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How to Report Bugs
You may find additional and/or more up-to-date instructions at
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 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
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.
To enable someone to investigate the bug, you should include all
* The version of GCC. You can get this by running it with the `-v'
Without this, we won't know whether there is any point in looking
for the bug in the current version of GCC.
* 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 (`cc1'),
send the preprocessor output generated by adding `-save-temps' to
the compilation command ( Debugging Options.). When you do
this, use the same `-I', `-D' or `-U' options that you used in
actual compilation. Then send the INPUT.i or INPUT.ii files
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 GCC
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 GCC 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.
* The command arguments you gave GCC 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.
* The operands you gave to the `configure' command when you installed
* 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
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
* 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 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
If the problem is a diagnostic when compiling GCC 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 GCC, 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 GCC 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 useful
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 GCC 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
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 GCC 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
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 cpp output, and that can't have
* 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 GCC 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
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.
Sending Patches, 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 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.
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