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 <[email protected]> Reviewed-by: Allan Sandfeld Jensen <[email protected]> Reviewed-by: Qt CI Bot <[email protected]> Reviewed-by: Thiago Macieira <[email protected]>
petrmanek · Oct 22, 2021 · a7d1c48 · a7d1c48
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commit a7d1c48
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diff --git a/src/corelib/tools/qvarlengtharray.h b/src/corelib/tools/qvarlengtharray.h
@@ -621,7 +621,8 @@ Q_OUTOFLINE_TEMPLATE auto QVarLengthArray<T, Prealloc>::emplace(const_iterator b
     Q_ASSERT_X(isValidIterator(before), "QVarLengthArray::insert", "The specified const_iterator argument 'before' is invalid");
 
     qsizetype offset = qsizetype(before - ptr);
-    reserve(s + 1);
+    if (s == a)
+        reserve(s * 2);
     if (!QTypeInfo<T>::isRelocatable) {
         T *b = ptr + offset;
         T *i = ptr + s;

diff --git 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);
     }
 }