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Bug
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Resolution: Done
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P2: Important
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5.0.1
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None
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Windows 8, 32-bit
MinGW build of Qt 5.0.1 (from qt-project.org) much worse performance when handling QByteArrays and I/O (and probably everything in QtCore) then Qt 4.8.4. In my application that loads a text-based data file line-by-line, splits it in fields, converts them to other data types and saves everything to a file slowdown is 2-3 times depending on what kind of processing is done. I'm attaching a simple benchmark that demonstrates the problem. On my system I have the following results:
********* Start testing of Qt5PerfTest *********
Config: Using QTest library 4.8.4, Qt 4.8.4
PASS : Qt5PerfTest::initTestCase()
RESULT : Qt5PerfTest::testCase1():
0.0062 msecs per iteration (total: 51, iterations: 8192)
PASS : Qt5PerfTest::testCase1()
PASS : Qt5PerfTest::cleanupTestCase()
Totals: 3 passed, 0 failed, 0 skipped
********* Finished testing of Qt5PerfTest *********
********* Start testing of Qt5PerfTest *********
Config: Using QTest library 5.0.1, Qt 5.0.1
PASS : Qt5PerfTest::initTestCase()
PASS : Qt5PerfTest::testCase1()
RESULT : Qt5PerfTest::testCase1():
0.018 msecs per iteration (total: 74, iterations: 4096)
PASS : Qt5PerfTest::cleanupTestCase()
Totals: 3 passed, 0 failed, 0 skipped
********* Finished testing of Qt5PerfTest *********
That's 2.9 times slowdown. I guess it is related to use of SJLJ exceptions with the new toolchain. I cannot reproduce the slowdown in Linux. Using CONFIG+=exceptions_off also does not significantly improve performance.
It would be good to have MinGW build without such large performance regression.