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 the 16bug `crashes the compiler`_ or if the compiler is `miscompiling`_ the program 17(i.e., the compiler successfully produces an executable, but it doesn't run 18right). Based on what type of bug it is, follow the instructions in the 19linked section to narrow down the bug so that the person who fixes it will be 20able to find the problem more easily. 21 22Once you have a reduced test-case, go to `the LLVM Bug Tracking System 23<https://github.com/llvm/llvm-project/issues>`_ and fill out the form with the 24necessary details (note that you don't need to pick a label, just use if you're 25not sure). The bug description should contain the following information: 26 27* All information necessary to reproduce the problem. 28* The reduced test-case that triggers the bug. 29* The location where you obtained LLVM (if not from our Git 30 repository). 31 32Thanks for helping us make LLVM better! 33 34.. _crashes the compiler: 35 36Crashing Bugs 37============= 38 39More often than not, bugs in the compiler cause it to crash---often due to 40an assertion failure of some sort. The most important piece of the puzzle 41is to figure out if it is crashing in the Clang front-end or if it is one of 42the LLVM libraries (e.g. the optimizer or code generator) that has 43problems. 44 45To figure out which component is crashing (the front-end, middle-end 46optimizer, or backend code generator), run the ``clang`` command line as you 47were when the crash occurred, but with the following extra command line 48options: 49 50* ``-emit-llvm -Xclang -disable-llvm-passes``: If ``clang`` still crashes when 51 passed these options (which disable the optimizer and code generator), then 52 the crash is in the front-end. Jump ahead to :ref:`front-end bugs 53 <frontend-crash>`. 54 55* ``-emit-llvm``: If ``clang`` crashes with this option (which disables 56 the code generator), you found a middle-end optimizer bug. Jump ahead to 57 :ref:`middle-end bugs <middleend-crash>`. 58 59* Otherwise, you have a backend code generator crash. Jump ahead to :ref:`code 60 generator bugs <backend-crash>`. 61 62.. _frontend-crash: 63 64Front-end bugs 65-------------- 66 67On a ``clang`` crash, the compiler will dump a preprocessed file and a script 68to replay the ``clang`` command. For example, you should see something like 69 70.. code-block:: text 71 72 PLEASE ATTACH THE FOLLOWING FILES TO THE BUG REPORT: 73 Preprocessed source(s) and associated run script(s) are located at: 74 clang: note: diagnostic msg: /tmp/foo-xxxxxx.c 75 clang: note: diagnostic msg: /tmp/foo-xxxxxx.sh 76 77The `creduce <https://github.com/csmith-project/creduce>`_ tool helps to 78reduce the preprocessed file down to the smallest amount of code that still 79replicates the problem. You're encouraged to use creduce to reduce the code 80to make the developers' lives easier. The 81``clang/utils/creduce-clang-crash.py`` script can be used on the files 82that clang dumps to help with automating creating a test to check for the 83compiler crash. 84 85`cvise <https://github.com/marxin/cvise>`_ is an alternative to ``creduce``. 86 87.. _middleend-crash: 88 89Middle-end optimization bugs 90---------------------------- 91 92If you find that a bug crashes in the optimizer, compile your test-case to a 93``.bc`` file by passing "``-emit-llvm -O1 -Xclang -disable-llvm-passes -c -o 94foo.bc``". The ``-O1`` is important because ``-O0`` adds the ``optnone`` 95function attribute to all functions and many passes don't run on ``optnone`` 96functions. Then run: 97 98.. code-block:: bash 99 100 opt -O3 foo.bc -disable-output 101 102If this doesn't crash, please follow the instructions for a :ref:`front-end 103bug <frontend-crash>`. 104 105If this does crash, then you should be able to debug this with the following 106:doc:`bugpoint <Bugpoint>` command: 107 108.. code-block:: bash 109 110 bugpoint foo.bc -O3 111 112Run this, then file a bug with the instructions and reduced .bc 113files that bugpoint emits. 114 115If bugpoint doesn't reproduce the crash, ``llvm-reduce`` is an alternative 116way to reduce LLVM IR. Create a script that repros the crash and run: 117 118.. code-block:: bash 119 120 llvm-reduce --test=path/to/script foo.bc 121 122which should produce reduced IR that reproduces the crash. Be warned the 123``llvm-reduce`` is still fairly immature and may crash. 124 125If none of the above work, you can get the IR before a crash by running the 126``opt`` command with the ``--print-before-all --print-module-scope`` flags to 127dump the IR before every pass. Be warned that this is very verbose. 128 129.. _backend-crash: 130 131Backend code generator bugs 132--------------------------- 133 134If you find a bug that crashes clang in the code generator, compile your 135source file to a .bc file by passing "``-emit-llvm -c -o foo.bc``" to 136clang (in addition to the options you already pass). Once your have 137foo.bc, one of the following commands should fail: 138 139#. ``llc foo.bc`` 140#. ``llc foo.bc -relocation-model=pic`` 141#. ``llc foo.bc -relocation-model=static`` 142 143If none of these crash, please follow the instructions for a :ref:`front-end 144bug<frontend-crash>`. If one of these do crash, you should be able to reduce 145this with one of the following :doc:`bugpoint <Bugpoint>` command lines (use 146the one corresponding to the command above that failed): 147 148#. ``bugpoint -run-llc foo.bc`` 149#. ``bugpoint -run-llc foo.bc --tool-args -relocation-model=pic`` 150#. ``bugpoint -run-llc foo.bc --tool-args -relocation-model=static`` 151 152Please run this, then file a bug with the instructions and reduced .bc file 153that bugpoint emits. If something goes wrong with bugpoint, please submit 154the "foo.bc" file and the option that llc crashes with. 155 156LTO bugs 157--------------------------- 158 159If you encounter a bug that leads to crashes in the LLVM LTO phase when using 160the ``-flto`` option, follow these steps to diagnose and report the issue: 161 162Compile your source file to a ``.bc`` (Bitcode) file with the following options, 163in addition to your existing compilation options: 164 165.. code-block:: bash 166 167 export CFLAGS="-flto -fuse-ld=lld" CXXFLAGS="-flto -fuse-ld=lld" LDFLAGS="-Wl,-plugin-opt=save-temps" 168 169These options enable LTO and save temporary files generated during compilation 170for later analysis. 171 172On Windows, you should be using lld-link as the linker. Adjust your compilation 173flags as follows: 174* Add ``/lldsavetemps`` to the linker flags. 175* When linking from the compiler driver, add ``/link /lldsavetemps`` in order to forward that flag to the linker. 176 177Using the specified flags will generate four intermediate bytecode files: 178 179#. a.out.0.0.preopt.bc (Before any link-time optimizations (LTO) are applied) 180#. a.out.0.2.internalize.bc (After initial optimizations are applied) 181#. a.out.0.4.opt.bc (After an extensive set of optimizations) 182#. a.out.0.5.precodegen.bc (After LTO but before translating into machine code) 183 184Execute one of the following commands to identify the source of the problem: 185 186#. ``opt "-passes=lto<O3>" a.out.0.2.internalize.bc`` 187#. ``llc a.out.0.5.precodegen.bc`` 188 189If one of these do crash, you should be able to reduce 190this with :program:`llvm-reduce` 191command line (use the bc file corresponding to the command above that failed): 192 193.. code-block:: bash 194 195 llvm-reduce --test reduce.sh a.out.0.2.internalize.bc 196 197Example of reduce.sh script 198 199.. code-block:: bash 200 201 $ cat reduce.sh 202 #!/bin/bash -e 203 204 path/to/not --crash path/to/opt "-passes=lto<O3>" $1 -o temp.bc 2> err.log 205 grep -q "It->second == &Insn" err.log 206 207Here we have grepped the failed assert message. 208 209Please run this, then file a bug with the instructions and reduced .bc file 210that llvm-reduce emits. 211 212.. _miscompiling: 213 214Miscompilations 215=============== 216 217If clang successfully produces an executable, but that executable doesn't run 218right, this is either a bug in the code or a bug in the compiler. The first 219thing to check is to make sure it is not using undefined behavior (e.g. 220reading a variable before it is defined). In particular, check to see if the 221program is clean under various `sanitizers 222<https://github.com/google/sanitizers>`_ (e.g. ``clang 223-fsanitize=undefined,address``) and `valgrind <http://valgrind.org/>`_. Many 224"LLVM bugs" that we have chased down ended up being bugs in the program being 225compiled, not LLVM. 226 227Once you determine that the program itself is not buggy, you should choose 228which code generator you wish to compile the program with (e.g. LLC or the JIT) 229and optionally a series of LLVM passes to run. For example: 230 231.. code-block:: bash 232 233 bugpoint -run-llc [... optzn passes ...] file-to-test.bc --args -- [program arguments] 234 235bugpoint will try to narrow down your list of passes to the one pass that 236causes an error, and simplify the bitcode file as much as it can to assist 237you. It will print a message letting you know how to reproduce the 238resulting error. 239 240The :doc:`OptBisect <OptBisect>` page shows an alternative method for finding 241incorrect optimization passes. 242 243Incorrect code generation 244========================= 245 246Similarly to debugging incorrect compilation by mis-behaving passes, you 247can debug incorrect code generation by either LLC or the JIT, using 248``bugpoint``. The process ``bugpoint`` follows in this case is to try to 249narrow the code down to a function that is miscompiled by one or the other 250method, but since for correctness, the entire program must be run, 251``bugpoint`` will compile the code it deems to not be affected with the C 252Backend, and then link in the shared object it generates. 253 254To debug the JIT: 255 256.. code-block:: bash 257 258 bugpoint -run-jit -output=[correct output file] [bitcode file] \ 259 --tool-args -- [arguments to pass to lli] \ 260 --args -- [program arguments] 261 262Similarly, to debug the LLC, one would run: 263 264.. code-block:: bash 265 266 bugpoint -run-llc -output=[correct output file] [bitcode file] \ 267 --tool-args -- [arguments to pass to llc] \ 268 --args -- [program arguments] 269 270**Special note:** if you are debugging MultiSource or SPEC tests that 271already exist in the ``llvm/test`` hierarchy, there is an easier way to 272debug the JIT, LLC, and CBE, using the pre-written Makefile targets, which 273will pass the program options specified in the Makefiles: 274 275.. code-block:: bash 276 277 cd llvm/test/../../program 278 make bugpoint-jit 279 280At the end of a successful ``bugpoint`` run, you will be presented 281with two bitcode files: a *safe* file which can be compiled with the C 282backend and the *test* file which either LLC or the JIT 283mis-codegenerates, and thus causes the error. 284 285To reproduce the error that ``bugpoint`` found, it is sufficient to do 286the following: 287 288#. Regenerate the shared object from the safe bitcode file: 289 290 .. code-block:: bash 291 292 llc -march=c safe.bc -o safe.c 293 gcc -shared safe.c -o safe.so 294 295#. If debugging LLC, compile test bitcode native and link with the shared 296 object: 297 298 .. code-block:: bash 299 300 llc test.bc -o test.s 301 gcc test.s safe.so -o test.llc 302 ./test.llc [program options] 303 304#. If debugging the JIT, load the shared object and supply the test 305 bitcode: 306 307 .. code-block:: bash 308 309 lli -load=safe.so test.bc [program options] 310