g77.info: Bug Reporting

How to Report Bugs

   The fundamental principle of reporting bugs usefully is this:
*report all the facts*.  If you are not sure whether to state a fact or
leave it out, state it!

   Often people omit facts because they think they know what causes the
problem and they conclude that some details don't matter.  Thus, you
might assume that the name of the variable you use in an example does
not matter.  Well, probably it doesn't, but one cannot be sure.
Perhaps the bug is a stray memory reference which happens to fetch from
the location where that name is stored in memory; perhaps, if the name
were different, the contents of that location would fool the compiler
into doing the right thing despite the bug.  Play it safe and give a
specific, complete example.  That is the easiest thing for you to do,
and the most helpful.

   Keep in mind that the purpose of a bug report is to enable someone to
fix the bug if it is not known.  It isn't very important what happens if
the bug is already known.  Therefore, always write your bug reports on
the assumption that the bug is not known.

   Sometimes people give a few sketchy facts and ask, "Does this ring a
bell?"  This cannot help us fix a bug, so it is rarely helpful.  We
respond by asking for enough details to enable us to investigate.  You
might as well expedite matters by sending them to begin with.
(Besides, there are enough bells ringing around here as it is.)

   Try to make your bug report self-contained.  If we have to ask you
for more information, it is best if you include all the previous
information in your response, as well as the information that was
missing.

   Please report each bug in a separate message.  This makes it easier
for us to track which bugs have been fixed and to forward your bugs
reports to the appropriate maintainer.

   Do not compress and encode any part of your bug report using programs
such as `uuencode'.  If you do so it will slow down the processing of
your bug.  If you must submit multiple large files, use `shar', which
allows us to read your message without having to run any decompression
programs.

   (As a special exception for GNU Fortran bug-reporting, at least for
now, if you are sending more than a few lines of code, if your
program's source file format contains "interesting" things like
trailing spaces or strange characters, or if you need to include binary
data files, it is acceptable to put all the files together in a `tar'
archive, and, whether you need to do that, it is acceptable to then
compress the single file (`tar' archive or source file) using `gzip'
and encode it via `uuencode'.  Do not use any MIME stuff--the current
maintainer can't decode this.  Using `compress' instead of `gzip' is
acceptable, assuming you have licensed the use of the patented
algorithm in `compress' from Unisys.)

   To enable someone to investigate the bug, you should include all
these things:

   * The version of GNU Fortran.  You can get this by running `g77'
     with the `-v' option.  (Ignore any error messages that might be
     displayed when the linker is run.)

     Without this, we won't know whether there is any point in looking
     for the bug in the current version of GNU Fortran.

   * A complete input file that will reproduce the bug.

     If your source file(s) require preprocessing (for example, their
     names have suffixes like `.F', `.fpp', `.FPP', and `.r'), and the
     bug is in the compiler proper (`f771') or in a subsequent phase of
     processing, run your source file through the C preprocessor by
     doing `g77 -E SOURCEFILE > NEWFILE'.  Then, include the contents
     of NEWFILE in the bug report.  (When you do this, use the same
     preprocessor options--such as `-I', `-D', and `-U'--that you used
     in actual compilation.)

     A single statement is not enough of an example.  In order to
     compile it, it must be embedded in a complete file of compiler
     input.  The bug might depend on the details of how this is done.

     Without a real example one can compile, all anyone can do about
     your bug report is wish you luck.  It would be futile to try to
     guess how to provoke the bug.  For example, bugs in register
     allocation and reloading can depend on every little detail of the
     source and include files that trigger them.

   * Note that you should include with your bug report any files
     included by the source file (via the `#include' or `INCLUDE'
     directive) that you send, and any files they include, and so on.

     It is not necessary to replace the `#include' and `INCLUDE'
     directives with the actual files in the version of the source file
     that you send, but it might make submitting the bug report easier
     in the end.  However, be sure to _reproduce_ the bug using the
     _exact_ version of the source material you submit, to avoid
     wild-goose chases.

   * The command arguments you gave GNU Fortran to compile that example
     and observe the bug.  For example, did you use `-O'?  To guarantee
     you won't omit something important, list all the options.

     If we were to try to guess the arguments, we would probably guess
     wrong and then we would not encounter the bug.

   * The type of machine you are using, and the operating system name
     and version number.  (Much of this information is printed by `g77
     -v'--if you include that, send along any additional info you have
     that you don't see clearly represented in that output.)

   * The operands you gave to the `configure' command when you installed
     the compiler.

   * A complete list of any modifications you have made to the compiler
     source.  (We don't promise to investigate the bug unless it
     happens in an unmodified compiler.  But if you've made
     modifications and don't tell us, then you are sending us on a
     wild-goose chase.)

     Be precise about these changes.  A description in English is not
     enough--send a context diff for them.

     Adding files of your own (such as a machine description for a
     machine we don't support) is a modification of the compiler source.

   * Details of any other deviations from the standard procedure for
     installing GNU Fortran.

   * A description of what behavior you observe that you believe is
     incorrect.  For example, "The compiler gets a fatal signal," or,
     "The assembler instruction at line 208 in the output is incorrect."

     Of course, if the bug is that the compiler gets a fatal signal,
     then one can't miss it.  But if the bug is incorrect output, the
     maintainer might not notice unless it is glaringly wrong.  None of
     us has time to study all the assembler code from a 50-line Fortran
     program just on the chance that one instruction might be wrong.
     We need _you_ to do this part!

     Even if the problem you experience is a fatal signal, you should
     still say so explicitly.  Suppose something strange is going on,
     such as, your copy of the compiler is out of synch, or you have
     encountered a bug in the C library on your system.  (This has
     happened!)  Your copy might crash and the copy here would not.  If
     you said to expect a crash, then when the compiler here fails to
     crash, we would know that the bug was not happening.  If you don't
     say to expect a crash, then we would not know whether the bug was
     happening.  We would not be able to draw any conclusion from our
     observations.

     If the problem is a diagnostic when building GNU Fortran with some
     other compiler, say whether it is a warning or an error.

     Often the observed symptom is incorrect output when your program
     is run.  Sad to say, this is not enough information unless the
     program is short and simple.  None of us has time to study a large
     program to figure out how it would work if compiled correctly,
     much less which line of it was compiled wrong.  So you will have
     to do that.  Tell us which source line it is, and what incorrect
     result happens when that line is executed.  A person who
     understands the program can find this as easily as finding a bug
     in the program itself.

   * If you send examples of assembler code output from GNU Fortran,
     please use `-g' when you make them.  The debugging information
     includes source line numbers which are essential for correlating
     the output with the input.

   * If you wish to mention something in the GNU Fortran source, refer
     to it by context, not by line number.

     The line numbers in the development sources don't match those in
     your sources.  Your line numbers would convey no convenient
     information to the maintainers.

   * Additional information from a debugger might enable someone to
     find a problem on a machine which he does not have available.
     However, you need to think when you collect this information if
     you want it to have any chance of being useful.

     For example, many people send just a backtrace, but that is never
     useful by itself.  A simple backtrace with arguments conveys little
     about GNU Fortran because the compiler is largely data-driven; the
     same functions are called over and over for different RTL insns,
     doing different things depending on the details of the insn.

     Most of the arguments listed in the backtrace are useless because
     they are pointers to RTL list structure.  The numeric values of the
     pointers, which the debugger prints in the backtrace, have no
     significance whatever; all that matters is the contents of the
     objects they point to (and most of the contents are other such
     pointers).

     In addition, most compiler passes consist of one or more loops that
     scan the RTL insn sequence.  The most vital piece of information
     about such a loop--which insn it has reached--is usually in a
     local variable, not in an argument.

     What you need to provide in addition to a backtrace are the values
     of the local variables for several stack frames up.  When a local
     variable or an argument is an RTX, first print its value and then
     use the GDB command `pr' to print the RTL expression that it points
     to.  (If GDB doesn't run on your machine, use your debugger to call
     the function `debug_rtx' with the RTX as an argument.)  In
     general, whenever a variable is a pointer, its value is no use
     without the data it points to.

   Here are some things that are not necessary:

   * A description of the envelope of the bug.

     Often people who encounter a bug spend a lot of time investigating
     which changes to the input file will make the bug go away and which
     changes will not affect it.

     This is often time consuming and not very useful, because the way
     we will find the bug is by running a single example under the
     debugger with breakpoints, not by pure deduction from a series of
     examples.  You might as well save your time for something else.

     Of course, if you can find a simpler example to report _instead_ of
     the original one, that is a convenience.  Errors in the output
     will be easier to spot, running under the debugger will take less
     time, etc.  Most GNU Fortran bugs involve just one function, so
     the most straightforward way to simplify an example is to delete
     all the function definitions except the one where the bug occurs.
     Those earlier in the file may be replaced by external declarations
     if the crucial function depends on them.  (Exception: inline
     functions might affect compilation of functions defined later in
     the file.)

     However, simplification is not vital; if you don't want to do this,
     report the bug anyway and send the entire test case you used.

   * In particular, some people insert conditionals `#ifdef BUG' around
     a statement which, if removed, makes the bug not happen.  These
     are just clutter; we won't pay any attention to them anyway.
     Besides, you should send us preprocessor output, and that can't
     have conditionals.

   * A patch for the bug.

     A patch for the bug is useful if it is a good one.  But don't omit
     the necessary information, such as the test case, on the
     assumption that a patch is all we need.  We might see problems
     with your patch and decide to fix the problem another way, or we
     might not understand it at all.

     Sometimes with a program as complicated as GNU Fortran it is very
     hard to construct an example that will make the program follow a
     certain path through the code.  If you don't send the example, we
     won't be able to construct one, so we won't be able to verify that
     the bug is fixed.

     And if we can't understand what bug you are trying to fix, or why
     your patch should be an improvement, we won't install it.  A test
     case will help us to understand.

     See `http://gcc.gnu.org/contribute.html' for guidelines on how to
     make it easy for us to understand and install your patches.

   * A guess about what the bug is or what it depends on.

     Such guesses are usually wrong.  Even the maintainer can't guess
     right about such things without first using the debugger to find
     the facts.

   * A core dump file.

     We have no way of examining a core dump for your type of machine
     unless we have an identical system--and if we do have one, we
     should be able to reproduce the crash ourselves.