Using and Porting the GNU Compiler Collection (GCC): Bugs

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# 8. Reporting Bugs

Your bug reports play an essential role in making GCC reliable.

When you encounter a problem, the first thing to do is to see if it is already known. See section 7. Known Causes of Trouble with GCC. If it isn't known, then you should report the problem.

Reporting a bug may help you by bringing a solution to your problem, or it may not. (If it does not, look in the service directory; see 9. How To Get Help with GCC.) In any case, the principal function of a bug report is to help the entire community by making the next version of GCC work better. Bug reports are your contribution to the maintenance of GCC.

Since the maintainers are very overloaded, we cannot respond to every bug report. However, if the bug has not been fixed, we are likely to send you a patch and ask you to tell us whether it works.

In order for a bug report to serve its purpose, you must include the information that makes for fixing the bug.

> 8.1 Have You Found a Bug? |  | Have you really found a bug?
> 8.2 Where to Report Bugs |  | Where to send your bug report.
> 8.3 How to Report Bugs |  | How to report a bug effectively.
> 8.4 Sending Patches for GCC |  | How to send a patch for GCC.
> 7. Known Causes of Trouble with GCC |  | Known problems.
> 9. How To Get Help with GCC |  | Where to ask for help.

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## 8.1 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.

- If the compiler produces invalid assembly code, for any input whatever (except an `asm` statement), 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 may have run into an incompatibility between GNU C and traditional C (see section 7.6 Incompatibilities of GCC). These incompatibilities might be considered bugs, but they are inescapable consequences of valuable features.

  Or you may have a program whose behavior is undefined, which happened by chance to give the desired results with another C or C++ compiler.

  For example, in many nonoptimizing compilers, you can write ``x;'` at the end of a function instead of ``return x;'`, with the same results. But the value of the function is undefined if `return` is omitted; it is not a bug when GCC produces different results.

  Problems often result from expressions with two increment operators, as in `f (*p++, *p++)`. Your previous compiler might have interpreted that expression the way you intended; GCC might interpret it another way. Neither compiler is wrong. The bug is in your code.

  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.

- 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 my idea of "an extension" or "support for traditional practice".

- If you are an experienced user of C or C++ (or Fortran or Objective-C) compilers, your suggestions for improvement of GCC are welcome in any case.

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## 8.2 Where to Report Bugs

Send bug reports for the GNU Compiler Collection to ``gcc-bugs@gcc.gnu.org'`. In accordance with the GNU-wide convention, in which bug reports for tool "foo" are sent to ``bug-foo@gnu.org'`, the address ``bug-gcc@gnu.org'` may also be used; it will forward to the address given above.

Please read ``<URL:http://www.gnu.org/software/gcc/bugs.html>'` for bug reporting instructions before you post a bug report.

Often people think of posting bug reports to the newsgroup instead of mailing them. This appears to work, but it has one problem which can be crucial: a newsgroup posting does not contain a mail path back to the sender. Thus, if maintainers need more information, they may be unable to reach you. For this reason, you should always send bug reports by mail to the proper mailing list.

As a last resort, send bug reports on paper to:

    GNU Compiler Bugs
    Free Software Foundation
    59 Temple Place - Suite 330
    Boston, MA 02111-1307, USA

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## 8.3 How to Report Bugs

You may find additional and/or more up-to-date instructions at ``<URL:http://www.gnu.org/software/gcc/bugs.html>'`.

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 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.

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

- The version of GCC. You can get this by running it with the ``-v'` option.

  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 (see section 2.7 Options for Debugging Your Program or GCC). 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 generated.

  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 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 GCC.

- 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 observations.

  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 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 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 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 cpp 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 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 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 section 8.4 Sending Patches for GCC, 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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## 8.4 Sending Patches for GCC

If you would like to write bug fixes or improvements for the GNU C compiler, that is very helpful. Send suggested fixes to the patches mailing list, `gcc-patches@gcc.gnu.org`.

Please follow these guidelines so we can study your patches efficiently. If you don't follow these guidelines, your information might still be useful, but using it will take extra work. Maintaining GNU C is a lot of work in the best of circumstances, and we can't keep up unless you do your best to help.

- Send an explanation with your changes of what problem they fix or what improvement they bring about. For a bug fix, just include a copy of the bug report, and explain why the change fixes the bug.

  (Referring to a bug report is not as good as including it, because then we will have to look it up, and we have probably already deleted it if we've already fixed the bug.)

- Always include a proper bug report for the problem you think you have fixed. We need to convince ourselves that the change is right before installing it. Even if it is right, we might have trouble judging it if we don't have a way to reproduce the problem.

- Include all the comments that are appropriate to help people reading the source in the future understand why this change was needed.

- Don't mix together changes made for different reasons. Send them individually.

  If you make two changes for separate reasons, then we might not want to install them both. We might want to install just one. If you send them all jumbled together in a single set of diffs, we have to do extra work to disentangle them--to figure out which parts of the change serve which purpose. If we don't have time for this, we might have to ignore your changes entirely.

  If you send each change as soon as you have written it, with its own explanation, then the two changes never get tangled up, and we can consider each one properly without any extra work to disentangle them.

  Ideally, each change you send should be impossible to subdivide into parts that we might want to consider separately, because each of its parts gets its motivation from the other parts.

- Send each change as soon as that change is finished. Sometimes people think they are helping us by accumulating many changes to send them all together. As explained above, this is absolutely the worst thing you could do.

  Since you should send each change separately, you might as well send it right away. That gives us the option of installing it immediately if it is important.

- Use ``diff -c'` to make your diffs. Diffs without context are hard for us to install reliably. More than that, they make it hard for us to study the diffs to decide whether we want to install them. Unidiff format is better than contextless diffs, but not as easy to read as ``-c'` format.

  If you have GNU diff, use ``diff -cp'`, which shows the name of the function that each change occurs in.

- Write the change log entries for your changes. We get lots of changes, and we don't have time to do all the change log writing ourselves.

  Read the ``ChangeLog'` file to see what sorts of information to put in, and to learn the style that we use. The purpose of the change log is to show people where to find what was changed. So you need to be specific about what functions you changed; in large functions, it's often helpful to indicate where within the function the change was.

  On the other hand, once you have shown people where to find the change, you need not explain its purpose. Thus, if you add a new function, all you need to say about it is that it is new. If you feel that the purpose needs explaining, it probably does--but the explanation will be much more useful if you put it in comments in the code.

  If you would like your name to appear in the header line for who made the change, send us the header line.

- When you write the fix, keep in mind that we can't install a change that would break other systems.

  People often suggest fixing a problem by changing machine-independent files such as ``toplev.c'` to do something special that a particular system needs. Sometimes it is totally obvious that such changes would break GCC for almost all users. We can't possibly make a change like that. At best it might tell us how to write another patch that would solve the problem acceptably.

  Sometimes people send fixes that might be an improvement in general--but it is hard to be sure of this. It's hard to install such changes because we have to study them very carefully. Of course, a good explanation of the reasoning by which you concluded the change was correct can help convince us.

  The safest changes are changes to the configuration files for a particular machine. These are safe because they can't create new bugs on other machines.

  Please help us keep up with the workload by designing the patch in a form that is good to install.

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