llvm/docs/HowToSubmitABug.rst - llvm-project - Git at Google

 ================================
 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 choose
 which code generator you wish to compile the program with (e.g., LLC or the JIT)
 and optionally a series of LLVM passes to run.  For example:

 .. code-block:: bash

    bugpoint -run-llc [... optzn passes ...] file-to-test.bc --args -- [program arguments]

 bugpoint will try to narrow down your list of passes to the one pass that
 causes an error, and simplify the bitcode file as much as it can to assist
 you. It will print a message letting you know how to reproduce the
 resulting error.

 The :doc:`OptBisect <OptBisect>` page shows an alternative method for finding
 incorrect optimization passes.
	================================
	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 choose
	which code generator you wish to compile the program with (e.g., LLC or the JIT)
	and optionally a series of LLVM passes to run. For example:

	.. code-block:: bash

	bugpoint -run-llc [... optzn passes ...] file-to-test.bc --args -- [program arguments]

	bugpoint will try to narrow down your list of passes to the one pass that
	causes an error, and simplify the bitcode file as much as it can to assist
	you. It will print a message letting you know how to reproduce the
	resulting error.

	The :doc:`OptBisect <OptBisect>` page shows an alternative method for finding
	incorrect optimization passes.