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230 lines
8.4 KiB
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================================
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How to submit an LLVM bug report
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================================
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Introduction - Got bugs?
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========================
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If you're working with LLVM and run into a bug, we definitely want to know
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about it. This document describes what you can do to increase the odds of
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getting it fixed quickly.
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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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Once you have a reduced test-case, go to `the LLVM Bug Tracking System
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<http://llvm.org/bugs/enter_bug.cgi>`_ and fill out the form with the
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necessary details (note that you don't need to pick a category, just use
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the "new-bugs" category if you're not sure). The bug description should
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contain the following information:
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* All information necessary to reproduce the problem.
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* The reduced test-case that triggers the bug.
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* The location where you obtained LLVM (if not from our Subversion
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repository).
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Thanks for helping us make LLVM better!
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.. _crashes the compiler:
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Crashing Bugs
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=============
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More often than not, bugs in the compiler cause it to crash---often due to
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an assertion failure of some sort. The most important piece of the puzzle
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is to figure out if it is crashing in the GCC 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 ``llvm-gcc`` 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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* ``-O0 -emit-llvm``: If ``llvm-gcc`` 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``: If ``llvm-gcc`` 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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* Otherwise, you have a code generator crash. Jump ahead to `code
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generator bugs`_.
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.. _front-end bug:
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.. _front-end:
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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 ``llvm-gcc``
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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 ``llvm-gcc``, and a brief description of the error it caused.
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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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.. _compile-time optimization bugs:
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Compile-time 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 -O0 -c -o foo.bc``".
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Then run:
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.. code-block:: bash
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opt -std-compile-opts -debug-pass=Arguments 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 llvm-gcc. If it doesn't crash, please
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follow the instructions for a `front-end bug`_.
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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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.. code-block:: bash
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bugpoint foo.bc <list of passes printed by opt>
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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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.. _code generator bugs:
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Code generator bugs
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-------------------
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If you find a bug that crashes llvm-gcc 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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llvm-gcc (in addition to the options you already pass). Once your have
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foo.bc, one of the following commands should fail:
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#. ``llc foo.bc``
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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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#. ``bugpoint -run-llc foo.bc``
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#. ``bugpoint -run-llc foo.bc --tool-args -relocation-model=pic``
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#. ``bugpoint -run-llc foo.bc --tool-args -relocation-model=static``
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Please run this, then file a bug with the instructions and reduced .bc file
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that bugpoint emits. If something goes wrong with bugpoint, please submit
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the "foo.bc" file and the option that llc crashes with.
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.. _miscompiling:
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Miscompilations
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===============
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If llvm-gcc 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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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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and optionally a series of LLVM passes to run. For example:
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.. code-block:: bash
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bugpoint -run-llc [... optzn passes ...] file-to-test.bc --args -- [program arguments]
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bugpoint will try to narrow down your list of passes to the one pass that
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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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Incorrect code generation
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=========================
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Similarly to debugging incorrect compilation by mis-behaving passes, you
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can debug incorrect code generation by either LLC or the JIT, using
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``bugpoint``. The process ``bugpoint`` follows in this case is to try to
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narrow the code down to a function that is miscompiled by one or the other
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method, but since for correctness, the entire program must be run,
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``bugpoint`` will compile the code it deems to not be affected with the C
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Backend, and then link in the shared object it generates.
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To debug the JIT:
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.. code-block:: bash
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bugpoint -run-jit -output=[correct output file] [bitcode file] \
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--tool-args -- [arguments to pass to lli] \
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--args -- [program arguments]
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Similarly, to debug the LLC, one would run:
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.. code-block:: bash
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bugpoint -run-llc -output=[correct output file] [bitcode file] \
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--tool-args -- [arguments to pass to llc] \
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--args -- [program arguments]
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**Special note:** if you are debugging MultiSource or SPEC tests that
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already exist in the ``llvm/test`` hierarchy, there is an easier way to
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debug the JIT, LLC, and CBE, using the pre-written Makefile targets, which
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will pass the program options specified in the Makefiles:
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.. code-block:: bash
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cd llvm/test/../../program
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make bugpoint-jit
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At the end of a successful ``bugpoint`` run, you will be presented
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with two bitcode files: a *safe* file which can be compiled with the C
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backend and the *test* file which either LLC or the JIT
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mis-codegenerates, and thus causes the error.
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To reproduce the error that ``bugpoint`` found, it is sufficient to do
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the following:
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#. Regenerate the shared object from the safe bitcode file:
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.. code-block:: bash
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llc -march=c safe.bc -o safe.c
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gcc -shared safe.c -o safe.so
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#. If debugging LLC, compile test bitcode native and link with the shared
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object:
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.. code-block:: bash
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llc test.bc -o test.s
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gcc test.s safe.so -o test.llc
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./test.llc [program options]
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#. If debugging the JIT, load the shared object and supply the test
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bitcode:
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.. code-block:: bash
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lli -load=safe.so test.bc [program options]
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