I’m trying to bring an old code circa 1975 that ran on an IBM system back to life and am battling a pesky seg fault. One thing I noticed is that one of the routine called occasionally has what in the old days would be a one trip do. ie DO I=1,N will sometimes have N=0. The code appears to be designe…

In most cases, you can change the code to something like this: i = 1 do ...loop code here i = i + 1 if ( i > N ) exit enddo

Yeah, but I believe this happens throughout the code so I want to try the compiler option approach first before I start looking at every do loop where this can occur. For the one loop I thought was the problem I did nc = n if (nc == 0) nc = 1 do i=1,nc but that didn’t eliminate my seg fault proble…

ifx has the -f66 option but only in a Linux system. I don’t know whether gfortran --std=legacy and lfortran --std=legacy would do what you want.

Note @JohnCampbell (correctly) does not change the value of N. Changing NC as show in the previous discussion could have unintended side effects and if attempted generically could have other issues like changing an intent(in) variable, etc. Rewriting the loop to test at the bottom as also shown or …

[image] rwmsu: … I presume the loop never gets iterated unless you set some kind of compiler flag… It was unspecified prior to F77, though most compilers implemented one trip minimum. From F77 onwards, it is required to do the zero trip test prior to entering the loop. One also has to look …

[image] urbanjost: Also note if do i=j,k,l is used that do i=-10,-4 or do i= -10,-20,-1 would require you to think about whether min() or max() was appropriate and whether some other value than zero is needed as the fixed value in the function call. In principle yes, but in practice the one-t…

Forgot they could only be a positive ascending sequence, at least in most compilers. Does spag or fpt or … have an option to handle replicating one-trip behavior? If g77 is available on a platform I think it had a -onetrip option I thought -Wzerotrip option on gfortran might at least give a warn…

Indeed, g77 manpage states: -fonetrip Executable iterative “DO” loops are to be executed at least once each time they are reached. ANSI FORTRAN 77 and more recent versions of the Fortran standard specify that the body of an iterative “DO” loop is not executed if the number of iterations calcul…

More debugging revealed that my problem was not the zero trip do problem after all but was an issue with how the loop counter is incremented and what value it has when it exits the loop. The old code I’m working on has a large integer buffer that is used to create an old school array based “linked l…

Compiler support for one-trip do loops

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ivanpribec May 4, 2026, 10:35pm 14

Also mentioned in this post: Poll: refactoring a chunk of legacy code - #12 by mecej4

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Compiler support for one-trip do loops

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