From cd0f0803955a04b9d7e3a24d4e40f8405df2bba8 Mon Sep 17 00:00:00 2001
From: Marc Mutz <marc.mutz@qt.io>
Date: Wed, 27 May 2026 16:30:01 +0200
Subject: [PATCH] QXmlStreamReader: shrink the size of AttributeName

When we introduced this class, we stored all the fields that the code
from which this was derived compared, too. But we can do better...

Observe that in the nsProcessing case, the qualifiedName member was
always set to {}. So we can remove it.

Likewise, in the !nsProcessing case, we were storing name() and
qualifiedName(), but 'name' is just the suffix of 'qualifiedName', so
we only need to store 'qualifiedName', not 'name'.

So reduce the number of fields in AttributeName by one, by storing
qualifiedName() in 'name', which reduces the memory requirements of
the QDuplicateTracker by ~30%, and therefore increases the effective
dcache size.

Observe that nsProcessing is constant for the document, certainly for
the attributes of a single element, which this code is concerned about.
In particular, we never mix name() and qualifiedName() when comparing
AttributeName objects.

Picking to all active branches, as this author considers this a
salient part of the fix for QTBUG-147009: we don't want to waste more
resources than necessary in the common case of just a handful of
attributes. The only reason why this is a separate commit is to have
more space to explain the change in the commit message.

Task-number: QTBUG-147009
Pick-to: 6.8 6.5 5.15
Change-Id: Id4169bf645208d2ef0a1a14c028d2e338413fb68
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Magdalena Stojek <magdalena.stojek@qt.io>
(cherry picked from commit 1bbc829c8cffb3adeeaabf31f24e8d00037287e5)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
(cherry picked from commit d106e9efea501939107d32dc8b2d8c027f5e3d37)
---

diff --git a/src/corelib/serialization/qxmlstream.cpp b/src/corelib/serialization/qxmlstream.cpp
index 8bd439c..731e7f8 100644
--- a/src/corelib/serialization/qxmlstream.cpp
+++ b/src/corelib/serialization/qxmlstream.cpp
@@ -1783,28 +1783,25 @@
 struct AttributeName
 {
     QStringView name;
-    QStringView qualifiedName;
     QStringView namespaceUri;
 
     static AttributeName fromXmlAttribute(const QXmlStreamAttribute &a, bool nsProcessing)
     {
         if (nsProcessing)
-            return {a.name(), {}, a.namespaceUri()};
+            return {a.name(), a.namespaceUri()};
         else
-            return {a.name(), a.qualifiedName(), a.namespaceUri()};
+            return {a.qualifiedName(), a.namespaceUri()};
     }
 
     friend bool operator==(const AttributeName &lhs, const AttributeName &rhs) noexcept
     {
         return lhs.name == rhs.name
-            && lhs.qualifiedName == rhs.qualifiedName
             && lhs.namespaceUri == rhs.namespaceUri;
     }
     friend size_t qHash(const AttributeName &key, size_t seed = 0) noexcept
     {
         return qHashMulti(seed,
                           key.name,
-                          key.qualifiedName,
                           key.namespaceUri);
     }
 };