How to Report Bugs
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 basically useless. We respond by asking
for enough details to enable us to investigate. You might as well expedite
matters by sending them to begin with.
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 ‘
To enable someone to investigate the bug, you should include all these things:
Without this, we won’t know whether there is any point in looking for the bug
in the current version of GNU CC.
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; and 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 frequently depend on
every little detail of the function they happen in.
Even if the input file that fails comes from a GNU program, you should still
send the complete test case. Don’t ask the GNU CC maintainers to do the extra
work of obtaining the program in question—they are all overworked as it is. Also,
the problem may depend on what is in the header files on your system; it is
unreliable for the GNU CC maintainers to try the problem with the header files
available to them. By sending CPP output, you can eliminate this source of
uncertainty and save us a certain percentage of wild goose chases.
If we were to try to guess the arguments, we would probably guess wrong and
then we would not encounter the bug.
Be precise about these changes. A description in English is not enough—send a
context
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 C 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 compiling GNU CC 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.
The line numbers in the development sources don’t match those in your sources.
Your line numbers would convey no useful 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 CC 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
What follows 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 de-bugger 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 CC 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 may
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.
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 CC 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 “Sending Patches for GNU CC” on page 276 for guidelines on how to make it
easy for us to understand and install your patches.
The version of GNU CC. You can get this by running it with the ‘
A complete input file that will reproduce the bug. If the bug is in the C
preprocessor, send a source file and any header files that it requires. If the bug
is in the compiler proper (
The command arguments you gave GNU CC or GNU C++ to compile that example and
observe the bug. For example, did you use ‘
The type of machine you are using, and the operating system name and version
number.
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.)
Details of any other deviations from the standard procedure for installing GNU
CC.
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.”
If you send examples of assembler code output from GNU CC or GNU C++, please
use ‘
If you wish to mention something in the GNU CC source, refer to it by context,
not by line number.
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.
A description of the envelope of the bug.
In particular, some people insert conditionals (
A patch for the bug.
A guess about what the bug is or what it depends on. Such guesses are usually
wrong. Even I 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.