On Tuesday 08 April 2014 07:42:46 Thiago Macieira wrote:
> Em ter 08 abr 2014, às 15:49:16, Volker Krause escreveu:
> > API-wise we are quite flexible (can't get any more dirty than what we do
> > now ;) ), I was mainly asking due to this being on a performance-relevant
> > path. The QInternal callback system is obviously more flexible (and thus
> > would be my preferred choice), but also slightly more expensive at
> > runtime. The signal spy approach would be slightly more light-weight, at
> > the cost of thread-safety. So, if from the performance point of view
> > there are no objections to the QInternal callback approach, I'll make
> > sure the Squish team is happy with that too, and propose a patch
> > accordingly.
>
> Performance-wise, I'd prefer empty functions that get overridden at runtime.
> That's the smallest overhead for when they are not present.
According to a quick benchmark (attached), it is clearly more efficient to
have:
if (callback) callback();
where callback is a function pointer than always calling callback even if it
is an empty function.
For this reason, I'd recommend an approach based on function pointer callbacks
such as QInternal::registerCallback
(but that one is using a QGlobalStatic, so there is room for improvement)
--
Olivier
Woboq - Qt services and support - http://woboq.com - http://code.woboq.org
#include <QtCore/QElapsedTimer>
#include <QtCore/QDebug>
extern "C" void noop();
void (*test1_ptr)() = noop;
void (*test2_ptr)() = 0;
static int test1() {
test1_ptr();
return 42;
}
static int test2() {
if (test2_ptr)
test2_ptr();
return 42;
}
static int test3() {
noop();
return 42;
}
int main() {
quint64 r;
QElapsedTimer t;
t.start();
for(quint64 i = 0; i < 500 * 1000 * 1000; ++i)
test3();
r = t.elapsed();
qDebug() << "test3: " << r;
t.start();
for(quint64 i = 0; i < 500 * 1000 * 1000; ++i)
test1();
r = t.elapsed();
qDebug() << "test1: " << r;
t.start();
for(quint64 i = 0; i < 500 * 1000 * 1000; ++i)
test2();
r = t.elapsed();
qDebug() << "test2: " << r;
t.start();
for(quint64 i = 0; i < 500 * 1000 * 1000; ++i)
test3();
r = t.elapsed();
qDebug() << "test3: " << r;
t.start();
for(quint64 i = 0; i < 500 * 1000 * 1000; ++i)
test3();
r = t.elapsed();
qDebug() << "test3: " << r;
t.start();
for(quint64 i = 0; i < 500 * 1000 * 1000; ++i)
test2();
r = t.elapsed();
qDebug() << "test2: " << r;
t.start();
for(quint64 i = 0; i < 500 * 1000 * 1000; ++i)
test1();
r = t.elapsed();
qDebug() << "test1: " << r;
}
extern "C" __attribute__((visibility("default"))) void noop() {};_______________________________________________
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