constant propagation optimization
Hi every one,
I have a question about gcc optimization, I hope I am writing to the right list
I compiled a simple c++ program using gcc 4.0.1
#include
using namespace std;
intc;
calss Calcul
{
public: void affich()
{
cout << "hello world" << endl;
}
public void inc (int& c)
{
switch (c)
{
case 1 : c=c+1; break;
case 2 : c=c+2; break;
case 3 : c=c+3; break;
default : c=c+0;
}
};
int main()
{
Calcul ca;
c=3;
ca.affich();
ca.inc(c);
cout << "the value of c is" << c << endl;
return 0;
}
I am interested in cpp optimisation (constant propagation)
normally, if I compile it using g++ test.cpp -O3 -fdump-tree-all, the c
variable will be considered as constant (ca.inc(c); will be replaced only by c
= 6; )
This kind of optimization is well done if I declare c just before ca.inc(c)):
compiling the code bellow, the ca.inc(c) is replaced by c=6; (in the test1.dom1
file)
int main()
{
Calcul ca;
ca.affich();
c=3;
ca.inc(c);
cout << "the value of c is" << c << endl;
return 0;
}
Why in the fist code, c is not considered as a constant (in spite that affich()
does not change c)
Thank you very much
Asma
Re : constant propagation optimization
thank you for your answer
If I change affich() code (I put as an example an incrementation of an other
variable named a),
the compiler consider c as a constant (he optimize well and remove all switch
cases in inc function).
So the problem comes from the <<() calls in affich.
Is the compiler lost when he find << calls ? (cause a lot of code is generated
to affich a string )
thank you for your help.
- Message d'origine
De : Nathan Froyd
À : charfi asma
Cc : gcc@gcc.gnu.org
Envoyé le : Jeudi, 5 Mars 2009, 17h33mn 28s
Objet : Re: constant propagation optimization
On Thu, Mar 05, 2009 at 11:39:45AM +, charfi asma wrote:
> intc;
> int main()
>
> {
>
> Calcul ca;
>
> c=3;
>
> ca.affich();
>
> ca.inc(c);
>
> cout << "the value of c is" << c << endl;
>
> return 0;
>
> }
[...]
> int main()
>
> {
>
> Calcul ca;
>
> ca.affich();
>
> c=3;
>
> ca.inc(c);
>
> cout << "the value of c is" << c << endl;
>
> return 0;
>
> }
>
> Why in the fist code, c is not considered as a constant (in spite that
> affich() does not change c)
Because GCC does not currently do the necessary analysis to know that
affich() does not change c; it therefore makes the conservative
assumption that it does.
-Nathan
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