Difference in output when f77 code compiled with ifort & gfortran

Question

I need some pointers to solve a problem that I can describe only in a limited way. I got a code written in f77 from a senior scientist. I can't give the code on a public forum for ownership issues. It's not big (750 lines) but given implicit declarations and gotos statements, it is very unreadable. Hence I am having trouble finding out the source of error. Here is the problem:

When I compile the code with ifort, it runs fine and gives me sensible numbers but when I compile it with gfortran, it compiles fine but does not give me the right answer. The code is a numerical root finder for a complex plasma physics problem. The ifort compiled version finds the root but the gfortran compiled version fails to find the root.

Any ideas on how to proceed looking for a solution? I will update the question to reflect the actual problem when I find one.

Answer 1

Some things to investigate, not necessarily in the order I would try them:

1. Use your compiler(s) to check everything that your compiler(s) are capable of checking including and especially array-bounds (for run-time confidence) and subroutine argument matching.
2. Use of uninitialised variables.
3. The kinds of real, complex and integer variables; the compilers (or your compilation options) may default to different kinds.
4. Common blocks, equivalences, entry, ... other now deprecated or obsolete features.

Finally, perhaps not a matter for immediate investigation but something you ought to do sooner (right choice) or later (wrong choice), make the effort to declare IMPLICIT NONE in all scopes and to write explicit declarations for all entities.