On Wed, 23 Feb 2005 10:36:07 -0800, Benjamin Redelings I
<[EMAIL PROTECTED]> wrote:
> Hi,
> I have a C++ program that runs slower under 4.0 CVS than 3.4. So, I
> am
> trying to make some test-cases that might help deduce the reason.
> However, when I reduced this testcase sufficiently, it began behaving
> badly under BOTH 3.4 and 4.0.... but I guess I should start with the
> most reduced case first.
>
> Basically, the code just does a lot of multiplies and adds. However,
> if I take the main loop outside of an if-block, it goes 5x faster.
> Also, if I implement an array as 'double*' instead of 'vector<double>'
> it also goes 5x faster. Using valarray<double> instead of
> vector<double> does not give any improvement.
I'm sure this is an aliasing problem. The compiler cannot deduce that storing
to result does not affect d. Otherwise the generated code looks reasonable.
What is interesting though is, that removing the if makes the compiler recognize
that result and d do not alias. In fact, the alias analysis seems to
be confused
by the scope of d - moving it outside of the if fixes the problem, too. Maybe
Diego can shed some light on this effect. The testcase looks like
#include <vector>
const int OUTER = 100000;
const int INNER = 1000;
using namespace std;
int main(int argn, char *argv[])
{
int s = atoi(argv[1]);
double result;
{
vector<double> d(INNER); // move outside of this scope to fix
// initialize d
for (int i = 0; i < INNER; i++)
d[i] = double(1+i) / INNER;
// calc result
result=0;
for (int i = 0; i < OUTER; ++i)
for (int j = 1; j < INNER; ++j)
result += d[j]*d[j-1] + d[j-1];
}
printf("result = %f\n",result);
return 0;
}
