-If you're working with LLVM and run into a bug, we definitely want to know about
-it. This document describes what you can do to increase the odds of getting it
-fixed quickly.
+
-Basically you have to do two things at a minimum. First, decide whether the bug
-crashes the compiler (or an LLVM pass), or if the
+
If you're working with LLVM and run into a bug, we definitely want to know
+about it. This document describes what you can do to increase the odds of
+getting it fixed quickly.
+
+
Basically you have to do two things at a minimum. First, decide whether the
+bug crashes the compiler (or an LLVM pass), or if the
compiler is miscompiling the program. Based on
what type of bug it is, follow the instructions in the linked section to narrow
down the bug so that the person who fixes it will be able to find the problem
-more easily.
+more easily.
-Once you have a reduced test-case, email information about the bug to: llvmbugs@cs.uiuc.edu. This should
-include all of the information necessary to reproduce the problem, including
-where you got the LLVM tree from (if you're not working out of CVS).
+
Once you have a reduced test-case, go to the LLVM Bug Tracking
+System, select the category in which the bug falls, and fill out the form
+with the necessary details. The bug description should contain the following
+information:
-Thanks for helping us make LLVM better!
+
+
All information necessary to reproduce the problem.
+
The reduced test-case that triggers the bug.
+
The location where you obtained LLVM (if not from our CVS
+ repository).
-More often than not, bugs in the compiler cause it to crash - often due to an
+
+
+
More often than not, bugs in the compiler cause it to crash - often due to an
assertion failure of some sort. If you are running opt or
analyze directly, and something crashes, jump to the section on
bugs in LLVM passes. Otherwise, the most important
piece of the puzzle is to figure out if it is the GCC-based front-end that is
-buggy or if it's one of the LLVM tools that has problems.
+buggy or if it's one of the LLVM tools that has problems.
-To figure out which program is crashing (the front-end, gccas,
-or gccld), run the llvm-gcc command line as you
-were when the crash occurred, but add a -v option to the command line.
-The compiler will print out a bunch of stuff, and should end with telling you
-that one of cc1, gccas, or gccld
-crashed.
+
To figure out which program is crashing (the front-end,
+gccas, or gccld), run the
+llvm-gcc command line as you were when the crash occurred, but
+add a -v option to the command line. The compiler will print out a
+bunch of stuff, and should end with telling you that one of
+cc1/cc1plus, gccas, or
+gccld crashed.
-
If cc1 crashed, you found a problem with the front-end.
-Jump ahead to the section on front-end bugs.
-
-If the problem is in the front-end, pretty much the only thing you can do is
-preprocess the input (compile with the -E option) and send us the
-results. There is no good way to reduce source-level test-cases that I know
-of... if you do know, send me information and we can extend this section. :)
+
If the problem is in the front-end, you should re-run the same
+llvm-gcc command that resulted in the crash, but add the
+-save-temps option. The compiler will crash again, but it will leave
+behind a foo.i file (containing preprocessed C source code) and
+possibly foo.s (containing LLVM assembly code), for each
+compiled foo.c file. Send us the foo.i file,
+along with a brief description of the error it caused.
If you find that a bug crashes in the gccas stage of
+compilation, compile your test-case to a .s file with the
+-save-temps option to llvm-gcc. Then run:
-... which will print a list of arguments, indicating the list of passes that
+
... which will print a list of arguments, indicating the list of passes that
gccas runs. Once you have the input file and the list of
passes, go to the section on debugging bugs in LLVM
-passes.
-If you find that a bug crashes in the gccld stage of
+
If you find that a bug crashes in the gccld stage of
compilation, gather all of the .o bytecode files and libraries that are
being linked together (the "llvm-gcc -v" output should include
-the full list of objects linked). Then run:
-... which will print a list of arguments, indicating the list of passes that
+
... which will print a list of arguments, indicating the list of passes that
gccld runs. Once you have the input files and the list of
passes, go to the section on debugging bugs in LLVM
-passes.
-At this point, you should have some number of LLVM assembly files or bytecode
+
At this point, you should have some number of LLVM assembly files or bytecode
files and a list of passes which crash when run on the specified input. In
order to reduce the list of passes (which is probably large) and the input to
-something tractable, use the bugpoint tool as follows:
+something tractable, use the bugpoint tool as follows:
-
- bugpoint <input files> <list of passes>
-
-
-bugpoint will print a bunch of output as it reduces the
-test-case, but it should eventually print something like this:
+
+
bugpoint <input files> <list of passes>
+
-
- ...
- Emitted bytecode to 'bugpoint-reduced-simplified.bc'
+
bugpoint will print a bunch of output as it reduces the
+test-case, but it should eventually print something like this:
- *** You can reproduce the problem with: opt bugpoint-reduced-simplified.bc -licm
-
+
+
+...
+Emitted bytecode to 'bugpoint-reduced-simplified.bc'
+
+*** You can reproduce the problem with: opt bugpoint-reduced-simplified.bc -licm
+
+
-Once you complete this, please send the LLVM bytecode file and the command line
-to reproduce the problem to the llvmbugs mailing list.
+
Once you complete this, please send the LLVM bytecode file and the command
+line to reproduce the problem to the llvmbugs mailing list.
-Fortunately we haven't had to many miscompilations. Because of this, this
-section is a TODO. Basically, use bugpoint to track down the problem.
+
+
A miscompilation occurs when a pass does not correctly transform a program,
+thus producing errors that are only noticed during execution. This is different
+from producing invalid LLVM code (i.e., code not in SSA form, using values
+before defining them, etc.) which the verifier will check for after a pass
+finishes its run.
+
+
If it looks like the LLVM compiler is miscompiling a program, the very first
+thing to check is to make sure it is not using undefined behavior. In
+particular, check to see if the program valgrinds clean, passes purify, or some
+other memory checker tool. Many of the "LLVM bugs" that we have chased down
+ended up being bugs in the program being compiled, not LLVM.
+
+
Once you determine that the program itself is not buggy, you should choose
+which code generator you wish to compile the program with (e.g. C backend, the
+JIT, or LLC) and optionally a series of LLVM passes to run. For example:
bugpoint will try to narrow down your list of passes to the one pass
+that causes an error, and simplify the bytecode file as much as it can to assist
+you. It will print a message letting you know how to reproduce the resulting
+error.
Similarly to debugging incorrect compilation by mis-behaving passes, you can
+debug incorrect code generation by either LLC or the JIT, using
+bugpoint. The process bugpoint follows in this case is to try
+to narrow the code down to a function that is miscompiled by one or the other
+method, but since for correctness, the entire program must be run,
+bugpoint will compile the code it deems to not be affected with the C
+Backend, and then link in the shared object it generates.
Special note: if you are debugging MultiSource or SPEC tests that
+already exist in the llvm/test hierarchy, there is an easier way to
+debug the JIT, LLC, and CBE, using the pre-written Makefile targets, which
+will pass the program options specified in the Makefiles:
+
+
+
+cd llvm/test/../../program
+make bugpoint-jit
+
+
+
+
At the end of a successful bugpoint run, you will be presented
+with two bytecode files: a safe file which can be compiled with the C
+backend and the test file which either LLC or the JIT
+mis-codegenerates, and thus causes the error.
+
+
To reproduce the error that bugpoint found, it is sufficient to do
+the following:
+
+
+
+
Regenerate the shared object from the safe bytecode file:
-
+