QString: use new assign() in operator=({QByteArray, QChar, char *})

operator=(~) and assign() share similar names but, until now, have not
shared the same functionality. This patch introduces the usage of
QString::assign() within the non-sharing assignment operators to
effectively boost efficiency by reusing the available capacity.

Since we're re-using the capacity we update the test case in places
where they don't hold true anymore.

Since these assignment operators are frequently used in many places,
both within Qt and non-Qt code, this patch comes with benchmarks.

The preview of the benchmark results are compared with this patch and
before this patch. The results show a boost in performance for the
QByteArray and 'const char*' overload. The QLatin1StringView overload
already preserved the capacity and has a better performance than the
assign() alternative, so don't us it there.

(x86_64-little_endian-lp64 shared (dynamic) release build (O3); by
gcc 13.2.1, endeavouros ; 13th Gen Intel(R) Core(TM) i9-13900K

benchmarks executed with -perf -iterations 1000000

  * The last value at the EOL represent the string size.

QString &operator=(const QByteArray &a) (current)
  64.3  cycles/iter; 300  instructions/iter; 17   nsec/iter (5)
  65.8  cycles/iter; 366  instructions/iter; 12   nsec/iter (10)
  62.9  cycles/iter; 301  instructions/iter; 11.5 nsec/iter (20)
  61.3  cycles/iter; 315  instructions/iter; 11.1 nsec/iter (50)
  71.4  cycles/iter; 386  instructions/iter; 13   nsec/iter (100)
  136.9 cycles/iter; 811  instructions/iter; 24.5 nsec/iter (500)
  245.8 cycles/iter; 1394 instructions/iter; 42.5 nsec/iter (1'000)

QString &operator=(const QByteArray &a) (before)
  78   cycles/iter; 399  instructions/iter; 15.3 nsec/iter (5)
  82.3 cycles/iter; 465  instructions/iter; 15   nsec/iter (10)
  76.7 cycles/iter; 400  instructions/iter; 14   nsec/iter (20)
  79.5 cycles/iter; 414  instructions/iter; 14.5 nsec/iter (50)
  91.4 cycles/iter; 485  instructions/iter; 16.7 nsec/iter (100)
  189  cycles/iter; 910  instructions/iter; 34.4 nsec/iter (500)
  320  cycles/iter; 1666 instructions/iter; 56   nsec/iter (1'000)

QString &operator=(const char *ch) (current)
  70  cycles/iter; 317  instructions/iter; 12   nsec/iter (5)
  71  cycles/iter; 383  instructions/iter; 12.3 nsec/iter (10)
  64  cycles/iter; 318  instructions/iter; 11.1 nsec/iter (20)
  69  cycles/iter; 340  instructions/iter; 12   nsec/iter (50)
  77  cycles/iter; 419  instructions/iter; 13.5 nsec/iter (100)
  141 cycles/iter; 899  instructions/iter; 24.4 nsec/iter (500)
  280 cycles/iter; 1518 instructions/iter; 48.4 nsec/iter (1'000)

QString &operator=(const char *ch) (before)
  86.7  cycles/iter; 416  instructions/iter; 15   nsec/iter (5)
  87.8  cycles/iter; 482  instructions/iter; 15.7 nsec/iter (10)
  82.4  cycles/iter; 417  instructions/iter; 14.3 nsec/iter (20)
  90.2  cycles/iter; 443  instructions/iter; 15.6 nsec/iter (50)
  101.4 cycles/iter; 518  instructions/iter; 17.7 nsec/iter (100)
  204.4 cycles/iter; 994  instructions/iter; 36.5 nsec/iter (500)
  337.9 cycles/iter; 1789 instructions/iter; 58.9 nsec/iter (1'000)

 * current implemented as: assign(other)
QString &operator=(QLatin1StringView other) (current)
  47.4 cycles/iter; 237 instructions/iter; 8.2  nsec/iter (5)
  46.2 cycles/iter; 237 instructions/iter; 7.9  nsec/iter (10)
  46.8 cycles/iter; 255 instructions/iter; 8    nsec/iter (20)
  59   cycles/iter; 273 instructions/iter; 10.2 nsec/iter (50)
  55   cycles/iter; 300 instructions/iter; 9.5  nsec/iter (100)
  94.3 cycles/iter; 525 instructions/iter; 16.3 nsec/iter (500)
  166  cycles/iter; 804 instructions/iter; 28.7 nsec/iter (1'000)

QString &operator=(QLatin1StringView other) (before)
  14  cycles/iter; 79  instructions/iter; 2.5  nsec/iter (5)
  14  cycles/iter; 79  instructions/iter; 2.6  nsec/iter (10)
  16  cycles/iter; 97  instructions/iter; 3    nsec/iter (20)
  19  cycles/iter; 115 instructions/iter; 3.5  nsec/iter (50)
  23  cycles/iter; 142 instructions/iter; 4.2  nsec/iter (100)
  91  cycles/iter; 367 instructions/iter; 16.6 nsec/iter (500)
  131 cycles/iter; 646 instructions/iter; 23.4 nsec/iter (1'000)

Task-number: QTBUG-106201
Change-Id: Ie852f6abd1cf16164802acddb048eae5df59758f
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

1 files changed, 14 insertions, 3 deletions

diff --git a/src/corelib/text/qstring.h b/src/corelib/text/qstring.h
index b2344f94f3..cc49eb2716 100644
--- a/src/corelib/text/qstring.h
+++ b/src/corelib/text/qstring.h
@@ -786,10 +786,21 @@ public:
         : QString(fromUtf8(a))
     {}
     QT_ASCII_CAST_WARN inline QString &operator=(const char *ch)
-    { return (*this = fromUtf8(ch)); }
+    {
+        if (!ch) {
+            clear();
+            return *this;
+        }
+        return assign(ch);
+    }
     QT_ASCII_CAST_WARN inline QString &operator=(const QByteArray &a)
-    { return (*this = fromUtf8(a)); }
-
+    {
+        if (a.isNull()) {
+            clear();
+            return *this;
+        }
+        return assign(a);
+    }
     // these are needed, so it compiles with STL support enabled
     QT_ASCII_CAST_WARN inline QString &prepend(const char *s)
     { return prepend(QUtf8StringView(s)); }