When you have a Fortran only program, moving to 64-bits should be straight forward. First, are you sure your program is crashing or is it just exiting? A ‘Fortran Stop’ message just means that you program exited at a ‘stop’ statement.
The Floating point overflow warning is a bit of a concern. This means that your using a constant real number that is too big for use in a specific context. For example, if your assigning a number bigger than 2Gb to a variable declared REAL*4. Since constants have the default kind of 4, it can even happen if you use a very large constant without changing its kind (i.e. append with an ‘_8’). Have you modified your program to use larger constants or are you compiling with “-r8”? The warning should print the line number, can you post this line?
What I’d do, is take a step back and compile your program in 64-bits with “-g”. If it still stops after you enter the input file, run again using pgdbg (the PGI debugger), and step through the code to better understand why it’s stopping. Once it works at “-g”, begin adding optimizations. Start with “-fastsse” and then compile with “-fastsse -Mconcur”, “-fastsse -Mconcur=innermost”, and “-fastsse -Mconcur=noaltcode”. Be sure to note the run times of each of the 4 flag sets to determine which option is best for your code.
Hope this helps,