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, 4 deletions

diff --git a/src/platformsupport/fontdatabases/fontconfig/qfontconfigdatabase.cpp b/src/platformsupport/fontdatabases/fontconfig/qfontconfigdatabase.cpp
index 7af5490963..af49ad6407 100644
--- a/src/platformsupport/fontdatabases/fontconfig/qfontconfigdatabase.cpp
+++ b/src/platformsupport/fontdatabases/fontconfig/qfontconfigdatabase.cpp
@@ -56,6 +56,8 @@
 
 #include <QtGui/qguiapplication.h>
 
+#include <QtCore/private/qduplicatetracker_p.h>
+
 #include <fontconfig/fontconfig.h>
 #if FC_VERSION >= 20402
 #include <fontconfig/fcfreetype.h>
@@ -778,9 +780,9 @@ QStringList QFontconfigDatabase::fallbacksForFamily(const QString &family, QFont
     FcPatternDestroy(pattern);
 
     if (fontSet) {
-        QSet<QString> duplicates;
+        QDuplicateTracker<QString> duplicates;
         duplicates.reserve(fontSet->nfont + 1);
-        duplicates.insert(family.toCaseFolded());
+        (void)duplicates.hasSeen(family.toCaseFolded());
         for (int i = 0; i < fontSet->nfont; i++) {
             FcChar8 *value = nullptr;
             if (FcPatternGetString(fontSet->fonts[i], FC_FAMILY, 0, &value) != FcResultMatch)
@@ -788,9 +790,8 @@ QStringList QFontconfigDatabase::fallbacksForFamily(const QString &family, QFont
             //         capitalize(value);
             const QString familyName = QString::fromUtf8((const char *)value);
             const QString familyNameCF = familyName.toCaseFolded();
-            if (!duplicates.contains(familyNameCF)) {
+            if (!duplicates.hasSeen(familyNameCF)) {
                 fallbackFamilies << familyName;
-                duplicates.insert(familyNameCF);
             }
         }
         FcFontSetDestroy(fontSet);