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:
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.
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.
#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.
-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.
g77 -v--if you
include that, send along any additional info you have that you
don't see clearly represented in that output.)
configure command when you installed
the compiler.
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.
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.
-g when you make them. The debugging information
includes source line numbers which are essential for correlating the
output with the input.
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.
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:
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.
#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 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.
Such guesses are usually wrong. Even the maintainer can't guess right about such things without first using the debugger to find the facts.
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.