QtCreator with MinGW : How to make compiler optimization - optimization

Question

I am using QtCreator on windows, and I would like to know how to make my compiler optimize the output.
My understanding is that MinGW is a port of GCC. So, I should be able to use arguments such as -O2. However, in the "Projects" bar, the only things I see are :
Build step for qmake (probably not here, qmake is about the .pro files / MOC / Qt stuff ...)
Build step for mingw32-make (probably)
Clean steps (probably not)
So, I tried to add -O2 in the "Make arguments" box, but unfortunately, I get an error "invalid option --O"
For anyone interested, I was trying to make an implementation of the Ackermann function because I read that :
The Ackermann function, due to its definition in terms of extremely
deep recursion, can be used as a benchmark of a compiler's ability to
optimize recursion
The code (which doesn't really use Qt) :
#include <QtCore/QCoreApplication>
#include <QDebug>
#include <ctime>
using namespace std;
int nbRecursion;
int nbRecursions9;
int Ackermann(int m, int n){
nbRecursion++;
if(nbRecursion % 1000000 == 0){
qDebug() << nbRecursions9 << nbRecursion;
}
if(nbRecursion == 1000000000){
nbRecursion = 0;
nbRecursions9++;
}
if(m==0){
return n+1;
}
if(m>0 && n>0){
return Ackermann(m-1,Ackermann(m, n-1));
}
if(m>0 && n==0){
return Ackermann(m-1,1);
}
qDebug() << "Bug at " << m << ", " << n;
}
int main(int argc, char *argv[])
{
QCoreApplication a(argc, argv);
nbRecursion = 0;
nbRecursions9 = 0;
int m = 3;
int n = 13;
clock_t begin = clock();
Ackermann(m,n);
clock_t end = clock();
double elapsed_secs = double(end - begin) / CLOCKS_PER_SEC;
qDebug() << "There are " << CLOCKS_PER_SEC << " CLOCKS_PER_SEC";
qDebug() << "There were " << nbRecursions9 << nbRecursion << " recursions in " << elapsed_secs << " seconds";
double timeX = 1000000000.0*((elapsed_secs)/(double)nbRecursion);
if(nbRecursions9>0){
timeX += elapsed_secs/(double)nbRecursions9;
}
qDebug() << "Time for a function call : " << timeX << " nanoseconds";
return a.exec();
}

Answer 1

-O2 is used by default when you do a release build. Only debug builds don't use optimization. Regardless, if you want to use specific compiler options, you do so in the project file (*.pro) itself, by appending your options to the QMAKE_CFLAGS_RELEASE (for C files) and QMAKE_CXXFLAGS_RELEASE (for C++ files). For example:
QMAKE_CFLAGS_RELEASE += -O3 -march=i686 -mtune=generic -fomit-frame-pointer
QMAKE_CXXFLAGS_RELEASE += -O3 -march=i686 -mtune=generic -fomit-frame-pointer
If you really want to use some specific options always, regardless of whether it's a debug or release build, then append to QMAKE_CFLAGS and QMAKE_CXXFLAGS instead. But usually, you'll only want optimization options in your release builds, not the debug ones.