From: David R. Jenni Date: Sun, 5 Feb 2017 10:02:03 +0000 (+0100) Subject: sort: optimize average calculation in binary search X-Git-Tag: go1.9beta1~1729 X-Git-Url: http://www.git.cypherpunks.su/?a=commitdiff_plain;h=fd37b8ccf2262bb3f0a608f7545f78a72e8d661f;p=gostls13.git sort: optimize average calculation in binary search Use fewer instructions to calculate the average of i and j without overflowing at the addition. Even if both i and j are math.MaxInt{32,64}, the sum fits into a uint{32,64}. Because the sum of i and j is always ≥ 0, the right shift by one does the same as a division by two. The result of the shift operation is at most math.MaxInt{32,64} and fits again into an int{32,64}. name old time/op new time/op delta SearchWrappers-4 153ns ± 3% 143ns ± 6% -6.33% (p=0.000 n=90+100) This calculation is documented in: https://research.googleblog.com/2006/06/extra-extra-read-all-about-it-nearly.html Change-Id: I2be7922afc03b3617fce32e59364606c37a83678 Reviewed-on: https://go-review.googlesource.com/36332 Reviewed-by: Robert Griesemer Run-TryBot: Robert Griesemer TryBot-Result: Gobot Gobot --- diff --git a/src/sort/search.go b/src/sort/search.go index b9640a40af..24cc90248c 100644 --- a/src/sort/search.go +++ b/src/sort/search.go @@ -61,7 +61,7 @@ func Search(n int, f func(int) bool) int { // Invariant: f(i-1) == false, f(j) == true. i, j := 0, n for i < j { - h := i + (j-i)/2 // avoid overflow when computing h + h := int(uint(i+j) >> 1) // avoid overflow when computing h // i ≤ h < j if !f(h) { i = h + 1 // preserves f(i-1) == false