1================================ 2How to submit an LLVM bug report 3================================ 4 5Introduction - Got bugs? 6======================== 7 8 9If you're working with LLVM and run into a bug, we definitely want to know 10about it. This document describes what you can do to increase the odds of 11getting it fixed quickly. 12 13�� If you believe that the bug is security related, please follow :ref:`report-security-issue`. �� 14 15Basically you have to do two things at a minimum. First, decide whether 16the bug `crashes the compiler`_ (or an LLVM pass), or if the 17compiler is `miscompiling`_ the program (i.e., the 18compiler successfully produces an executable, but it doesn't run right). 19Based on what type of bug it is, follow the instructions in the linked 20section to narrow down the bug so that the person who fixes it will be able 21to find the problem more easily. 22 23Once you have a reduced test-case, go to `the LLVM Bug Tracking System 24<https://bugs.llvm.org/enter_bug.cgi>`_ and fill out the form with the 25necessary details (note that you don't need to pick a category, just use 26the "new-bugs" category if you're not sure). The bug description should 27contain the following information: 28 29* All information necessary to reproduce the problem. 30* The reduced test-case that triggers the bug. 31* The location where you obtained LLVM (if not from our Git 32 repository). 33 34Thanks for helping us make LLVM better! 35 36.. _crashes the compiler: 37 38Crashing Bugs 39============= 40 41More often than not, bugs in the compiler cause it to crash---often due to 42an assertion failure of some sort. The most important piece of the puzzle 43is to figure out if it is crashing in the Clang front-end or if it is one of 44the LLVM libraries (e.g. the optimizer or code generator) that has 45problems. 46 47To figure out which component is crashing (the front-end, optimizer or code 48generator), run the ``clang`` command line as you were when the crash 49occurred, but with the following extra command line options: 50 51* ``-O0 -emit-llvm``: If ``clang`` still crashes when passed these 52 options (which disable the optimizer and code generator), then the crash 53 is in the front-end. Jump ahead to the section on :ref:`front-end bugs 54 <front-end>`. 55 56* ``-emit-llvm``: If ``clang`` crashes with this option (which disables 57 the code generator), you found an optimizer bug. Jump ahead to 58 `compile-time optimization bugs`_. 59 60* Otherwise, you have a code generator crash. Jump ahead to `code 61 generator bugs`_. 62 63.. _front-end bug: 64.. _front-end: 65 66Front-end bugs 67-------------- 68 69If the problem is in the front-end, you should re-run the same ``clang`` 70command that resulted in the crash, but add the ``-save-temps`` option. 71The compiler will crash again, but it will leave behind a ``foo.i`` file 72(containing preprocessed C source code) and possibly ``foo.s`` for each 73compiled ``foo.c`` file. Send us the ``foo.i`` file, along with the options 74you passed to ``clang``, and a brief description of the error it caused. 75 76The `delta <http://delta.tigris.org/>`_ tool helps to reduce the 77preprocessed file down to the smallest amount of code that still replicates 78the problem. You're encouraged to use delta to reduce the code to make the 79developers' lives easier. `This website 80<http://gcc.gnu.org/wiki/A_guide_to_testcase_reduction>`_ has instructions 81on the best way to use delta. 82 83.. _compile-time optimization bugs: 84 85Compile-time optimization bugs 86------------------------------ 87 88If you find that a bug crashes in the optimizer, compile your test-case to a 89``.bc`` file by passing "``-emit-llvm -O1 -Xclang -disable-llvm-passes -c -o 90foo.bc``". Then run: 91 92.. code-block:: bash 93 94 opt -O3 -debug-pass=Arguments foo.bc -disable-output 95 96This command should do two things: it should print out a list of passes, and 97then it should crash in the same way as clang. If it doesn't crash, please 98follow the instructions for a `front-end bug`_. 99 100If this does crash, then you should be able to debug this with the following 101bugpoint command: 102 103.. code-block:: bash 104 105 bugpoint foo.bc <list of passes printed by opt> 106 107Please run this, then file a bug with the instructions and reduced .bc 108files that bugpoint emits. If something goes wrong with bugpoint, please 109submit the "foo.bc" file and the list of passes printed by ``opt``. 110 111.. _code generator bugs: 112 113Code generator bugs 114------------------- 115 116If you find a bug that crashes clang in the code generator, compile your 117source file to a .bc file by passing "``-emit-llvm -c -o foo.bc``" to 118clang (in addition to the options you already pass). Once your have 119foo.bc, one of the following commands should fail: 120 121#. ``llc foo.bc`` 122#. ``llc foo.bc -relocation-model=pic`` 123#. ``llc foo.bc -relocation-model=static`` 124 125If none of these crash, please follow the instructions for a `front-end 126bug`_. If one of these do crash, you should be able to reduce this with 127one of the following bugpoint command lines (use the one corresponding to 128the command above that failed): 129 130#. ``bugpoint -run-llc foo.bc`` 131#. ``bugpoint -run-llc foo.bc --tool-args -relocation-model=pic`` 132#. ``bugpoint -run-llc foo.bc --tool-args -relocation-model=static`` 133 134Please run this, then file a bug with the instructions and reduced .bc file 135that bugpoint emits. If something goes wrong with bugpoint, please submit 136the "foo.bc" file and the option that llc crashes with. 137 138.. _miscompiling: 139 140Miscompilations 141=============== 142 143If clang successfully produces an executable, but that executable 144doesn't run right, this is either a bug in the code or a bug in the 145compiler. The first thing to check is to make sure it is not using 146undefined behavior (e.g. reading a variable before it is defined). In 147particular, check to see if the program `valgrind 148<http://valgrind.org/>`_'s clean, passes purify, or some other memory 149checker tool. Many of the "LLVM bugs" that we have chased down ended up 150being bugs in the program being compiled, not LLVM. 151 152Once you determine that the program itself is not buggy, you should choose 153which code generator you wish to compile the program with (e.g. LLC or the JIT) 154and optionally a series of LLVM passes to run. For example: 155 156.. code-block:: bash 157 158 bugpoint -run-llc [... optzn passes ...] file-to-test.bc --args -- [program arguments] 159 160bugpoint will try to narrow down your list of passes to the one pass that 161causes an error, and simplify the bitcode file as much as it can to assist 162you. It will print a message letting you know how to reproduce the 163resulting error. 164 165Incorrect code generation 166========================= 167 168Similarly to debugging incorrect compilation by mis-behaving passes, you 169can debug incorrect code generation by either LLC or the JIT, using 170``bugpoint``. The process ``bugpoint`` follows in this case is to try to 171narrow the code down to a function that is miscompiled by one or the other 172method, but since for correctness, the entire program must be run, 173``bugpoint`` will compile the code it deems to not be affected with the C 174Backend, and then link in the shared object it generates. 175 176To debug the JIT: 177 178.. code-block:: bash 179 180 bugpoint -run-jit -output=[correct output file] [bitcode file] \ 181 --tool-args -- [arguments to pass to lli] \ 182 --args -- [program arguments] 183 184Similarly, to debug the LLC, one would run: 185 186.. code-block:: bash 187 188 bugpoint -run-llc -output=[correct output file] [bitcode file] \ 189 --tool-args -- [arguments to pass to llc] \ 190 --args -- [program arguments] 191 192**Special note:** if you are debugging MultiSource or SPEC tests that 193already exist in the ``llvm/test`` hierarchy, there is an easier way to 194debug the JIT, LLC, and CBE, using the pre-written Makefile targets, which 195will pass the program options specified in the Makefiles: 196 197.. code-block:: bash 198 199 cd llvm/test/../../program 200 make bugpoint-jit 201 202At the end of a successful ``bugpoint`` run, you will be presented 203with two bitcode files: a *safe* file which can be compiled with the C 204backend and the *test* file which either LLC or the JIT 205mis-codegenerates, and thus causes the error. 206 207To reproduce the error that ``bugpoint`` found, it is sufficient to do 208the following: 209 210#. Regenerate the shared object from the safe bitcode file: 211 212 .. code-block:: bash 213 214 llc -march=c safe.bc -o safe.c 215 gcc -shared safe.c -o safe.so 216 217#. If debugging LLC, compile test bitcode native and link with the shared 218 object: 219 220 .. code-block:: bash 221 222 llc test.bc -o test.s 223 gcc test.s safe.so -o test.llc 224 ./test.llc [program options] 225 226#. If debugging the JIT, load the shared object and supply the test 227 bitcode: 228 229 .. code-block:: bash 230 231 lli -load=safe.so test.bc [program options] 232