QVarLengthArray: Reduce memory allocations in emplace()

Currently, we allocate memory for elements one by one which can get
pretty slow when adding many elements.

[ChangeLog][QtCore][QVarLengthArray] Reduced number of memory
allocations in emplace() by allocating more memory at once.

Fixes: QTBUG-97489
Pick-to: 6.2
Change-Id: Idfb5b5946b047d5215c8ed00770574249f9f5d40
Reviewed-by: Andrei Golubev <andrei.golubev@qt.io>
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

1 files changed, 27 insertions, 0 deletions

diff --git a/tests/auto/corelib/tools/qvarlengtharray/tst_qvarlengtharray.cpp b/tests/auto/corelib/tools/qvarlengtharray/tst_qvarlengtharray.cpp
index 6e95577655..1d30ca3d03 100644
--- a/tests/auto/corelib/tools/qvarlengtharray/tst_qvarlengtharray.cpp
+++ b/tests/auto/corelib/tools/qvarlengtharray/tst_qvarlengtharray.cpp
@@ -180,6 +180,33 @@ void tst_QVarLengthArray::emplace()
         QCOMPARE(&r, &strings.back());
         QCOMPARE(strings.size(), 3);
         QCOMPARE(strings.back(), QString(42, u'y'));
+
+        // test growing from empty arrays
+        QVarLengthArray<QString> emptyArrDefaultPrealloc;
+        QCOMPARE(emptyArrDefaultPrealloc.size(), 0);
+        emptyArrDefaultPrealloc.emplace_back();
+        QCOMPARE(emptyArrDefaultPrealloc.size(), 1);
+        emptyArrDefaultPrealloc.resize(1024);
+        QCOMPARE(emptyArrDefaultPrealloc.size(), 1024);
+        emptyArrDefaultPrealloc.resize(0);
+        QCOMPARE(emptyArrDefaultPrealloc.size(), 0);
+        emptyArrDefaultPrealloc.squeeze();
+        QCOMPARE(emptyArrDefaultPrealloc.size(), 0);
+        emptyArrDefaultPrealloc.emplace_back();
+        QCOMPARE(emptyArrDefaultPrealloc.size(), 1);
+
+        QVarLengthArray<QString, 1> emptyArrSmallPrealloc;
+        QCOMPARE(emptyArrSmallPrealloc.size(), 0);
+        emptyArrSmallPrealloc.emplace_back();
+        QCOMPARE(emptyArrSmallPrealloc.size(), 1);
+        emptyArrSmallPrealloc.resize(1024);
+        QCOMPARE(emptyArrSmallPrealloc.size(), 1024);
+        emptyArrSmallPrealloc.resize(0);
+        QCOMPARE(emptyArrSmallPrealloc.size(), 0);
+        emptyArrSmallPrealloc.squeeze();
+        QCOMPARE(emptyArrSmallPrealloc.size(), 0);
+        emptyArrSmallPrealloc.emplace_back();
+        QCOMPARE(emptyArrSmallPrealloc.size(), 1);
     }
 }