diff --git a/rand_core/src/impls.rs b/rand_core/src/impls.rs
new file mode 100644
index 00000000000..fb45a3ecee2
--- /dev/null
+++ b/rand_core/src/impls.rs
@@ -0,0 +1,78 @@
+// Copyright 2013-2017 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+//! Helper functions for implementing `Rng` functions.
+//! 
+//! For cross-platform reproducibility, these functions all use Little Endian:
+//! least-significant part first. For example, `next_u64_via_u32` takes `u32`
+//! values `x, y`, then outputs `(y << 32) | x`. To implement `next_u32`
+//! from `next_u64` in little-endian order, one should use `next_u64() as u32`.
+//! 
+//! Byte-swapping (like the std `to_le` functions) is only needed to convert
+//! to/from byte sequences, and since its purpose is reproducibility,
+//! non-reproducible sources (e.g. `OsRng`) need not bother with it.
+
+use core::intrinsics::transmute;
+use {Rng, Result};
+
+/// Implement `next_u64` via `next_u32`, little-endian order.
+pub fn next_u64_via_u32<R: Rng+?Sized>(rng: &mut R) -> u64 {
+    // Use LE; we explicitly generate one value before the next.
+    let x = rng.next_u32() as u64;
+    let y = rng.next_u32() as u64;
+    (y << 32) | x
+}
+
+/// Implement `next_u128` via `next_u64`, little-endian order.
+#[cfg(feature = "i128_support")]
+pub fn next_u128_via_u64<R: Rng+?Sized>(rng: &mut R) -> u128 {
+    // Use LE; we explicitly generate one value before the next.
+    let x = rng.next_u64() as u128;
+    let y = rng.next_u64() as u128;
+    (y << 64) | x
+}
+
+macro_rules! try_fill_via {
+    ($rng:ident, $next_u:ident, $BYTES:expr, $dest:ident) => {{
+        let mut left = $dest;
+        while left.len() >= $BYTES {
+            let (l, r) = {left}.split_at_mut($BYTES);
+            left = r;
+            let chunk: [u8; $BYTES] = unsafe {
+                transmute($rng.$next_u().to_le())
+            };
+            l.copy_from_slice(&chunk);
+        }
+        let n = left.len();
+        if n > 0 {
+            let chunk: [u8; $BYTES] = unsafe {
+                transmute($rng.$next_u().to_le())
+            };
+            left.copy_from_slice(&chunk[..n]);
+        }
+        Ok(())
+    }}
+}
+
+/// Implement `try_fill` via `next_u32`, little-endian order.
+pub fn try_fill_via_u32<R: Rng+?Sized>(rng: &mut R, dest: &mut [u8]) -> Result<()> {
+    try_fill_via!(rng, next_u32, 4, dest)
+}
+
+/// Implement `try_fill` via `next_u64`, little-endian order.
+pub fn try_fill_via_u64<R: Rng+?Sized>(rng: &mut R, dest: &mut [u8]) -> Result<()> {
+    try_fill_via!(rng, next_u64, 8, dest)
+}
+
+/// Implement `try_fill` via `next_u128`, little-endian order.
+#[cfg(feature = "i128_support")]
+pub fn try_fill_via_u128<R: Rng+?Sized>(rng: &mut R, dest: &mut [u8]) -> Result<()> {
+    try_fill_via!(rng, next_u128, 16, dest)
+}
diff --git a/rand_core/src/lib.rs b/rand_core/src/lib.rs
index afa681641b3..7094b8ebe5e 100644
--- a/rand_core/src/lib.rs
+++ b/rand_core/src/lib.rs
@@ -35,6 +35,7 @@
 #[cfg(feature="std")]
 extern crate core;
 
+pub mod impls;
 pub mod mock;
 
 
@@ -75,63 +76,21 @@ pub trait Rng {
     fn next_u32(&mut self) -> u32;
 
     /// Return the next random u64.
-    ///
-    /// By default this is implemented in terms of `next_u32` in LE order. An
-    /// implementation of this trait must provide at least one of
-    /// these two methods. Similarly to `next_u32`, this rarely needs
-    /// to be called directly, prefer `r.gen()` to `r.next_u64()`.
-    fn next_u64(&mut self) -> u64 {
-        // Use LE; we explicitly generate one value before the next.
-        let x = self.next_u32() as u64;
-        let y = self.next_u32() as u64;
-        (y << 32) | x
-    }
+    fn next_u64(&mut self) -> u64;
 
-    #[cfg(feature = "i128_support")]
     /// Return the next random u128.
-    /// 
-    /// By default this is implemented in terms of `next_u64` in LE order.
-    fn next_u128(&mut self) -> u128 {
-        // Use LE; we explicitly generate one value before the next.
-        let x = self.next_u64() as u128;
-        let y = self.next_u64() as u128;
-        (y << 64) | x
-    }
+    #[cfg(feature = "i128_support")]
+    fn next_u128(&mut self) -> u128;
     
     /// Fill `dest` entirely with random data.
     ///
-    /// This has a default implementation in terms of `next_u64` in LE order,
-    /// but should be overridden by implementations that
-    /// offer a more efficient solution than just calling `next_u64`
-    /// repeatedly.
-    ///
     /// This method does *not* have any requirement on how much of the
-    /// generated random number stream is consumed; e.g. the default
+    /// generated random number stream is consumed; e.g. `try_fill_via_u64`
     /// implementation uses `next_u64` thus consuming 8 bytes even when only
     /// 1 is required. A different implementation might use `next_u32` and
     /// only consume 4 bytes; *however* any change affecting *reproducibility*
     /// of output must be considered a breaking change.
-    fn try_fill(&mut self, dest: &mut [u8]) -> Result<()> {
-        use core::intrinsics::transmute;
-        
-        let mut left = dest;
-        while left.len() >= 8 {
-            let (l, r) = {left}.split_at_mut(8);
-            left = r;
-            let chunk: [u8; 8] = unsafe {
-                transmute(self.next_u64().to_le())
-            };
-            l.copy_from_slice(&chunk);
-        }
-        let n = left.len();
-        if n > 0 {
-            let chunk: [u8; 8] = unsafe {
-                transmute(self.next_u64().to_le())
-            };
-            left.copy_from_slice(&chunk[..n]);
-        }
-        Ok(())
-    }
+    fn try_fill(&mut self, dest: &mut [u8]) -> Result<()>;
 }
 
 #[cfg(feature="std")]
diff --git a/rand_core/src/mock.rs b/rand_core/src/mock.rs
index a4fead00e4a..c32e9310d84 100644
--- a/rand_core/src/mock.rs
+++ b/rand_core/src/mock.rs
@@ -16,7 +16,7 @@
 //! Instead maybe this should be yet another crate? Or just leave it here?
 
 use core::num::Wrapping as w;
-use {Rng, SeedableRng};
+use {Rng, SeedableRng, impls, Result};
 
 /// A simple implementation of `Rng`, purely for testing.
 /// Returns an arithmetic sequence (i.e. adds a constant each step).
@@ -49,6 +49,17 @@ impl Rng for MockAddRng<u32> {
         self.v += self.a;
         result
     }
+    fn next_u64(&mut self) -> u64 {
+        impls::next_u64_via_u32(self)
+    }
+    #[cfg(feature = "i128_support")]
+    fn next_u128(&mut self) -> u128 {
+        impls::next_u128_via_u64(self)
+    }
+    
+    fn try_fill(&mut self, dest: &mut [u8]) -> Result<()> {
+        impls::try_fill_via_u32(self, dest)
+    }
 }
 
 impl Rng for MockAddRng<u64> {
@@ -60,6 +71,14 @@ impl Rng for MockAddRng<u64> {
         self.v += self.a;
         result
     }
+    #[cfg(feature = "i128_support")]
+    fn next_u128(&mut self) -> u128 {
+        impls::next_u128_via_u64(self)
+    }
+    
+    fn try_fill(&mut self, dest: &mut [u8]) -> Result<()> {
+        impls::try_fill_via_u32(self, dest)
+    }
 }
 
 impl<T> SeedableRng<T> for MockAddRng<T> where
diff --git a/src/lib.rs b/src/lib.rs
index 91c829ae0a1..53b2216a45e 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -404,15 +404,19 @@ pub struct StdRng {
 }
 
 impl Rng for StdRng {
-    #[inline]
     fn next_u32(&mut self) -> u32 {
         self.rng.next_u32()
     }
-
-    #[inline]
     fn next_u64(&mut self) -> u64 {
         self.rng.next_u64()
     }
+    #[cfg(feature = "i128_support")]
+    fn next_u128(&mut self) -> u128 {
+        self.rng.next_u128()
+    }
+    fn try_fill(&mut self, dest: &mut [u8]) -> Result<()> {
+        self.rng.try_fill(dest)
+    }
 }
 
 impl SeedFromRng for StdRng {
@@ -424,7 +428,7 @@ impl SeedFromRng for StdRng {
 
 #[cfg(test)]
 mod test {
-    use {Rng, thread_rng, Sample};
+    use {Rng, thread_rng, Sample, Result};
     use rand_core::mock::MockAddRng;
     use distributions::{uniform};
     use distributions::{Uniform, Range, Exp};
@@ -436,10 +440,17 @@ mod test {
 
     impl<R: Rng+?Sized> Rng for MyRng<R> {
         fn next_u32(&mut self) -> u32 {
-            fn next<T: Rng+?Sized>(t: &mut T) -> u32 {
-                t.next_u32()
-            }
-            next(&mut self.inner)
+            self.inner.next_u32()
+        }
+        fn next_u64(&mut self) -> u64 {
+            self.inner.next_u64()
+        }
+        #[cfg(feature = "i128_support")]
+        fn next_u128(&mut self) -> u128 {
+            self.inner.next_u128()
+        }
+        fn try_fill(&mut self, dest: &mut [u8]) -> Result<()> {
+            self.inner.try_fill(dest)
         }
     }
 
diff --git a/src/os.rs b/src/os.rs
index 9e46560b9c9..eeb4c13d1eb 100644
--- a/src/os.rs
+++ b/src/os.rs
@@ -55,6 +55,12 @@ impl Rng for OsRng {
         self.try_fill(&mut buf).unwrap_or_else(|e| panic!("try_fill failed: {:?}", e));
         unsafe{ *(buf.as_ptr() as *const u64) }
     }
+    #[cfg(feature = "i128_support")]
+    fn next_u128(&mut self) -> u128 {
+        let mut buf: [u8; 16] = [0; 16];
+        self.try_fill(&mut buf).unwrap_or_else(|e| panic!("try_fill failed: {:?}", e));
+        unsafe{ *(buf.as_ptr() as *const u128) }
+    }
     fn try_fill(&mut self, v: &mut [u8]) -> Result<()> {
         self.0.try_fill(v)
     }
diff --git a/src/prng/chacha.rs b/src/prng/chacha.rs
index 0b0a83b6c1f..2f0a5f78794 100644
--- a/src/prng/chacha.rs
+++ b/src/prng/chacha.rs
@@ -198,6 +198,14 @@ impl Rng for ChaChaRng {
         value.0
     }
     
+    fn next_u64(&mut self) -> u64 {
+        ::rand_core::impls::next_u64_via_u32(self)
+    }
+    #[cfg(feature = "i128_support")]
+    fn next_u128(&mut self) -> u128 {
+        ::rand_core::impls::next_u128_via_u64(self)
+    }
+    
     // Custom implementation allowing larger reads from buffer is about 8%
     // faster than default implementation in my tests
     fn try_fill(&mut self, dest: &mut [u8]) -> Result<()> {
diff --git a/src/prng/isaac.rs b/src/prng/isaac.rs
index f7d206c629f..2e866028552 100644
--- a/src/prng/isaac.rs
+++ b/src/prng/isaac.rs
@@ -222,27 +222,16 @@ impl Rng for IsaacRng {
         self.rsl[(self.cnt % RAND_SIZE) as usize].0
     }
     
-    // Default impl adjusted for native byte size; approx 18% faster in tests
+    fn next_u64(&mut self) -> u64 {
+        ::rand_core::impls::next_u64_via_u32(self)
+    }
+    #[cfg(feature = "i128_support")]
+    fn next_u128(&mut self) -> u128 {
+        ::rand_core::impls::next_u128_via_u64(self)
+    }
+    
     fn try_fill(&mut self, dest: &mut [u8]) -> Result<()> {
-        use core::intrinsics::transmute;
-        
-        let mut left = dest;
-        while left.len() >= 4 {
-            let (l, r) = {left}.split_at_mut(4);
-            left = r;
-            let chunk: [u8; 4] = unsafe {
-                transmute(self.next_u32().to_le())
-            };
-            l.copy_from_slice(&chunk);
-        }
-        let n = left.len();
-        if n > 0 {
-            let chunk: [u8; 4] = unsafe {
-                transmute(self.next_u32().to_le())
-            };
-            left.copy_from_slice(&chunk[..n]);
-        }
-        Ok(())
+        ::rand_core::impls::try_fill_via_u32(self, dest)
     }
 }
 
@@ -492,6 +481,14 @@ impl Rng for Isaac64Rng {
         debug_assert!(self.cnt < RAND_SIZE_64);
         self.rsl[(self.cnt % RAND_SIZE_64) as usize].0
     }
+    #[cfg(feature = "i128_support")]
+    fn next_u128(&mut self) -> u128 {
+        ::rand_core::impls::next_u128_via_u64(self)
+    }
+    
+    fn try_fill(&mut self, dest: &mut [u8]) -> Result<()> {
+        ::rand_core::impls::try_fill_via_u64(self, dest)
+    }
 }
 
 impl SeedFromRng for Isaac64Rng {
diff --git a/src/prng/xorshift.rs b/src/prng/xorshift.rs
index dbfa24a26a0..1e2d6cca252 100644
--- a/src/prng/xorshift.rs
+++ b/src/prng/xorshift.rs
@@ -75,6 +75,18 @@ impl Rng for XorShiftRng {
         self.w = w_ ^ (w_ >> 19) ^ (t ^ (t >> 8));
         self.w.0
     }
+    
+    fn next_u64(&mut self) -> u64 {
+        ::rand_core::impls::next_u64_via_u32(self)
+    }
+    #[cfg(feature = "i128_support")]
+    fn next_u128(&mut self) -> u128 {
+        ::rand_core::impls::next_u128_via_u64(self)
+    }
+    
+    fn try_fill(&mut self, dest: &mut [u8]) -> Result<()> {
+        ::rand_core::impls::try_fill_via_u32(self, dest)
+    }
 }
 
 impl SeedableRng<[u32; 4]> for XorShiftRng {