From 3a48320fd6335d21451f07cb68336404177e2d87 Mon Sep 17 00:00:00 2001
From: Lars Knoll <lars.knoll@nokia.com>
Date: Fri, 7 Sep 2012 10:02:04 +0200
Subject: Fix performance regression when appending one vector to another

When appending a QVector to an existing vector the code would
unconditionally realloc the vector instead of first checking
whether we can do without. This gives a quadratic behaviour when
repeatedly appending a vector to another.

Change-Id: I2cd81253e6a8aec0bc4402e6fbda262435080966
Reviewed-by: Qt Doc Bot <qt_docbot@qt-project.org>
Reviewed-by: Simon Hausmann <simon.hausmann@nokia.com>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
---
 src/corelib/tools/qvector.h | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

(limited to 'src/corelib/tools/qvector.h')

diff --git a/src/corelib/tools/qvector.h b/src/corelib/tools/qvector.h
index f3ef9b97c3..e52fb9a1be 100644
--- a/src/corelib/tools/qvector.h
+++ b/src/corelib/tools/qvector.h
@@ -650,8 +650,12 @@ QVector<T> &QVector<T>::fill(const T &from, int asize)
 template <typename T>
 QVector<T> &QVector<T>::operator+=(const QVector &l)
 {
-    int newSize = d->size + l.d->size;
-    reallocData(d->size, newSize);
+    uint newSize = d->size + l.d->size;
+    const bool isTooSmall = newSize > d->alloc;
+    if (!isDetached() || isTooSmall) {
+        QArrayData::AllocationOptions opt(isTooSmall ? QArrayData::Grow : QArrayData::Default);
+        reallocData(d->size, isTooSmall ? newSize : d->alloc, opt);
+    }
 
     if (d->alloc) {
         T *w = d->begin() + newSize;
-- 
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