17.1 Have You Found a Bug?

If you are not sure whether you have found a bug, here are some guidelines:

• If the compiler gets a fatal signal, for any input whatever, that is a compiler bug. Reliable compilers never crash—they just remain obsolete.

• If the compiler produces invalid assembly code, for any input whatever, that is a compiler bug, unless the compiler reports errors (not just warnings) which would ordinarily prevent the assembler from being run.

• If the compiler produces valid assembly code that does not correctly execute the input source code, that is a compiler bug.

  However, you must double-check to make sure, because you might have run into an incompatibility between GNU Fortran and traditional Fortran. These incompatibilities might be considered bugs, but they are inescapable consequences of valuable features.

  Or you might have a program whose behavior is undefined, which happened by chance to give the desired results with another Fortran compiler. It is best to check the relevant Fortran standard thoroughly if it is possible that the program indeed does something undefined.

  After you have localized the error to a single source line, it should be easy to check for these things. If your program is correct and well defined, you have found a compiler bug.

  It might help if, in your submission, you identified the specific language in the relevant Fortran standard that specifies the desired behavior, if it isn't likely to be obvious and agreed-upon by all Fortran users.

• If the compiler produces an error message for valid input, that is a compiler bug.

• If the compiler does not produce an error message for invalid input, that is a compiler bug. However, you should note that your idea of “invalid input” might be someone else's idea of “an extension” or “support for traditional practice”.
• If you are an experienced user of Fortran compilers, your suggestions for improvement of GNU Fortran are welcome in any case.

Many, perhaps most, bug reports against g77 turn out to be bugs in the user's code. While we find such bug reports educational, they sometimes take a considerable amount of time to track down or at least respond to—time we could be spending making g77, not some user's code, better.

Some steps you can take to verify that the bug is not certainly in the code you're compiling with g77:

• Compile your code using the g77 options `-W -Wall -O'. These options enable many useful warning; the -O option enables flow analysis that enables the uninitialized-variable warning.

  If you investigate the warnings and find evidence of possible bugs in your code, fix them first and retry g77.

• Compile your code using the g77 options -finit-local-zero, -fno-automatic, -ffloat-store, and various combinations thereof.

  If your code works with any of these combinations, that is not proof that the bug isn't in g77—a g77 bug exposed by your code might simply be avoided, or have a different, more subtle effect, when different options are used—but it can be a strong indicator that your code is making unwarranted assumptions about the Fortran dialect and/or underlying machine it is being compiled and run on.

  See Overly Convenient Command-Line Options, for information on the -fno-automatic and -finit-local-zero options and how to convert their use into selective changes in your own code.

• Validate your code with ftnchek or a similar code-checking tool. ftnchek can be found at ftp://ftp.netlib.org/fortran or ftp://ftp.dsm.fordham.edu.

  Here are some sample Makefile rules using ftnchek “project” files to do cross-file checking and sfmakedepend (from ftp://ahab.rutgers.edu/pub/perl/sfmakedepend) to maintain dependencies automatically. These assume the use of GNU make.

            # Dummy suffix for ftnchek targets:
            .SUFFIXES: .chek
            .PHONY: chekall
            
            # How to compile .f files (for implicit rule):
            FC = g77
            # Assume `include' directory:
            FFLAGS = -Iinclude -g -O -Wall
            
            # Flags for ftnchek:
            CHEK1 = -array=0 -include=includes -noarray
            CHEK2 = -nonovice -usage=1 -notruncation
            CHEKFLAGS = $(CHEK1) $(CHEK2)
            
            # Run ftnchek with all the .prj files except the one corresponding
            # to the target's root:
            %.chek : %.f ; \
              ftnchek $(filter-out $*.prj,$(PRJS)) $(CHEKFLAGS) \
                -noextern -library $<
            
            # Derive a project file from a source file:
            %.prj : %.f ; \
              ftnchek $(CHEKFLAGS) -noextern -project -library $<
            
            # The list of objects is assumed to be in variable OBJS.
            # Sources corresponding to the objects:
            SRCS = $(OBJS:%.o=%.f)
            # ftnchek project files:
            PRJS = $(OBJS:%.o=%.prj)
            
            # Build the program
            prog: $(OBJS) ; \
              $(FC) -o $ $(OBJS)
            
            chekall: $(PRJS) ; \
              ftnchek $(CHEKFLAGS) $(PRJS)
            
            prjs: $(PRJS)
            
            # For Emacs M-x find-tag:
            TAGS: $(SRCS) ; \
              etags $(SRCS)
            
            # Rebuild dependencies:
            depend: ; \
              sfmakedepend -I $(PLTLIBDIR) -I includes -a prj $(SRCS1)
       

• Try your code out using other Fortran compilers, such as f2c. If it does not work on at least one other compiler (assuming the compiler supports the features the code needs), that is a strong indicator of a bug in the code.

  However, even if your code works on many compilers except g77, that does not mean the bug is in g77. It might mean the bug is in your code, and that g77 simply exposes it more readily than other compilers.