different results with different f77 optimizers

  I am running some molecular dynamics simulations on an SGI 150 MHz
'Challenger' with IRIX 5.2. I compile the Fortran source with f77
-O3 -non_shared;if I try -O3 alone I get a message that the system does
not support -O3 in 'shared' mode, please use non_shared. The program
compiles and runs fine. Recently I recompiled with no optimizer,which I
understand leads to the default '-O1'. Again the program compiled and
ran fine-the same identical program-but now one of the calculated
quantities was radically different. I have checked enough now to verify
that these two different optimizers really do lead to different results.


  I would greatly appreciate any suggestions, Tom Keyes
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