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

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

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


 If you're working with LLVM and run into 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 figure out which component is crashing (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 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 should be able to debug this with the following
 :doc:`bugpoint <Bugpoint>` command:

 .. code-block:: bash

    bugpoint foo.bc -O3

 Run this, then file a bug with the instructions and reduced .bc
 files that bugpoint emits.

 If bugpoint doesn't reproduce the crash, ``llvm-reduce`` is an alternative
 way to reduce LLVM IR. Create a script that repros the crash and run:

 .. code-block:: bash

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

 which should produce reduced IR that reproduces the crash. Be warned the
 ``llvm-reduce`` is still fairly immature and may crash.

 If none of the above work, you can get the IR before a crash by running the
 ``opt`` command with 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 your 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 do crash, you should be able to reduce
 this with one of the following :doc:`bugpoint <Bugpoint>` command lines (use
 the one corresponding to the command above that failed):

 #. ``bugpoint -run-llc foo.bc``
 #. ``bugpoint -run-llc foo.bc --tool-args -relocation-model=pic``
 #. ``bugpoint -run-llc foo.bc --tool-args -relocation-model=static``

 Please run this, then file a bug with the instructions and reduced .bc file
 that bugpoint emits.  If something goes wrong with bugpoint, 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, you should be using 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 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.

 Incorrect code generation
 =========================

 Similarly to debugging incorrect compilation by mis-behaving passes, you
 can debug incorrect code generation by either LLC or the JIT, using
 ``bugpoint``. The process ``bugpoint`` follows in this case is to try to
 narrow the code down to a function that is miscompiled by one or the other
 method, but since for correctness, the entire program must be run,
 ``bugpoint`` will compile the code it deems to not be affected with the C
 Backend, and then link in the shared object it generates.

 To debug the JIT:

 .. code-block:: bash

    bugpoint -run-jit -output=[correct output file] [bitcode file]  \
             --tool-args -- [arguments to pass to lli]              \
             --args -- [program arguments]

 Similarly, to debug the LLC, one would run:

 .. code-block:: bash

    bugpoint -run-llc -output=[correct output file] [bitcode file]  \
             --tool-args -- [arguments to pass to llc]              \
             --args -- [program arguments]

 **Special note:** if you are debugging MultiSource or SPEC tests that
 already exist in the ``llvm/test`` hierarchy, there is an easier way to
 debug the JIT, LLC, and CBE, using the pre-written Makefile targets, which
 will pass the program options specified in the Makefiles:

 .. code-block:: bash

    cd llvm/test/../../program
    make bugpoint-jit

 At the end of a successful ``bugpoint`` run, you will be presented
 with two bitcode files: a *safe* file which can be compiled with the C
 backend and the *test* file which either LLC or the JIT
 mis-codegenerates, and thus causes the error.

 To reproduce the error that ``bugpoint`` found, it is sufficient to do
 the following:

 #. Regenerate the shared object from the safe bitcode file:

    .. code-block:: bash

       llc -march=c safe.bc -o safe.c
       gcc -shared safe.c -o safe.so

 #. If debugging LLC, compile test bitcode native and link with the shared
    object:

    .. code-block:: bash

       llc test.bc -o test.s
       gcc test.s safe.so -o test.llc
       ./test.llc [program options]

 #. If debugging the JIT, load the shared object and supply the test
    bitcode:

    .. code-block:: bash

       lli -load=safe.so test.bc [program options]
	================================
	How to submit an LLVM bug report
	================================

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


	If you're working with LLVM and run into 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 figure out which component is crashing (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 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 should be able to debug this with the following
	:doc:`bugpoint <Bugpoint>` command:

	.. code-block:: bash

	bugpoint foo.bc -O3

	Run this, then file a bug with the instructions and reduced .bc
	files that bugpoint emits.

	If bugpoint doesn't reproduce the crash, ``llvm-reduce`` is an alternative
	way to reduce LLVM IR. Create a script that repros the crash and run:

	.. code-block:: bash

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

	which should produce reduced IR that reproduces the crash. Be warned the
	``llvm-reduce`` is still fairly immature and may crash.

	If none of the above work, you can get the IR before a crash by running the
	``opt`` command with 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 your 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 do crash, you should be able to reduce
	this with one of the following :doc:`bugpoint <Bugpoint>` command lines (use
	the one corresponding to the command above that failed):

	#. ``bugpoint -run-llc foo.bc``
	#. ``bugpoint -run-llc foo.bc --tool-args -relocation-model=pic``
	#. ``bugpoint -run-llc foo.bc --tool-args -relocation-model=static``

	Please run this, then file a bug with the instructions and reduced .bc file
	that bugpoint emits. If something goes wrong with bugpoint, 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, you should be using 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 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.

	Incorrect code generation
	=========================

	Similarly to debugging incorrect compilation by mis-behaving passes, you
	can debug incorrect code generation by either LLC or the JIT, using
	``bugpoint``. The process ``bugpoint`` follows in this case is to try to
	narrow the code down to a function that is miscompiled by one or the other
	method, but since for correctness, the entire program must be run,
	``bugpoint`` will compile the code it deems to not be affected with the C
	Backend, and then link in the shared object it generates.

	To debug the JIT:

	.. code-block:: bash

	bugpoint -run-jit -output=[correct output file] [bitcode file] \
	--tool-args -- [arguments to pass to lli] \
	--args -- [program arguments]

	Similarly, to debug the LLC, one would run:

	.. code-block:: bash

	bugpoint -run-llc -output=[correct output file] [bitcode file] \
	--tool-args -- [arguments to pass to llc] \
	--args -- [program arguments]

	Special note: if you are debugging MultiSource or SPEC tests that
	already exist in the ``llvm/test`` hierarchy, there is an easier way to
	debug the JIT, LLC, and CBE, using the pre-written Makefile targets, which
	will pass the program options specified in the Makefiles:

	.. code-block:: bash

	cd llvm/test/../../program
	make bugpoint-jit

	At the end of a successful ``bugpoint`` run, you will be presented
	with two bitcode files: a safe file which can be compiled with the C
	backend and the test file which either LLC or the JIT
	mis-codegenerates, and thus causes the error.

	To reproduce the error that ``bugpoint`` found, it is sufficient to do
	the following:

	#. Regenerate the shared object from the safe bitcode file:

	.. code-block:: bash

	llc -march=c safe.bc -o safe.c
	gcc -shared safe.c -o safe.so

	#. If debugging LLC, compile test bitcode native and link with the shared
	object:

	.. code-block:: bash

	llc test.bc -o test.s
	gcc test.s safe.so -o test.llc
	./test.llc [program options]

	#. If debugging the JIT, load the shared object and supply the test
	bitcode:

	.. code-block:: bash

	lli -load=safe.so test.bc [program options]