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7. What to do when something goes wrong

If you still have a problem after consulting this section, then you may have found a bug---please report it! See Section How to report a bug in the GHC system for a list of things we'd like to know about your bug. If in doubt, send a report---we love mail from irate users :-!

(Section Haskell 98 vs. Glasgow Haskell: language non-compliance, which describes Glasgow Haskell's shortcomings vs. the Haskell language definition, may also be of interest.)

7.1 When the compiler ``does the wrong thing''

``Help! The compiler crashed (or `panic'd)!''

These events are always bugs in the GHC system---please report them.

``The compiler ran out of heap (or stack) when compiling itself!''

It happens. We try to supply reasonable -H<n> flags for ghc/compiler/ and ghc/lib/, but GHC's memory consumption can vary by platform (e.g., on a 64-bit machine).

Just say make all EXTRA_HC_OPTS=-H<a reasonable number> and see how you get along.

Note that this is less likely to happen if you are compiling with GHC 4.00 or later, since the introduction of the dynamically expanding heap.

``The compiler died with a pattern-matching error.''

This is a bug just as surely as a ``panic.'' Please report it.

``This is a terrible error message.''

If you think that GHC could have produced a better error message, please report it as a bug.

``What about these `trace' messages from GHC?''

Almost surely not a problem. About some specific cases...

Simplifier still going after N iterations:

Sad, but harmless. You can change the number with a -fmax-simplifier-iterations<N> option (no space); and you can see what actions took place in each iteration by turning on the -fshow-simplifier-progress option.

If the simplifier definitely seems to be ``looping,'' please report it.

``What about this warning from the C compiler?''

For example: ``...warning: `Foo' declared `static' but never defined.'' Unsightly, but shouldn't be a problem.

Sensitivity to .hi interface files:

GHC is very sensitive about interface files. For example, if it picks up a non-standard Prelude.hi file, pretty terrible things will happen. If you turn on -fno-implicit-prelude , the compiler will almost surely die, unless you know what you are doing.

Furthermore, as sketched below, you may have big problems running programs compiled using unstable interfaces.

``I think GHC is producing incorrect code'':

Unlikely :-) A useful be-more-paranoid option to give to GHC is -dcore-lint ; this causes a ``lint'' pass to check for errors (notably type errors) after each Core-to-Core transformation pass. We run with -dcore-lint on all the time; it costs about 5% in compile time.

``Why did I get a link error?''

If the linker complains about not finding _<something>_fast, then something is inconsistent: you probably didn't compile modules in the proper dependency order.

``What's a `consistency error'?''

(These are reported just after linking your program.)

You tried to link incompatible object files, e.g., normal ones (registerised, Appel garbage-collector) with profiling ones (two-space collector). Or those compiled by a previous version of GHC with an incompatible newer version.

If you run nm -o *.o | egrep 't (cc|hsc)\.' (or, on unregisterised files: what *.o), you'll see all the consistency tags/strings in your object files. They must all be the same! (ToDo: tell you what they mean...)

``Is this line number right?''

On this score, GHC usually does pretty well, especially if you ``allow'' it to be off by one or two. In the case of an instance or class declaration, the line number may only point you to the declaration, not to a specific method.

Please report line-number errors that you find particularly unhelpful.

7.2 When your program ``does the wrong thing''

(For advice about overly slow or memory-hungry Haskell programs, please see Section Advice on: sooner, faster, smaller, stingier).

``Help! My program crashed!''

(e.g., a `segmentation fault' or `core dumped')

If your program has no _ccall_s/_casm_s in it, then a crash is always a BUG in the GHC system, except in one case: If your program is made of several modules, each module must have been compiled after any modules on which it depends (unless you use .hi-boot files, in which case these must be correct with respect to the module source).

For example, if an interface is lying about the type of an imported value then GHC may well generate duff code for the importing module. This applies to pragmas inside interfaces too! If the pragma is lying (e.g., about the ``arity'' of a value), then duff code may result. Furthermore, arities may change even if types do not.

In short, if you compile a module and its interface changes, then all the modules that import that interface must be re-compiled.

A useful option to alert you when interfaces change is -hi-diffs . It will run diff on the changed interface file, before and after, when applicable.

If you are using make, a useful tool to make sure that every module is up-to-date with respect to its imported interfaces is mkdependHS (which comes with GHC). Please see Section Makefile dependencies in Haskell: using mkdependHS.

If you are down to your last-compile-before-a-bug-report, we would recommend that you add a -dcore-lint option (for extra checking) to your compilation options.

So, before you report a bug because of a core dump, you should probably:

% rm *.o        # scrub your object files
% make my_prog  # re-make your program; use -hi-diffs to highlight changes;
                # as mentioned above, use -dcore-lint to be more paranoid
% ./my_prog ... # retry...

Of course, if you have _ccall_s/_casm_s in your program then all bets are off, because you can trash the heap, the stack, or whatever.

If you are interested in hard-core debugging of a crashing GHC-compiled program, please see Section Hard-core debugging of GHC-compiled programs.

``My program entered an `absent' argument.''

This is definitely caused by a bug in GHC. Please report it.

``What's with this `arithmetic (or `floating') exception' ''?

Int, Float, and Double arithmetic is unchecked. Overflows, underflows and loss of precision are either silent or reported as an exception by the operating system (depending on the architecture). Divide-by-zero may cause an untrapped exception (please report it if it does).

7.3 How to report a bug in the GHC system

Glasgow Haskell is a changing system so there are sure to be bugs in it. Please report them to! (However, please check the earlier part of this section to be sure it's not a known not-really-a problem.)

The name of the bug-reporting game is: facts, facts, facts. Don't omit them because ``Oh, they won't be interested...''

  1. What kind of machine are you running on, and exactly what version of the operating system are you using? (uname -a or cat /etc/motd will show the desired information.)
  2. What version of GCC are you using? gcc -v will tell you.
  3. Run the sequence of compiles/runs that caused the offending behaviour, capturing all the input/output in a ``script'' (a UNIX command) or in an Emacs shell window. We'd prefer to see the whole thing.
  4. Be sure any Haskell compilations are run with a -v (verbose) flag, so we can see exactly what was run, what versions of things you have, etc.
  5. What is the program behaviour that is wrong, in your opinion?
  6. If practical, please send enough source files/interface files for us to duplicate the problem.
  7. If you are a Hero and track down the problem in the compilation-system sources, please send us patches relative to a known released version of GHC, or whole files if you prefer.

7.4 Hard-core debugging of GHC-compiled programs

If your program is crashing, you should almost surely file a bug report, as outlined in previous sections.

This section suggests ways to Make Further Progress Anyway.

The first thing to establish is: Is it a garbage-collection (GC) bug? Try your program with a very large heap and a -Sstderr RTS flag.

If it is a GC bug, you may be able to avoid it by using a particular heap size or by using a -F2s runtime flag. (But don't forget to report the bug!!!)

ToDo: more here?

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