10.3 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 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 3.9 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 10.5 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.