From 22722ac55d0dbd26a50c9b405153beca88a44e24 Mon Sep 17 00:00:00 2001
From: Josh Stone <cuviper@gmail.com>
Date: Wed, 6 Jan 2016 18:27:50 -0800
Subject: [PATCH] pow: shift exp powers of 2 before accumulating

We can save a multiplication if we start the accumulation basically at
the first set bit of the exponent, rather than starting at one and
waiting to multiply.  We only need one itself if the exponent is zero,
which is easy to pre-check.

Before:
    test pow_bench         ... bench:   8,267,370 ns/iter (+/- 93,319)

After:
    test pow_bench         ... bench:   7,506,463 ns/iter (+/- 116,311)
---
 src/lib.rs | 29 +++++++++++++++--------------
 1 file changed, 15 insertions(+), 14 deletions(-)

diff --git a/src/lib.rs b/src/lib.rs
index 6ded371..b8c8aa7 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -141,22 +141,23 @@ pub fn abs_sub<T: Signed>(x: T, y: T) -> T {
 /// ```
 #[inline]
 pub fn pow<T: Clone + One + Mul<T, Output = T>>(mut base: T, mut exp: usize) -> T {
-    if exp == 1 { base }
-    else {
-        let mut acc = one::<T>();
-        while exp > 0 {
-            if (exp & 1) == 1 {
-                acc = acc * base.clone();
-            }
+    if exp == 0 { return T::one() }
 
-            // avoid overflow if we won't need it
-            if exp > 1 {
-                base = base.clone() * base;
-            }
-            exp = exp >> 1;
-        }
-        acc
+    while exp & 1 == 0 {
+        base = base.clone() * base;
+        exp >>= 1;
     }
+    if exp == 1 { return base }
+
+    let mut acc = base.clone();
+    while exp > 1 {
+        exp >>= 1;
+        base = base.clone() * base;
+        if exp & 1 == 1 {
+            acc = acc * base.clone();
+        }
+    }
+    acc
 }
 
 #[cfg(test)]