Fix quadratic performance hit in Q(Multi)Map::insert() with hint
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>
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@ -1,7 +1,7 @@
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/****************************************************************************
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**
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** Copyright (C) 2020 Klarälvdalens Datakonsult AB, a KDAB Group company, info@kdab.com, author Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
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** Copyright (C) 2016 The Qt Company Ltd.
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** Copyright (C) 2021 The Qt Company Ltd.
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** Contact: https://www.qt.io/licensing/
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**
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** This file is part of the QtCore module of the Qt Toolkit.
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@ -668,10 +668,15 @@ public:
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iterator insert(const_iterator pos, const Key &key, const T &value)
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{
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// TODO: improve. In case of assignment, why copying first?
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typename Map::const_iterator dpos;
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if (!d || d.isShared()) {
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auto posDistance = d ? std::distance(d->m.cbegin(), pos.i) : 0;
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detach();
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auto detachedPos = std::next(d->m.cbegin(), posDistance);
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return iterator(d->m.insert_or_assign(detachedPos, key, value));
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dpos = std::next(d->m.cbegin(), posDistance);
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} else {
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dpos = pos.i;
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}
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return iterator(d->m.insert_or_assign(dpos, key, value));
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}
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void insert(const QMap<Key, T> &map)
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@ -1335,10 +1340,15 @@ public:
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iterator insert(const_iterator pos, const Key &key, const T &value)
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{
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typename Map::const_iterator dpos;
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if (!d || d.isShared()) {
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auto posDistance = d ? std::distance(d->m.cbegin(), pos.i) : 0;
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detach();
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auto detachedPos = std::next(d->m.cbegin(), posDistance);
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return iterator(d->m.insert(detachedPos, {key, value}));
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dpos = std::next(d->m.cbegin(), posDistance);
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} else {
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dpos = pos.i;
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}
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return iterator(d->m.insert(dpos, {key, value}));
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}
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#if QT_DEPRECATED_SINCE(6, 0)
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