From a7d1c48ca3162a805708d059da87ca3d742863f1 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Robert=20L=C3=B6hning?= <robert.loehning@qt.io>
Date: Mon, 18 Oct 2021 15:00:09 +0200
Subject: [PATCH] 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>
---
 src/corelib/tools/qvarlengtharray.h           |  3 ++-
 .../qvarlengtharray/tst_qvarlengtharray.cpp   | 27 +++++++++++++++++++
 2 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/src/corelib/tools/qvarlengtharray.h b/src/corelib/tools/qvarlengtharray.h
index 90be3b32996..6ecc22213f4 100644
--- 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
index 6e955776558..1d30ca3d035 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);
     }
 }