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Fix quadratic performance hit in Q(Multi)Map::insert() with hint

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The insert() overloads that took a const_iterator started by calling
std::distance(begin(), pos) - which has a cost linear in how far pos
is from begin() - in order to, after detach()ing, obtain an iterator
at the same offset from the new begin(), using std::next() - also
linear. This leads to quadratic behavior when large numbers of entries
are added with constEnd() as the hint, which happened to be tested by
tst_bench_qmap. That wasn't running, due to some assertion failures,
but once those were fixed the hinted tests timed out after five
minutes, where their unhinted peers completed comfortably within a
second.

Check whether detach() is even needed and bypass the std::distance() /
std::next() linear delay when it isn't. This brings the hinted tests
down to running faster than their unhinted equivalents.

Task-number: QTBUG-91713
Change-Id: I6b705bf8fc34e67aed2ac4b3312a836e105ca2f2
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
Reviewed-by: Andrei Golubev <andrei.golubev@qt.io>
(cherry picked from commit 33c916577389fa6607b0b2f6a78da4a0eb485000)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
This commit is contained in:
Edward Welbourne 2021-07-19 17:17:36 +02:00 committed by Qt Cherry-pick Bot
parent a83cf2c10e
commit 160daf5d63
1 changed files with 19 additions and 9 deletions
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28
src/corelib/tools/qmap.h
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@ -1,7 +1,7 @@
/**************************************************************************** /****************************************************************************
** **
** Copyright (C) 2020 Klarälvdalens Datakonsult AB, a KDAB Group company, info@kdab.com, author Giuseppe D'Angelo <giuseppe.dangelo@kdab.com> ** Copyright (C) 2020 Klarälvdalens Datakonsult AB, a KDAB Group company, info@kdab.com, author Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
** Copyright (C) 2016 The Qt Company Ltd. ** Copyright (C) 2021 The Qt Company Ltd.
** Contact: https://www.qt.io/licensing/ ** Contact: https://www.qt.io/licensing/
** **
** This file is part of the QtCore module of the Qt Toolkit. ** This file is part of the QtCore module of the Qt Toolkit.
@ -668,10 +668,15 @@ public:
iterator insert(const_iterator pos, const Key &key, const T &value) iterator insert(const_iterator pos, const Key &key, const T &value)
{ {
// TODO: improve. In case of assignment, why copying first? // TODO: improve. In case of assignment, why copying first?
auto posDistance = d ? std::distance(d->m.cbegin(), pos.i) : 0; typename Map::const_iterator dpos;
detach(); if (!d || d.isShared()) {
auto detachedPos = std::next(d->m.cbegin(), posDistance); auto posDistance = d ? std::distance(d->m.cbegin(), pos.i) : 0;
return iterator(d->m.insert_or_assign(detachedPos, key, value)); detach();
dpos = std::next(d->m.cbegin(), posDistance);
} else {
dpos = pos.i;
}
return iterator(d->m.insert_or_assign(dpos, key, value));
} }
void insert(const QMap<Key, T> &map) void insert(const QMap<Key, T> &map)
@ -1335,10 +1340,15 @@ public:
iterator insert(const_iterator pos, const Key &key, const T &value) iterator insert(const_iterator pos, const Key &key, const T &value)
{ {
auto posDistance = d ? std::distance(d->m.cbegin(), pos.i) : 0; typename Map::const_iterator dpos;
detach(); if (!d || d.isShared()) {
auto detachedPos = std::next(d->m.cbegin(), posDistance); auto posDistance = d ? std::distance(d->m.cbegin(), pos.i) : 0;
return iterator(d->m.insert(detachedPos, {key, value})); detach();
dpos = std::next(d->m.cbegin(), posDistance);
} else {
dpos = pos.i;
}
return iterator(d->m.insert(dpos, {key, value}));
} }
#if QT_DEPRECATED_SINCE(6, 0) #if QT_DEPRECATED_SINCE(6, 0)