-O option gives different running time!

[aamir]

Hi,

When I compile my program adding -O option, the running time of my
program is much smaller. For example if i compile my program like this

g++ -Wall -g  prog2.cc avltree.cc cells_list.cc hashsep_query.cc
hashsep_point.cc int_list.cc loadavltree.cc -o p2 -lm

its run time is around 12 seconds.

but if i compile it with

g++ -Wall -g -O prog2.cc avltree.cc cells_list.cc hashsep_query.cc
hashsep_point.cc int_list.cc loadavltree.cc -o p2 -lm

its run time is  around 7 seconds.
I am really new at C++ so can't figure it out why is it so different.
Any idea?

Thanks,
Aamir