Here's help in converting to Fortran 90

FORTRAN 77 is a subset of Fortran 90. Thus, if you are using standard FORTRAN 77, you can easily switch to Fortran 90. While there are many new language features in Fortran 90, you are not required to use them.

This article emphasizes the differences between the FORTRAN 77 and Fortran 90 standards and gives some compiling examples to get you started down the road to converting to Fortran 90. To complement this article, the SCD Technical Consulting Office has created a new set of Fortran 90 information on its online ConsultWeb. The URL for ConsultWeb is:

http://www.scd.ucar.edu/tcg/consweb/Top.html

Select the "Fortran 90 Info" link.

The ConsultWeb Fortran 90 help site emphasizes the practical issues of migrating from the Cray CF77 to CF90 compiling systems, including checking your code, compiling and linking, language changes, UNIX environment, performance issues, new features, source management using make, reprints from SCD Computing News of Fortran 90 articles by Jeanne Adams, a Fortran 90 Tutorial by Zane Dodson (University of New Mexico), and a Fortran 90 Frequently Asked Questions (FAQ) list.

Why convert now?

When the Cray programming environment 2.0 was released this spring, the Cray Fortran 90 (CF90) compiler replaced the CF77 compiling system for CRAY PVP systems as the supported Fortran compiler.

Converting to Fortran 90 has become a high priority because SCD has acquired a Cray C90 that replaced the CRAY Y-MP (antero). SCD has also acquired a Cray J90se, ouray. The only Fortran compiler on these machines is the Fortran 90 compiler (CF90). If other new Crays are acquired, they too will have CF90 as the only Fortran compiler.

What will happen to CF77?

Although SCD may still be able to provide the CF77 compiling system on our older systems, such as the CRAY Y-MP (shavano), all new Cray systems come with CF90 as the default compiling system. CF77 should be considered as a transitional compiler only, with emphasis on getting codes running using CF90 as soon as possible. Users planning to run on the C90 (antero) MUST convert their codes to Fortran 90 now.

Conversion to Fortran 90: Issues and answers

Are you using standard Fortran (77 or 90)? Then conversion should be a breeze. FORTRAN 77 users will need to make very few changes to a FORTRAN 77 program to convert it to Fortran 90, provided all the statements used are bona fide FORTRAN 77 statements.

As mentioned above, Fortran 90 is a superset of the FORTRAN 77 language. Some of the difficulty of converting programs comes from statements that are Cray extensions to the FORTRAN 77 standard that are not in the Fortran 90 standard. If the statements are in the FORTRAN 77 standard, there will be very few changes (if any) to run using the Fortran 90 compiler. Of course, there are control file and command line changes that inevitably come with the change to any new compiler. And any new compiler will have optimization features that may be dissimilar to any that have been used in the past. The job of conversion does not require changing DO loops to the Fortran 90 vector notation (which, by the way, can be done using various conversion routines). You may take advantage of the new features at a later time, after your programs are running on the Fortran 90 compiler.

There are different approaches to conversion of a source code depending on what kind of statement is in question. Standard FORTRAN 77 codes should compile without difficulty, with the exception of a few caveats discussed below.

Standard FORTRAN 77 statements: A few caveats

• FORTRAN 77 standard statements will compile, with a few exceptions, on standard Fortran 90. One of the exceptions is with names of your user-supplied functions that are the same as new names in the list of standard Fortran 90 functions. An example might be MATMUL in Fortran 90, which does a matrix multiply on numeric or logical matrices. If you also have a function named MATMUL and want to use your own routine, you must declare your routine EXTERNAL or change its name. Another commonly used name that may cause conflicts is the Fortran 90 intrinsic function SUM.

• The G format edit descriptor produces 0.0E+00 on output, whereas Fortran 90 produces a 0.0.

• Fortran 90 does not blank pad after a list-directed character if the number of characters in the list is less than the number of characters in the record. FORTRAN 77 left this processor dependent. If you depend on this feature, use PAD = NO in the OPEN statement.

• When a real constant is used to initialize a double precision constant, only the precision of the real constant is used in Fortran 90. FORTRAN 77 permitted more precision to be used.

• If data is initialized in a DATA statement, Fortran 90 states that the data has the SAVE attribute; FORTRAN 77 makes this processor dependent.

Cray Fortran statements that extend Fortran 90 and Cray Fortran statements that extend FORTRAN 77, but which are not in Fortran 90:

• There is no double complex type in a Fortran standard.

• Cray NAMELIST is different from the Fortran 90 NAMELIST. Both forms are accepted in the Fortran 90 compiler. If you plan to port your program to another system, use the standard conforming NAMELIST.

• Single-letter abbreviations like .T. for .TRUE. and .A. for .AND., etc. are not in the standard, but are allowed in the Cray CF77 compiler. The Cray CF77 compiler is permissive in these cases.

• The interpretation of the logical intrinsic operators results in a logical value (true or false) in Fortran standards. These are not interpreted as a masking operator in a boolean expression as in Cray Fortran. .XOR. is an additional operator added by Cray.

• There are a number of system library routines in Cray CF77 Fortran that are not in the standard. However, these routines will remain in the Cray Fortran 90 compiler for compatibility.

• Some of the tasking directives in previous Cray CF77 Fortrans have been replaced by EXIT and the CASE construct in standard Fortran 90. The DO GLOBAL has been replaced by the DO ALL statement, which is not in standard Fortran 90. The DO ALL statement is Cray Fortran 90.

• REAL *4, etc. are not standard conforming in Fortran 90; the KIND parameter is used instead in the standard. However, both forms are in Cray Fortran 90.

• Recursion was in Cray FORTRAN 77, but in a slightly different form. In Fortran 90, the word RECURSION must appear on recursive procedures. It was required in the standard because of efficiency considerations for compiler writers, since recursion is generally inefficient and impacts optimization.

• Both Cray pointers and Fortran 90 pointers are in Cray Fortran 90. While these two implementations are different, they do not conflict if the user sticks to one form or the other. Cray pointers are non-standard conforming, however.

• Fortran 90 is more strict with type checking than CF77. Pointers are not interchangeable with the integer type as in CF77. In Fortran 90, pointers are their own type and point to a particular type of data.

• Straight translation of Cray Fortran pointers to Fortran 90 standard pointers may add significant overhead due to creating, copying to, and deallocating temporary arrays.

Fortran 95 draft standard statements

Fortran 95 is still being reviewed and balloted. The final balloting should be completed in early 1997. There are only a few new features in this standard. They include the FORALL statement and construct, PURE and ELEMENTAL functions, a few new intrinsic functions (NULL and CPU_TIME) and pointer default initialization. Many clarifications were made to Fortran 90 in preparing the Fortran 95 draft document.

Fortran 90/95 language architecture: Moving on

How to keep a language from growing indefinitely to a very large size and how to allow old software to run on any new system are two issues that are difficult to resolve. For this reason, a language architecture was developed for Fortran 90 that defined what features were old, new, obsolescent, and not standard anymore (deleted). The list of obsolescent features was derived from those that were little used, and/or redundant with another feature, possibly a new one. The deleted features were chosen each standard revision cycle from the list of current obsolescent features. However, not all of these features are deleted, especially if users complain in a public review cycle. At least one standard revision cycle is required before a feature is removed from the language.

Table 1. DELETED FEATURES

---------------------------------------------------
From Fortran 95    From Fortran 90  From FORTRAN 77  
---------------------------------------------------
Real and double         None        Hollerith Data   
   precision DO                                         
   variables                                            
Branching to an                                      
   ENDIF statement                                      
   from outside the                                     
   IF block                                             
PAUSE statement                                      
ASSIGN, ASSIGNED                                     
   GOTO, and                                            
   assigned FORMAT                                      
H edit descriptor                                    
---------------------------------------------------

Dealing with deleted and obsolete features

All obsolete features are still in the standard.

However, we have some recommendations on dealing with deleted and obsolete features.

Add new Fortran 90 features gradually

You now have been introduced to the basic differences between FORTRAN 77 and Fortran 90 and should be able to begin converting to Fortran 90. If you have a standard conforming FORTRAN 77 program to convert, you should not need to use any of the new features of Fortran 90 initially. These new features can be introduced in the program as it is convenient. The array notation, modules, nested WHERE constructs, and the use of many new intrinsic procedures are among the list of things new in Fortran 90. There are many textbooks and tutorials available to help you begin to use new features. Copies of the Fortran 90 standard are available in the SCD Consulting Office, Room 17 of the Mesa Laboratory.

Below are some examples showing how to invoke the Cray Fortran 90 compiler.

Control file changes and command lines: Using the Fortran 90 compiler on the Cray systems

Detailed information on the Cray Fortran 90 compiler can be found online in the f90 man page, and case-by-case assistance can be obtained from the SCD Technical Consulting Office.
1. Simple compile:

   cf77: cf77 file.f
   

   cf90: f90 file.f
   

   Note: The default level of optimization under cf77 and f90 on Cray systems is -O 2 (moderate optimization). By comparison, no optimization is performed under f77 on a Sparc platform.

2. Rename executable:

   cf77: cf77 -o file.exe file.f
   

   cf90: f90 -o file.exe file.f
   

3. Turn on automatic multitasking:

   cf77: cf77 -Zp file.f
   

   cf90: f90 -O task2 file.f
   

4. Do not interpret user multitasking directives:

   cf77: cf77 file.f
   

   cf90: f90 -O task0 file.f
   

5. Interpret user multitasking directives:

   cf77: cf77 -Zu file.f
   

   cf90: f90 file.f
   

   cf90: f90 -O task1 file.f
   

6. Add libraries:

   cf77: cf77 file.f -L/usr/local/lib -lncarm -lncaro
   

   cf90: f90 file.f -L/usr/local/lib -lncarm -lncaro
   

7. Compile with NCAR Graphics libraries:

   cf77: cf77 file.f -L/usr/local/lib -lncarg -lncarg_gks \
   

   -lncarg_c -lX11
   

   cf90: cf90 file.f -L/usr/local/lib -lncarg -lncarg_gks \
   

   -lncarg_c -lX11
   

8. Compile-only (for use with make):

   cf77: cf77 -c file.f
   

   cf90: f90 -c file.f
   

9. Link-only (for use with make):

   cf77: cf77 file1.o file2.o
   

   cf90: f90 file1.o file2.o
   

10. Compile with static memory:

    cf77: cf77: cf77 -a static file.f
    

    cf90: f90 -e v file.f
    

11. Compile with variables preset to indefinite values:

    cf77: cf77 -e i file.f
    

    cf90: f90 -e i file.f
    

For further information

For further information and assistance in converting codes from CF77 to CF90, call the SCD Consultants at 303-497-1278, drop by the Consulting Office at Mesa Lab Room 17, or send e-mail to consult1@ncar.ucar.edu.