main.cpp

// qmake -project QT="core testlib" CONFIG+=release
#include <QtCore>
#include <QtTest>

class tst_Likely : public QObject
{
    Q_OBJECT

private slots:
    void test_normal();
    void test_unlikely();
    void test_likely();
};

#define test_(name, expr) \
void tst_Likely::test_##name() { \
    int sum = 0; \
    QBENCHMARK { \
        for (int i = 0; i < 0x7fffffff; i++) { \
            if (expr) \
                sum++; \
            else \
                sum--; \
        } \
    } \
    if (sum < 0) \
        qDebug() << "unlikely :("; \
    else \
        qDebug() << "likely :)"; \
}

test_(normal, i % 100000 == 0)
test_(unlikely, Q_UNLIKELY(i % 100000 == 0))
test_(likely, Q_LIKELY(i % 100000 == 0))

#undef test_

QTEST_MAIN(tst_Likely)
#include "main.moc"