From 78e6abd244eb4f75180fdb3bc72d69a2f471feca Mon Sep 17 00:00:00 2001
From: "David R. Jenni" <david.r.jenni@gmail.com>
Date: Wed, 8 Feb 2017 11:32:58 +0100
Subject: [PATCH] sort: optimize average calculation in symMerge and doPivot.
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Change code of the form `i + (j-i)/2` to `int(uint(i+j) >> 1)`.
The optimized average calculation uses fewer instructions to calculate
the average without overflowing at the addition.
Analogous to https://golang.org/cl/36332.
name old time/op new time/op delta
StableString1K-4 49.6µs ± 3% 49.1µs ± 8% ~ (p=0.659 n=16+19)
StableInt1K-4 160µs ±10% 148µs ± 5% -7.29% (p=0.000 n=20+16)
StableInt1K_Slice-4 139µs ± 4% 136µs ± 3% -2.52% (p=0.000 n=20+16)
StableInt64K-4 8.84ms ± 6% 8.57ms ± 5% -3.07% (p=0.001 n=20+19)
Stable1e2-4 162µs ±19% 147µs ±16% -8.79% (p=0.002 n=20+20)
Stable1e4-4 31.0ms ± 5% 30.6ms ± 5% ~ (p=0.221 n=20+20)
Stable1e6-4 6.37s ± 3% 6.27s ± 2% -1.67% (p=0.000 n=19+20)
Change-Id: I1cea0bcb9ace8ef7e48b8fab772e41b4b2170da9
Reviewed-on: https://go-review.googlesource.com/36570
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
---
src/sort/genzfunc.go | 4 ++++
src/sort/sort.go | 10 +++++-----
src/sort/zfuncversion.go | 10 +++++-----
3 files changed, 14 insertions(+), 10 deletions(-)
diff --git a/src/sort/genzfunc.go b/src/sort/genzfunc.go
index 6d2b471b62e..3bb7691f6a8 100644
--- a/src/sort/genzfunc.go
+++ b/src/sort/genzfunc.go
@@ -115,6 +115,10 @@ func rewriteCall(ce *ast.CallExpr) {
// e.g. skip SelectorExpr (data.Less(..) calls)
return
}
+ // skip casts
+ if ident.Name == "int" || ident.Name == "uint" {
+ return
+ }
if len(ce.Args) < 1 {
return
}
diff --git a/src/sort/sort.go b/src/sort/sort.go
index 72d24efceab..54f92a42176 100644
--- a/src/sort/sort.go
+++ b/src/sort/sort.go
@@ -96,7 +96,7 @@ func swapRange(data Interface, a, b, n int) {
}
func doPivot(data Interface, lo, hi int) (midlo, midhi int) {
- m := lo + (hi-lo)/2 // Written like this to avoid integer overflow.
+ m := int(uint(lo+hi) >> 1) // Written like this to avoid integer overflow.
if hi-lo > 40 {
// Tukey's ``Ninther,'' median of three medians of three.
s := (hi - lo) / 8
@@ -447,7 +447,7 @@ func symMerge(data Interface, a, m, b int) {
i := m
j := b
for i < j {
- h := i + (j-i)/2
+ h := int(uint(i+j) >> 1)
if data.Less(h, a) {
i = h + 1
} else {
@@ -471,7 +471,7 @@ func symMerge(data Interface, a, m, b int) {
i := a
j := m
for i < j {
- h := i + (j-i)/2
+ h := int(uint(i+j) >> 1)
if !data.Less(m, h) {
i = h + 1
} else {
@@ -485,7 +485,7 @@ func symMerge(data Interface, a, m, b int) {
return
}
- mid := a + (b-a)/2
+ mid := int(uint(a+b) >> 1)
n := mid + m
var start, r int
if m > mid {
@@ -498,7 +498,7 @@ func symMerge(data Interface, a, m, b int) {
p := n - 1
for start < r {
- c := start + (r-start)/2
+ c := int(uint(start+r) >> 1)
if !data.Less(p-c, c) {
start = c + 1
} else {
diff --git a/src/sort/zfuncversion.go b/src/sort/zfuncversion.go
index 7abb18a24d5..99c95a22c1f 100644
--- a/src/sort/zfuncversion.go
+++ b/src/sort/zfuncversion.go
@@ -70,7 +70,7 @@ func swapRange_func(data lessSwap, a, b, n int) {
// Auto-generated variant of sort.go:doPivot
func doPivot_func(data lessSwap, lo, hi int) (midlo, midhi int) {
- m := lo + (hi-lo)/2
+ m := int(uint(lo+hi) >> 1)
if hi-lo > 40 {
s := (hi - lo) / 8
medianOfThree_func(data, lo, lo+s, lo+2*s)
@@ -189,7 +189,7 @@ func symMerge_func(data lessSwap, a, m, b int) {
i := m
j := b
for i < j {
- h := i + (j-i)/2
+ h := int(uint(i+j) >> 1)
if data.Less(h, a) {
i = h + 1
} else {
@@ -205,7 +205,7 @@ func symMerge_func(data lessSwap, a, m, b int) {
i := a
j := m
for i < j {
- h := i + (j-i)/2
+ h := int(uint(i+j) >> 1)
if !data.Less(m, h) {
i = h + 1
} else {
@@ -217,7 +217,7 @@ func symMerge_func(data lessSwap, a, m, b int) {
}
return
}
- mid := a + (b-a)/2
+ mid := int(uint(a+b) >> 1)
n := mid + m
var start, r int
if m > mid {
@@ -229,7 +229,7 @@ func symMerge_func(data lessSwap, a, m, b int) {
}
p := n - 1
for start < r {
- c := start + (r-start)/2
+ c := int(uint(start+r) >> 1)
if !data.Less(p-c, c) {
start = c + 1
} else {
--
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