QStringList: use local storage in removeDuplicates()

If available, use a C++17 std::pmr::unordered_set with a monotonic
buffer resource and a 256-byte stack buffer to avoid the per-element
allocations of QSet.

Results on my machine:

 RESULT : tst_QStringList::removeDuplicates():"empty":
-     0.00014 msecs per iteration (total: 74, iterations: 524288)
+     0.000031 msecs per iteration (total: 66, iterations: 2097152)
 RESULT : tst_QStringList::removeDuplicates():"short-dup-0.00":
-     0.00043 msecs per iteration (total: 57, iterations: 131072)
+     0.00013 msecs per iteration (total: 69, iterations: 524288)
 RESULT : tst_QStringList::removeDuplicates():"short-dup-0.50":
-     0.00049 msecs per iteration (total: 65, iterations: 131072)
+     0.00032 msecs per iteration (total: 85, iterations: 262144)
 RESULT : tst_QStringList::removeDuplicates():"short-dup-0.66":
-     0.00057 msecs per iteration (total: 75, iterations: 131072)
+     0.00039 msecs per iteration (total: 52, iterations: 131072)
 RESULT : tst_QStringList::removeDuplicates():"short-dup-0.75":
-     0.00064 msecs per iteration (total: 85, iterations: 131072)
+     0.00048 msecs per iteration (total: 63, iterations: 131072)
 RESULT : tst_QStringList::removeDuplicates():"long-dup-0.00":
-     0.083 msecs per iteration (total: 85, iterations: 1024)
+     0.039 msecs per iteration (total: 80, iterations: 2048)
 RESULT : tst_QStringList::removeDuplicates():"long-dup-0.50":
-     0.11 msecs per iteration (total: 58, iterations: 512)
+     0.078 msecs per iteration (total: 80, iterations: 1024)
 RESULT : tst_QStringList::removeDuplicates():"long-dup-0.66":
-     0.13 msecs per iteration (total: 70, iterations: 512)
+     0.10 msecs per iteration (total: 53, iterations: 512)
 RESULT : tst_QStringList::removeDuplicates():"long-dup-0.75":
-     0.16 msecs per iteration (total: 86, iterations: 512)
+     0.13 msecs per iteration (total: 69, iterations: 512)

When interpreting the data, take into account that each iteration
contains _also_ a deep copy of the QStringList d/t the detach from
'input'.

The pattern is used elsewhere in Qt, so I've put the class that
implements the seen set into a private header file and used in some
other places I found.

Change-Id: I1f71a82008a16d5a3818f91f290ade21d837805e
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>

1 files changed, 5 insertions, 3 deletions

diff --git a/src/xml/dom/qdom.cpp b/src/xml/dom/qdom.cpp
index bbc877eead..26c1a2122b 100644
--- a/src/xml/dom/qdom.cpp
+++ b/src/xml/dom/qdom.cpp
@@ -60,6 +60,9 @@
 #include <qshareddata.h>
 #include <qdebug.h>
 #include <qxmlstream.h>
+#include <private/qduplicatetracker_p.h>
+
+
 #include <stdio.h>
 
 QT_BEGIN_NAMESPACE
@@ -4081,10 +4084,10 @@ void QDomElementPrivate::save(QTextStream& s, int depth, int indent) const
     }
     s << '<' << qName << nsDecl;
 
-    QSet<QString> outputtedPrefixes;
 
     /* Write out attributes. */
     if (!m_attr->map.isEmpty()) {
+        QDuplicateTracker<QString> outputtedPrefixes;
         QHash<QString, QDomNodePrivate *>::const_iterator it = m_attr->map.constBegin();
         for (; it != m_attr->map.constEnd(); ++it) {
             s << ' ';
@@ -4105,9 +4108,8 @@ void QDomElementPrivate::save(QTextStream& s, int depth, int indent) const
                  * arrive in those situations. */
                 if((!it.value()->ownerNode ||
                    it.value()->ownerNode->prefix != it.value()->prefix) &&
-                   !outputtedPrefixes.contains(it.value()->prefix)) {
+                   !outputtedPrefixes.hasSeen(it.value()->prefix)) {
                     s << " xmlns:" << it.value()->prefix << "=\"" << encodeText(it.value()->namespaceURI, s, true, true) << '\"';
-                    outputtedPrefixes.insert(it.value()->prefix);
                 }
             }
         }