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[Docs] Update HowToSubmitABug
With the new PM imminent, bugpoint will diverge from opt, meaning it may not reproduce a crash with the same arguments passed to opt. We need to specify alternatives to bugpoint for reducing crashes. I looked at the rest of the document to see if anything could be improved. Major highlights: * Run -Xclang -disable-llvm-passes instead of -O0 for skipping IR passes * Mention the files that clang dumps on a crash * Remove outdated reference to `delta` and plug `creduce` instead * Mention llvm-reduce on top of bugpoint * Mention --print-before-all --print-module-scope * Mention sanitizers in addition to valgrind * Mention opt-bisect for miscompiles Reviewed By: fhahn, MaskRay Differential Revision: https://reviews.llvm.org/D95578
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Arthur Eubanks
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@ -12,13 +12,12 @@ getting it fixed quickly.
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🔒 If you believe that the bug is security related, please follow :ref:`report-security-issue`. 🔒
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Basically you have to do two things at a minimum. First, decide whether
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the bug `crashes the compiler`_ (or an LLVM pass), or if the
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compiler is `miscompiling`_ the program (i.e., the
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compiler successfully produces an executable, but it doesn't run right).
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Based on what type of bug it is, follow the instructions in the linked
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section to narrow down the bug so that the person who fixes it will be able
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to find the problem more easily.
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Basically you have to do two things at a minimum. First, decide whether the
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bug `crashes the compiler`_ or if the compiler is `miscompiling`_ the program
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(i.e., the compiler successfully produces an executable, but it doesn't run
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right). Based on what type of bug it is, follow the instructions in the
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linked section to narrow down the bug so that the person who fixes it will be
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able to find the problem more easily.
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Once you have a reduced test-case, go to `the LLVM Bug Tracking System
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<https://bugs.llvm.org/enter_bug.cgi>`_ and fill out the form with the
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@ -44,74 +43,94 @@ is to figure out if it is crashing in the Clang front-end or if it is one of
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the LLVM libraries (e.g. the optimizer or code generator) that has
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problems.
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To figure out which component is crashing (the front-end, optimizer or code
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generator), run the ``clang`` command line as you were when the crash
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occurred, but with the following extra command line options:
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To figure out which component is crashing (the front-end, middle-end
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optimizer, or backend code generator), run the ``clang`` command line as you
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were when the crash occurred, but with the following extra command line
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options:
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* ``-O0 -emit-llvm``: If ``clang`` still crashes when passed these
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options (which disable the optimizer and code generator), then the crash
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is in the front-end. Jump ahead to the section on :ref:`front-end bugs
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<front-end>`.
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* ``-emit-llvm -Xclang -disable-llvm-passes``: If ``clang`` still crashes when
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passed these options (which disable the optimizer and code generator), then
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the crash is in the front-end. Jump ahead to :ref:`front-end bugs
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<frontend-crash>`.
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* ``-emit-llvm``: If ``clang`` crashes with this option (which disables
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the code generator), you found an optimizer bug. Jump ahead to
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`compile-time optimization bugs`_.
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the code generator), you found a middle-end optimizer bug. Jump ahead to
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:ref:`middle-end bugs <middleend-crash>`.
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* Otherwise, you have a code generator crash. Jump ahead to `code
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generator bugs`_.
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* Otherwise, you have a backend code generator crash. Jump ahead to :ref:`code
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generator bugs <backend-crash>`.
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.. _front-end bug:
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.. _front-end:
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.. _frontend-crash:
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Front-end bugs
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--------------
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If the problem is in the front-end, you should re-run the same ``clang``
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command that resulted in the crash, but add the ``-save-temps`` option.
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The compiler will crash again, but it will leave behind a ``foo.i`` file
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(containing preprocessed C source code) and possibly ``foo.s`` for each
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compiled ``foo.c`` file. Send us the ``foo.i`` file, along with the options
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you passed to ``clang``, and a brief description of the error it caused.
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On a ``clang`` crash, the compiler will dump a preprocessed file and a script
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to replay the ``clang`` command. For example, you should see something like
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The `delta <http://delta.tigris.org/>`_ tool helps to reduce the
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preprocessed file down to the smallest amount of code that still replicates
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the problem. You're encouraged to use delta to reduce the code to make the
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developers' lives easier. `This website
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<http://gcc.gnu.org/wiki/A_guide_to_testcase_reduction>`_ has instructions
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on the best way to use delta.
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.. code-block:: text
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.. _compile-time optimization bugs:
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PLEASE ATTACH THE FOLLOWING FILES TO THE BUG REPORT:
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Preprocessed source(s) and associated run script(s) are located at:
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clang: note: diagnostic msg: /tmp/foo-xxxxxx.c
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clang: note: diagnostic msg: /tmp/foo-xxxxxx.sh
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Compile-time optimization bugs
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------------------------------
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The `creduce <https://github.com/csmith-project/creduce>`_ tool helps to
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reduce the preprocessed file down to the smallest amount of code that still
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replicates the problem. You're encouraged to use creduce to reduce the code
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to make the developers' lives easier. The
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``clang/utils/creduce-clang-crash.py`` script can be used on the files
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that clang dumps to help with automating creating a test to check for the
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compiler crash.
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`cvise <https://github.com/marxin/cvise>`_ is an alternative to ``creduce``.
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.. _middleend-crash:
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Middle-end optimization bugs
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----------------------------
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If you find that a bug crashes in the optimizer, compile your test-case to a
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``.bc`` file by passing "``-emit-llvm -O1 -Xclang -disable-llvm-passes -c -o
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foo.bc``". Then run:
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foo.bc``". The ``-O1`` is important because ``-O0`` adds the ``optnone``
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function attribute to all functions and many passes don't run on ``optnone``
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functions. Then run:
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.. code-block:: bash
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opt -O3 -debug-pass=Arguments foo.bc -disable-output
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opt -O3 foo.bc -disable-output
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This command should do two things: it should print out a list of passes, and
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then it should crash in the same way as clang. If it doesn't crash, please
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follow the instructions for a `front-end bug`_.
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If this doesn't crash, please follow the instructions for a :ref:`front-end
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bug <frontend-crash>`.
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If this does crash, then you should be able to debug this with the following
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bugpoint command:
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:doc:`bugpoint <Bugpoint>` command:
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.. code-block:: bash
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bugpoint foo.bc <list of passes printed by opt>
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bugpoint foo.bc -O3
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Please run this, then file a bug with the instructions and reduced .bc
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files that bugpoint emits. If something goes wrong with bugpoint, please
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submit the "foo.bc" file and the list of passes printed by ``opt``.
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Run this, then file a bug with the instructions and reduced .bc
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files that bugpoint emits.
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.. _code generator bugs:
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If bugpoint doesn't reproduce the crash, ``llvm-reduce`` is an alternative
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way to reduce LLVM IR. Create a script that repros the crash and run:
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Code generator bugs
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-------------------
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.. code-block:: bash
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llvm-reduce --test=path/to/script foo.bc
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which should produce reduced IR that reproduces the crash. Be warned the
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``llvm-reduce`` is still fairly immature and may crash.
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If none of the above work, you can get the IR before a crash by running the
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``opt`` command with the ``--print-before-all --print-module-scope`` flags to
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dump the IR before every pass. Be warned that this is very verbose.
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.. _backend-crash:
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Backend code generator bugs
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---------------------------
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If you find a bug that crashes clang in the code generator, compile your
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source file to a .bc file by passing "``-emit-llvm -c -o foo.bc``" to
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@ -122,10 +141,10 @@ foo.bc, one of the following commands should fail:
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#. ``llc foo.bc -relocation-model=pic``
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#. ``llc foo.bc -relocation-model=static``
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If none of these crash, please follow the instructions for a `front-end
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bug`_. If one of these do crash, you should be able to reduce this with
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one of the following bugpoint command lines (use the one corresponding to
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the command above that failed):
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If none of these crash, please follow the instructions for a :ref:`front-end
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bug<frontend-crash>`. If one of these do crash, you should be able to reduce
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this with one of the following :doc:`bugpoint <Bugpoint>` command lines (use
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the one corresponding to the command above that failed):
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#. ``bugpoint -run-llc foo.bc``
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#. ``bugpoint -run-llc foo.bc --tool-args -relocation-model=pic``
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@ -140,14 +159,15 @@ the "foo.bc" file and the option that llc crashes with.
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Miscompilations
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===============
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If clang successfully produces an executable, but that executable
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doesn't run right, this is either a bug in the code or a bug in the
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compiler. The first thing to check is to make sure it is not using
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undefined behavior (e.g. reading a variable before it is defined). In
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particular, check to see if the program `valgrind
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<http://valgrind.org/>`_'s clean, passes purify, or some other memory
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checker tool. Many of the "LLVM bugs" that we have chased down ended up
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being bugs in the program being compiled, not LLVM.
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If clang successfully produces an executable, but that executable doesn't run
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right, this is either a bug in the code or a bug in the compiler. The first
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thing to check is to make sure it is not using undefined behavior (e.g.
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reading a variable before it is defined). In particular, check to see if the
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program is clean under various `sanitizers
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<https://github.com/google/sanitizers>`_ (e.g. ``clang
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-fsanitize=undefined,address``) and `valgrind <http://valgrind.org/>`_. Many
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"LLVM bugs" that we have chased down ended up being bugs in the program being
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compiled, not LLVM.
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Once you determine that the program itself is not buggy, you should choose
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which code generator you wish to compile the program with (e.g. LLC or the JIT)
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@ -162,6 +182,9 @@ causes an error, and simplify the bitcode file as much as it can to assist
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you. It will print a message letting you know how to reproduce the
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resulting error.
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The :doc:`OptBisect <OptBisect>` page shows an alternative method for finding
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incorrect optimization passes.
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Incorrect code generation
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=========================
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