[Development] New assign() Functionality Added to Qt Containers

[Dennis Oberst]

Hello everyone,

I would like to inform you that we've added assign(~) functionality to the following
Qt containers:

    QVarLengthArray, QList, QByteArray, and QString.

It is now incorporated into certain non-sharing operator=(~) implementations.

As a quick heads up: assign() offers a convenient way to recycle allocated
memory, potentially resulting in greater efficiency compared to its
memory-allocating alternatives.

We observed that the efficiency of QString::operator=(~) could be boosted of up
to 30%! Here are some benchmarks to support this statement:


(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)


You can find the patch here: https://codereview.qt-project.org/c/qt/qtbase/+/494280

Best,


________________________
Dennis Oberst
Software Engineer

The Qt Company GmbH
Erich-Thilo-Str. 10
12489 Berlin, Germany
dennis.oberst at qt.io
https://www.qt.io

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Juha Varelius, Jouni Lintunen
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