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, 4 insertions, 6 deletions

diff --git a/src/corelib/text/qstringlist.cpp b/src/corelib/text/qstringlist.cpp
index 4bbe424ed2..4b9dcee169 100644
--- a/src/corelib/text/qstringlist.cpp
+++ b/src/corelib/text/qstringlist.cpp
@@ -43,9 +43,9 @@
 #if QT_CONFIG(regularexpression)
 #  include <qregularexpression.h>
 #endif
+#include <private/qduplicatetracker_p.h>
 
 #include <algorithm>
-
 QT_BEGIN_NAMESPACE
 
 /*! \typedef QStringListIterator
@@ -885,15 +885,13 @@ int QtPrivate::QStringList_removeDuplicates(QStringList *that)
 {
     int n = that->size();
     int j = 0;
-    QSet<QString> seen;
+
+    QDuplicateTracker<QString> seen;
     seen.reserve(n);
-    int setSize = 0;
     for (int i = 0; i < n; ++i) {
         const QString &s = that->at(i);
-        seen.insert(s);
-        if (setSize == seen.size()) // unchanged size => was already seen
+        if (seen.hasSeen(s))
             continue;
-        ++setSize;
         if (j != i)
             that->swapItemsAt(i, j);
         ++j;