llvm/docs/HowToSubmitABug.rst

================================
How to submit an LLVM bug report
================================

Introduction - Got bugs?
========================


If you're working with LLVM and encounter 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.

🔒 If you believe that the bug is security related, please follow :ref:`report-security-issue`. 🔒

Basically, you have to do two things at a minimum. First, decide whether the
bug `crashes the compiler`_ or if the compiler is `miscompiling`_ the program
(i.e., the compiler successfully produces an executable, but it doesn't run
right). 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.

Once you have a reduced test case, go to `the LLVM Bug Tracking System
<https://github.com/llvm/llvm-project/issues>`_ and fill out the form with the
necessary details (note that you don't need to pick a label, just use if you're
not sure).  The bug description should contain the following information:

* 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 Git
  repository).

Thanks for helping us make LLVM better!

.. _crashes the compiler:

Crashing Bugs
=============

More often than not, bugs in the compiler cause it to crash---often due to
an assertion failure of some sort. The most important piece of the puzzle
is to figure out if it is crashing in the Clang front-end or if it is one of
the LLVM libraries (e.g., the optimizer or code generator) that has
problems.

To identify the crashing component (the front-end, middle-end
optimizer, or backend code generator), run the ``clang`` command line as you
were when the crash occurred, but with the following extra command line
options:

* ``-emit-llvm -Xclang -disable-llvm-passes``: If ``clang`` still crashes when
  passed these options (which disable the optimizer and code generator), then
  the crash is in the front-end. Jump ahead to :ref:`front-end bugs
  <frontend-crash>`.

* ``-emit-llvm``: If ``clang`` crashes with this option (which disables
  the code generator), you've found a middle-end optimizer bug. Jump ahead to
  :ref:`middle-end bugs <middleend-crash>`.

* Otherwise, you have a backend code generator crash. Jump ahead to :ref:`code
  generator bugs <backend-crash>`.

.. _frontend-crash:

Front-end bugs
--------------

On a ``clang`` crash, the compiler will dump a preprocessed file and a script
to replay the ``clang`` command. For example, you should see something like

.. code-block:: text

   PLEASE ATTACH THE FOLLOWING FILES TO THE BUG REPORT:
   Preprocessed source(s) and associated run script(s) are located at:
   clang: note: diagnostic msg: /tmp/foo-xxxxxx.c
   clang: note: diagnostic msg: /tmp/foo-xxxxxx.sh

The `creduce <https://github.com/csmith-project/creduce>`_ tool helps to
reduce the preprocessed file down to the smallest amount of code that still
replicates the problem. You're encouraged to use creduce to reduce the code
to make the developers' lives easier. The
``clang/utils/creduce-clang-crash.py`` script can be used on the files
that clang dumps to help with automating creating a test to check for the
compiler crash.

`cvise <https://github.com/marxin/cvise>`_ is an alternative to ``creduce``.

.. _middleend-crash:

Middle-end optimization bugs
----------------------------

If you find that a bug crashes in the optimizer, compile your test-case to a
``.bc`` file by passing "``-emit-llvm -O1 -Xclang -disable-llvm-passes -c -o
foo.bc``". The ``-O1`` is important because ``-O0`` adds the ``optnone``
function attribute to all functions and many passes don't run on ``optnone``
functions. Then run:

.. code-block:: bash

   opt -O3 foo.bc -disable-output

If this doesn't crash, please follow the instructions for a :ref:`front-end
bug <frontend-crash>`.

If this does crash, then you can debug this with the :doc:`llvm-reduce
<CommandGuide/llvm-reduce>` tool. Create a script that reproduces the
crash and run:

.. code-block:: bash

   llvm-reduce --test=path/to/script foo.bc

which should produce reduced IR that reproduces the crash.

.. TIP::
   ``llvm-reduce -j $NUM_THREADS`` is multi-threaded and can therefore
   potentially be much faster.

.. TIP::
   Reduction is fastest and most effective the simpler the
   reproduction script is. Ideally, this will be running `opt` with a
   single pass. The most effective way to extract the IR before a
   specific point is a two step process. First, run the testcase with
   the ``-print-pass-numbers`` flag. This will print the name of a
   pass and an integer ID. You can then use the last ID printed before
   the crash and add 3 flags,
   ``-print-before-pass-number=<integer-id> -print-module-scope -ir-dump-directory=/my/debug/path``.
   This will place failing IR files in the given directory.
   ``-print-before-pass-number`` is the minimum required flag, but
   will not produce an output directly consumable by a tool. It will
   print to stderr, and will be incomplete in most situations without
   ``-print-module-scope``.

   A more brute force approach is to use the ``--print-before-all
   --print-module-scope`` flags to dump the IR before every pass. Be
   warned that this is very verbose.

.. _backend-crash:

Backend code generator bugs
---------------------------

If you find a bug that crashes clang in the code generator, compile your
source file to a ``.bc`` file by passing "``-emit-llvm -c -o foo.bc``" to
clang (in addition to the options you already pass).  Once you have
``foo.bc``, one of the following commands should fail:

#. ``llc foo.bc``
#. ``llc foo.bc -relocation-model=pic``
#. ``llc foo.bc -relocation-model=static``

If none of these crash, please follow the instructions for a
:ref:`front-end bug<frontend-crash>`. If one of these crashes, you
should be able to reduce this with :doc:`llvm-reduce
<CommandGuide/llvm-reduce>`, similar to middle end bugs. In this
case, your test script should use :doc:`llc <CommandGuide/llc>`
instead of `opt`.

Please run this, then file a bug with the instructions and reduced
``.bc`` file that `llvm-reduce` emits.  If something goes wrong with
`llvm-reduce`, please submit the ``foo.bc`` file and the option that
`llc` crashes with.

LTO bugs
---------------------------

If you encounter a bug that leads to crashes in the LLVM LTO phase when using
the ``-flto`` option, follow these steps to diagnose and report the issue:

Compile your source file to a ``.bc`` (Bitcode) file with the following options,
in addition to your existing compilation options:

.. code-block:: bash

   export CFLAGS="-flto -fuse-ld=lld" CXXFLAGS="-flto -fuse-ld=lld" LDFLAGS="-Wl,-plugin-opt=save-temps"

These options enable LTO and save temporary files generated during compilation
for later analysis.

On Windows, use lld-link as the linker. Adjust your compilation 
flags as follows:
* Add ``/lldsavetemps`` to the linker flags.
* When linking from the compiler driver, add ``/link /lldsavetemps`` in order to forward that flag to the linker.

Using the specified flags will generate four intermediate bytecode files:

#. a.out.0.0.preopt.bc (Before any link-time optimizations (LTO) are applied)
#. a.out.0.2.internalize.bc (After initial optimizations are applied)
#. a.out.0.4.opt.bc (After an extensive set of optimizations)
#. a.out.0.5.precodegen.bc (After LTO but before translating into machine code)

Execute one of the following commands to identify the source of the problem:

#. ``opt "-passes=lto<O3>" a.out.0.2.internalize.bc``
#. ``llc a.out.0.5.precodegen.bc``

If one of these do crash, you should be able to reduce
this with :program:`llvm-reduce`
command line (use the bc file corresponding to the command above that failed):

.. code-block:: bash

   llvm-reduce --test reduce.sh a.out.0.2.internalize.bc

Example of ``reduce.sh`` script

.. code-block:: bash

   $ cat reduce.sh
   #!/bin/bash -e

   path/to/not --crash path/to/opt "-passes=lto<O3>" $1 -o temp.bc  2> err.log
   grep -q "It->second == &Insn" err.log

Here we have grepped for the failed assert message.

Please run this, then file a bug with the instructions and reduced ``.bc`` file
that llvm-reduce emits.

.. _miscompiling:

Miscompilations
===============

If clang successfully produces an executable, but that executable doesn't run
right, this is either a bug in the code or a bug in the compiler. The first
thing to check is to make sure it is not using undefined behavior (e.g.,
reading a variable before it is defined). In particular, check to see if the
program is clean under various `sanitizers
<https://github.com/google/sanitizers>`_ (e.g., ``clang
-fsanitize=undefined,address``) and `valgrind <http://valgrind.org/>`_. Many
"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 work on
reducing the inputs required to reproduce the miscompilation. The
:doc:`OptBisect <OptBisect>` page shows how to find the optimization pass
causing the miscompile. You can use :doc:`llvm-reduce <CommandGuide/llvm-reduce>`
to minimize the bitcode necessary to reproduce the miscompilation.