On Sun, 23 May 2010, me wrote:
if I compile with "g++ speed_test.cpp"
I get some ticks out of the following
#include "iostream"
#include <time.h>
using namespace std;
int main(){
float x = 1;
float y = 1.000001;
clock_t ticks_at_start = clock();
for(long i=0;i<100000000;i++){
x = x * y;
}
clock_t ticks_taken = clock() - ticks_at_start;
cout << "took " << ticks_taken << " ticks" << endl;
//ticks_taken was 640,000 & macro "CLOCKS_PER_SEC"=1000000
//so g++ (without -O2 & -03) doing this in .64 seconds
return 0;
}
Whereas, if I say "g++ -O2 speed_test.cpp"..
I don't ??
Any advice much appreciated
You mean it becomes instantaneous with -O2? That's normal. The compiler
detects that the loop only modifies x, which is unused afterwards, so the
loop is useless and the compiler removes it. Try printing x after the
loop.
