1.. _how-to-submit-a-bug-report: 2 3================================ 4How to submit an LLVM bug report 5================================ 6 7.. sectionauthor:: Chris Lattner <sabre@nondot.org> and Misha Brukman <http://misha.brukman.net> 8 9Introduction - Got bugs? 10======================== 11 12 13If you're working with LLVM and run into a bug, we definitely want to know 14about it. This document describes what you can do to increase the odds of 15getting it fixed quickly. 16 17Basically you have to do two things at a minimum. First, decide whether 18the bug `crashes the compiler`_ (or an LLVM pass), or if the 19compiler is `miscompiling`_ the program (i.e., the 20compiler successfully produces an executable, but it doesn't run right). 21Based on what type of bug it is, follow the instructions in the linked 22section to narrow down the bug so that the person who fixes it will be able 23to find the problem more easily. 24 25Once you have a reduced test-case, go to `the LLVM Bug Tracking System 26<http://llvm.org/bugs/enter_bug.cgi>`_ and fill out the form with the 27necessary details (note that you don't need to pick a category, just use 28the "new-bugs" category if you're not sure). The bug description should 29contain the following information: 30 31* All information necessary to reproduce the problem. 32* The reduced test-case that triggers the bug. 33* The location where you obtained LLVM (if not from our Subversion 34 repository). 35 36Thanks for helping us make LLVM better! 37 38.. _crashes the compiler: 39 40Crashing Bugs 41============= 42 43More often than not, bugs in the compiler cause it to crash---often due to 44an assertion failure of some sort. The most important piece of the puzzle 45is to figure out if it is crashing in the GCC front-end or if it is one of 46the LLVM libraries (e.g. the optimizer or code generator) that has 47problems. 48 49To figure out which component is crashing (the front-end, optimizer or code 50generator), run the ``llvm-gcc`` command line as you were when the crash 51occurred, but with the following extra command line options: 52 53* ``-O0 -emit-llvm``: If ``llvm-gcc`` still crashes when passed these 54 options (which disable the optimizer and code generator), then the crash 55 is in the front-end. Jump ahead to the section on :ref:`front-end bugs 56 <front-end>`. 57 58* ``-emit-llvm``: If ``llvm-gcc`` crashes with this option (which disables 59 the code generator), you found an optimizer bug. Jump ahead to 60 `compile-time optimization bugs`_. 61 62* Otherwise, you have a code generator crash. Jump ahead to `code 63 generator bugs`_. 64 65.. _front-end bug: 66.. _front-end: 67 68Front-end bugs 69-------------- 70 71If the problem is in the front-end, you should re-run the same ``llvm-gcc`` 72command that resulted in the crash, but add the ``-save-temps`` option. 73The compiler will crash again, but it will leave behind a ``foo.i`` file 74(containing preprocessed C source code) and possibly ``foo.s`` for each 75compiled ``foo.c`` file. Send us the ``foo.i`` file, along with the options 76you passed to ``llvm-gcc``, and a brief description of the error it caused. 77 78The `delta <http://delta.tigris.org/>`_ tool helps to reduce the 79preprocessed file down to the smallest amount of code that still replicates 80the problem. You're encouraged to use delta to reduce the code to make the 81developers' lives easier. `This website 82<http://gcc.gnu.org/wiki/A_guide_to_testcase_reduction>`_ has instructions 83on the best way to use delta. 84 85.. _compile-time optimization bugs: 86 87Compile-time optimization bugs 88------------------------------ 89 90If you find that a bug crashes in the optimizer, compile your test-case to a 91``.bc`` file by passing "``-emit-llvm -O0 -c -o foo.bc``". 92Then run: 93 94.. code-block:: bash 95 96 opt -std-compile-opts -debug-pass=Arguments foo.bc -disable-output 97 98This command should do two things: it should print out a list of passes, and 99then it should crash in the same way as llvm-gcc. If it doesn't crash, please 100follow the instructions for a `front-end bug`_. 101 102If this does crash, then you should be able to debug this with the following 103bugpoint command: 104 105.. code-block:: bash 106 107 bugpoint foo.bc <list of passes printed by opt> 108 109Please run this, then file a bug with the instructions and reduced .bc 110files that bugpoint emits. If something goes wrong with bugpoint, please 111submit the "foo.bc" file and the list of passes printed by ``opt``. 112 113.. _code generator bugs: 114 115Code generator bugs 116------------------- 117 118If you find a bug that crashes llvm-gcc in the code generator, compile your 119source file to a .bc file by passing "``-emit-llvm -c -o foo.bc``" to 120llvm-gcc (in addition to the options you already pass). Once your have 121foo.bc, one of the following commands should fail: 122 123#. ``llc foo.bc`` 124#. ``llc foo.bc -relocation-model=pic`` 125#. ``llc foo.bc -relocation-model=static`` 126 127If none of these crash, please follow the instructions for a `front-end 128bug`_. If one of these do crash, you should be able to reduce this with 129one of the following bugpoint command lines (use the one corresponding to 130the command above that failed): 131 132#. ``bugpoint -run-llc foo.bc`` 133#. ``bugpoint -run-llc foo.bc --tool-args -relocation-model=pic`` 134#. ``bugpoint -run-llc foo.bc --tool-args -relocation-model=static`` 135 136Please run this, then file a bug with the instructions and reduced .bc file 137that bugpoint emits. If something goes wrong with bugpoint, please submit 138the "foo.bc" file and the option that llc crashes with. 139 140.. _miscompiling: 141 142Miscompilations 143=============== 144 145If llvm-gcc successfully produces an executable, but that executable 146doesn't run right, this is either a bug in the code or a bug in the 147compiler. The first thing to check is to make sure it is not using 148undefined behavior (e.g. reading a variable before it is defined). In 149particular, check to see if the program `valgrind 150<http://valgrind.org/>`_'s clean, passes purify, or some other memory 151checker tool. Many of the "LLVM bugs" that we have chased down ended up 152being bugs in the program being compiled, not LLVM. 153 154Once you determine that the program itself is not buggy, you should choose 155which code generator you wish to compile the program with (e.g. LLC or the JIT) 156and optionally a series of LLVM passes to run. For example: 157 158.. code-block:: bash 159 160 bugpoint -run-llc [... optzn passes ...] file-to-test.bc --args -- [program arguments] 161 162bugpoint will try to narrow down your list of passes to the one pass that 163causes an error, and simplify the bitcode file as much as it can to assist 164you. It will print a message letting you know how to reproduce the 165resulting error. 166 167Incorrect code generation 168========================= 169 170Similarly to debugging incorrect compilation by mis-behaving passes, you 171can debug incorrect code generation by either LLC or the JIT, using 172``bugpoint``. The process ``bugpoint`` follows in this case is to try to 173narrow the code down to a function that is miscompiled by one or the other 174method, but since for correctness, the entire program must be run, 175``bugpoint`` will compile the code it deems to not be affected with the C 176Backend, and then link in the shared object it generates. 177 178To debug the JIT: 179 180.. code-block:: bash 181 182 bugpoint -run-jit -output=[correct output file] [bitcode file] \ 183 --tool-args -- [arguments to pass to lli] \ 184 --args -- [program arguments] 185 186Similarly, to debug the LLC, one would run: 187 188.. code-block:: bash 189 190 bugpoint -run-llc -output=[correct output file] [bitcode file] \ 191 --tool-args -- [arguments to pass to llc] \ 192 --args -- [program arguments] 193 194**Special note:** if you are debugging MultiSource or SPEC tests that 195already exist in the ``llvm/test`` hierarchy, there is an easier way to 196debug the JIT, LLC, and CBE, using the pre-written Makefile targets, which 197will pass the program options specified in the Makefiles: 198 199.. code-block:: bash 200 201 cd llvm/test/../../program 202 make bugpoint-jit 203 204At the end of a successful ``bugpoint`` run, you will be presented 205with two bitcode files: a *safe* file which can be compiled with the C 206backend and the *test* file which either LLC or the JIT 207mis-codegenerates, and thus causes the error. 208 209To reproduce the error that ``bugpoint`` found, it is sufficient to do 210the following: 211 212#. Regenerate the shared object from the safe bitcode file: 213 214 .. code-block:: bash 215 216 llc -march=c safe.bc -o safe.c 217 gcc -shared safe.c -o safe.so 218 219#. If debugging LLC, compile test bitcode native and link with the shared 220 object: 221 222 .. code-block:: bash 223 224 llc test.bc -o test.s 225 gcc test.s safe.so -o test.llc 226 ./test.llc [program options] 227 228#. If debugging the JIT, load the shared object and supply the test 229 bitcode: 230 231 .. code-block:: bash 232 233 lli -load=safe.so test.bc [program options] 234