[Docs] Update HowToSubmitABug

[aeubanks]

Summary

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• Remove outdated reference to delta and plug creduce instead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

[xbolva00]

llvm/docs/HowToSubmitABug.rst

79

maybe mention cvise too? Should be better than creduce.

https://github.com/marxin/cvise

[MaskRay]

@nickdesaulniers

llvm/docs/HowToSubmitABug.rst

94

Worth mentioning that -Xclang -disable-O0-optnone can disable emitted optnone.

134

I only use --print-module-scope with -filter-print-funcs...

165

Perhaps also: -fsanitize=address,undefined && -fsanitize=memory

[MaskRay]

llvm/docs/HowToSubmitABug.rst

75

Perhaps your toolchain has changed the default. For many *NIX platforms I think this is just /tmp/foo-xxxxx.c

[aeubanks]

address comments

[aeubanks]

llvm/docs/HowToSubmitABug.rst

94

I think that's too much information. Clang can generate different code for -O0 than -O1 and above even besides the optnone attribute.

134

-print-before-all won't print the entire module before a function pass, and you need the whole module.

165

added a link to sanitizers and mentioned -fsanitize=address,undefined. I'm worried about msan because you need to build the C++ standard library with it, or else you get false positives. It's discoverable through the link anyway.

[aeubanks]

ping

[MaskRay]

LG, but you may need to add more CCes to get wider audience for their experiences on troubleshooting and wait a day or two. If people want to contribute after this is submitted, they can send separate patches.

[fhahn]

Thanks for the update, LGTM

llvm/docs/HowToSubmitABug.rst

69

nit: `clang` (in double ticks) ?

[aeubanks]

address comments

[MaskRay]

llvm/docs/HowToSubmitABug.rst

107

Might worth a link to the bugpoint documentation page.

[aeubanks]

add links to bugpoint

llvm/docs/HowToSubmitABug.rst

 ================================
 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 an LLVM pass), 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.
+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://bugs.llvm.org/enter_bug.cgi>`_ and fill out the form with the
 necessary details (note that you don't need to pick a category, just use
 the "new-bugs" category if you're not sure).  The bug description should
 contain the following information:
 
 * All information necessary to reproduce the problem.

 =============
 
 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, optimizer or code
-generator), run the ``clang`` command line as you were when the crash
-occurred, but with the following extra command line options:
-
-* ``-O0 -emit-llvm``: 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 the section on :ref:`front-end bugs
-  <front-end>`.
+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 an optimizer bug.  Jump ahead to
-  `compile-time optimization bugs`_.
+  the code generator), you found a middle-end optimizer bug. Jump ahead to
+  :ref:`middle-end bugs <middleend-crash>`.
 
-* Otherwise, you have a code generator crash. Jump ahead to `code
-  generator bugs`_.
+* Otherwise, you have a backend code generator crash. Jump ahead to :ref:`code
+  generator bugs <backend-crash>`.
 
-.. _front-end bug:
-.. _front-end:
+.. _frontend-crash:
 
 Front-end bugs
 --------------
 
-If the problem is in the front-end, you should re-run the same ``clang``
-command that resulted in the crash, but add the ``-save-temps`` option.
-The compiler will crash again, but it will leave behind a ``foo.i`` file
-(containing preprocessed C source code) and possibly ``foo.s`` for each
-compiled ``foo.c`` file. Send us the ``foo.i`` file, along with the options
-you passed to ``clang``, and a brief description of the error it caused.
-
-The `delta <http://delta.tigris.org/>`_ tool helps to reduce the
-preprocessed file down to the smallest amount of code that still replicates
-the problem. You're encouraged to use delta to reduce the code to make the
-developers' lives easier. `This website
-<http://gcc.gnu.org/wiki/A_guide_to_testcase_reduction>`_ has instructions
-on the best way to use delta.
+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
 
-.. _compile-time optimization bugs:
+.. code-block:: text
 
-Compile-time optimization bugs
-------------------------------
+   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``".  Then run:
+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 -debug-pass=Arguments foo.bc -disable-output
+   opt -O3 foo.bc -disable-output
 
-This command should do two things: it should print out a list of passes, and
-then it should crash in the same way as clang.  If it doesn't crash, please
-follow the instructions for a `front-end bug`_.
+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
 bugpoint command:
 
 .. code-block:: bash
 
-   bugpoint foo.bc <list of passes printed by opt>
+   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
 
-Please run this, then file a bug with the instructions and reduced .bc
-files that bugpoint emits.  If something goes wrong with bugpoint, please
-submit the "foo.bc" file and the list of passes printed by ``opt``.
+which should produce reduced IR that reproduces the crash. Be warned the
+``llvm-reduce`` is still fairly immature and may crash.
 
-.. _code generator bugs:
+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.
 
-Code generator bugs
--------------------
+.. _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 `front-end
-bug`_.  If one of these do crash, you should be able to reduce this with
-one of the following bugpoint command lines (use the one corresponding to
-the command above that failed):
+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 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.
 
 .. _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 `valgrind
-<http://valgrind.org/>`_'s clean, passes purify, or some other memory
-checker tool. Many of the "LLVM bugs" that we have chased down ended up
-being bugs in the program being compiled, not LLVM.
+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` 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,