANTISSA_DIGITS - sd;
+        }
+        unsafe {
+            ::core::mem::transmute(((e + 127) << 23) | (a as u32 & 0x007f_ffff))
+        }
+    }
+}