bisect: avoid copying ancestor list for non-merge commits
During a bisection, hg needs to compute a list of all ancestors for every
candidate commit. This is accomplished via a bottom-up traversal of the set of
candidates, during which each revision's ancestor list is populated using the
ancestor list of its parent(s). Previously, this involved copying the entire
list, which could be very long in if the bisection range was large.
To help improve this, we can observe that each candidate commit is visited
exactly once, at which point its ancestor list is copied into its children's
lists and then dropped. In the case of non-merge commits, a commit's ancestor
list consists exactly of its parent's list plus itself. This means that we can
trivially reuse the parent's existing list for one of its non-merge children,
which avoids copying entirely if that commit is the parent's only child. This
makes bisections over linear ranges of commits much faster.
During some informal testing in the large publicly-available `mozilla-central`
repository, this noticeably sped up bisections over large ranges of history:
Setup:
$ cd mozilla-central
$ hg bisect --reset
$ hg bisect --good 0
$ hg log -r tip -T '{rev}\n'
628417
Test:
$ time hg bisect --bad tip --noupdate
Before:
real 3m35.927s
user 3m35.553s
sys 0m0.319s
After:
real 1m41.142s
user 1m40.810s
sys 0m0.285s
#require serve
setting up repo
$ hg init test
$ cd test
$ echo a > a
$ hg ci -Ama
adding a
$ hg rm a
$ hg ci -mdel
set up hgweb
$ hg serve -n test -p $HGPORT -d --pid-file=hg.pid -A access.log -E errors.log
$ cat hg.pid >> $DAEMON_PIDS
revision
$ get-with-headers.py localhost:$HGPORT 'rev/tip'
200 Script output follows
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en-US">
<head>
<link rel="icon" href="/static/hgicon.png" type="image/png" />
<meta name="robots" content="index, nofollow" />
<link rel="stylesheet" href="/static/style-paper.css" type="text/css" />
<script type="text/javascript" src="/static/mercurial.js"></script>
<title>test: c78f6c5cbea9</title>
</head>
<body>
<div class="container">
<div class="menu">
<div class="logo">
<a href="https://mercurial-scm.org/">
<img src="/static/hglogo.png" alt="mercurial" /></a>
</div>
<ul>
<li><a href="/shortlog/tip">log</a></li>
<li><a href="/graph/tip">graph</a></li>
<li><a href="/tags">tags</a></li>
<li><a href="/bookmarks">bookmarks</a></li>
<li><a href="/branches">branches</a></li>
</ul>
<ul>
<li class="active">changeset</li>
<li><a href="/raw-rev/tip">raw</a></li>
<li><a href="/file/tip">browse</a></li>
</ul>
<ul>
</ul>
<ul>
<li><a href="/help">help</a></li>
</ul>
</div>
<div class="main">
<h2 class="breadcrumb"><a href="/">Mercurial</a> </h2>
<h3>
changeset 1:<a href="/rev/c78f6c5cbea9">c78f6c5cbea9</a>
<span class="phase">draft</span> <span class="branchhead">default</span> <span class="tag">tip</span>
</h3>
<form class="search" action="/log">
<p><input name="rev" id="search1" type="text" size="30" value="" /></p>
<div id="hint">Find changesets by keywords (author, files, the commit message), revision
number or hash, or <a href="/help/revsets">revset expression</a>.</div>
</form>
<div class="description">del</div>
<table id="changesetEntry">
<tr>
<th class="author">author</th>
<td class="author">test</td>
</tr>
<tr>
<th class="date">date</th>
<td class="date age">Thu, 01 Jan 1970 00:00:00 +0000</td>
</tr>
<tr>
<th class="author">parents</th>
<td class="author"><a href="/rev/cb9a9f314b8b">cb9a9f314b8b</a> </td>
</tr>
<tr>
<th class="author">children</th>
<td class="author"></td>
</tr>
<tr>
<th class="files">files</th>
<td class="files">a </td>
</tr>
<tr>
<th class="diffstat">diffstat</th>
<td class="diffstat">
1 files changed, 0 insertions(+), 1 deletions(-)
<a id="diffstatexpand" class="diffstattoggle" href="#">[<tt>+</tt>]</a>
<div id="diffstatdetails" style="display:none;">
<a class="diffstattoggle" href="#">[<tt>-</tt>]</a>
<table class="diffstat-table stripes2"> <tr>
<td class="diffstat-file"><a href="#l1.1">a</a></td>
<td class="diffstat-total" align="right">1</td>
<td class="diffstat-graph">
<span class="diffstat-add" style="width:0.0%;"> </span>
<span class="diffstat-remove" style="width:100.0%;"> </span>
</td>
</tr>
</table>
</div>
</td>
</tr>
</table>
<div class="overflow">
<div class="sourcefirst linewraptoggle">line wrap: <a class="linewraplink" href="#">on</a></div>
<div class="sourcefirst"> line diff</div>
<div class="stripes2 diffblocks">
<div class="bottomline inc-lineno"><pre class="sourcelines wrap">
<span id="l1.1" class="minusline">--- a/a Thu Jan 01 00:00:00 1970 +0000</span><a href="#l1.1"></a>
<span id="l1.2" class="plusline">+++ /dev/null Thu Jan 01 00:00:00 1970 +0000</span><a href="#l1.2"></a>
<span id="l1.3" class="atline">@@ -1,1 +0,0 @@</span><a href="#l1.3"></a>
<span id="l1.4" class="minusline">-a</span><a href="#l1.4"></a></pre></div>
</div>
</div>
</div>
</div>
</body>
</html>
diff removed file
$ get-with-headers.py localhost:$HGPORT 'diff/tip/a'
200 Script output follows
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en-US">
<head>
<link rel="icon" href="/static/hgicon.png" type="image/png" />
<meta name="robots" content="index, nofollow" />
<link rel="stylesheet" href="/static/style-paper.css" type="text/css" />
<script type="text/javascript" src="/static/mercurial.js"></script>
<title>test: a diff</title>
</head>
<body>
<div class="container">
<div class="menu">
<div class="logo">
<a href="https://mercurial-scm.org/">
<img src="/static/hglogo.png" alt="mercurial" /></a>
</div>
<ul>
<li><a href="/shortlog/tip">log</a></li>
<li><a href="/graph/tip">graph</a></li>
<li><a href="/tags">tags</a></li>
<li><a href="/bookmarks">bookmarks</a></li>
<li><a href="/branches">branches</a></li>
</ul>
<ul>
<li><a href="/rev/tip">changeset</a></li>
<li><a href="/file/tip">browse</a></li>
</ul>
<ul>
<li><a href="/file/tip/a">file</a></li>
<li><a href="/file/tip/a">latest</a></li>
<li class="active">diff</li>
<li><a href="/comparison/tip/a">comparison</a></li>
<li><a href="/annotate/tip/a">annotate</a></li>
<li><a href="/log/tip/a">file log</a></li>
<li><a href="/raw-file/tip/a">raw</a></li>
</ul>
<ul>
<li><a href="/help">help</a></li>
</ul>
</div>
<div class="main">
<h2 class="breadcrumb"><a href="/">Mercurial</a> </h2>
<h3>
diff a @ 1:<a href="/rev/c78f6c5cbea9">c78f6c5cbea9</a>
<span class="phase">draft</span> <span class="branchhead">default</span> <span class="tag">tip</span>
</h3>
<form class="search" action="/log">
<p><input name="rev" id="search1" type="text" size="30" value="" /></p>
<div id="hint">Find changesets by keywords (author, files, the commit message), revision
number or hash, or <a href="/help/revsets">revset expression</a>.</div>
</form>
<div class="description">del</div>
<table id="changesetEntry">
<tr>
<th>author</th>
<td>test</td>
</tr>
<tr>
<th>date</th>
<td class="date age">Thu, 01 Jan 1970 00:00:00 +0000</td>
</tr>
<tr>
<th>parents</th>
<td><a href="/file/cb9a9f314b8b/a">cb9a9f314b8b</a> </td>
</tr>
<tr>
<th>children</th>
<td></td>
</tr>
</table>
<div class="overflow">
<div class="sourcefirst linewraptoggle">line wrap: <a class="linewraplink" href="#">on</a></div>
<div class="sourcefirst"> line diff</div>
<div class="stripes2 diffblocks">
<div class="bottomline inc-lineno"><pre class="sourcelines wrap">
<span id="l1.1" class="minusline">--- a/a Thu Jan 01 00:00:00 1970 +0000</span><a href="#l1.1"></a>
<span id="l1.2" class="plusline">+++ /dev/null Thu Jan 01 00:00:00 1970 +0000</span><a href="#l1.2"></a>
<span id="l1.3" class="atline">@@ -1,1 +0,0 @@</span><a href="#l1.3"></a>
<span id="l1.4" class="minusline">-a</span><a href="#l1.4"></a></pre></div>
</div>
</div>
</div>
</div>
</body>
</html>
$ cd ..