I use PGI fortran 90 on a 64-bit AMD Linux system.
A program that finds fastest paths through a sparse graph is giving
me weird behavior.
I have two different models, one about twice the size as the other.
With no optimization the larger model takes longer to run, as expected.
With O2 both run faster but the larger model very much so; it is now
MUCH quicker than the smaller model!
I get the same behavior on a different machine, Lahey fortran compiler.
If I knew why this happens I could try to exploit it more often.
-info with -O2 lists only a few very minor optimizations, the same for both