Add documentation on git bisecting across the MLIR merge.

[mehdi_amini]

Nice doc! Thanks.

llvm/docs/GitBisecting.rst

98

Why does it confuse git bisect? I would expect your run script to try ninja -C ../llvm-build-project clang || exit 125 and just skip the revisions?

108

Using directory filtering to drive the bisection and skip unrelated revisions seems like a good general recommendation independently of MLIR or merges.
Someone bisecting in lld for example could skip most revisions.

118

Seems like it answers my previous question (should this explanation be moved there?).

The part about git being confused about directories appearing and disappearing is a bit strange: surely there are many directories that will appear and disappear during the bisection?

[thakis]

Thanks for the review, and sorry about dragging my feet -- my llvm repo was busy most of last week doing bisects :P

llvm/docs/GitBisecting.rst

98

I tried to reproduce the confused git bisect, but I only managed to do it if I hit ctrl-c during a bisect step. Let me replace this with a "don't hit ctrl-c while a bisect is running".

118

See above. I'll remove this paragraph.

llvm/docs/GitBisecting.rst

+===================
+Bisecting LLVM code
+===================
+
+Introduction
+============
+
+``git bisect`` is a useful tool for finding which revision caused a bug.
+
+This document describes how to use ``git bisect``. In particular, while LLVM
+has a mostly linear history, it has a few merge commits that added projects --
+and these merged the linear history of those projects. As a consequence, the
+LLVM repository has multiple roots: One "normal" root, and then one for each
+toplevel project that was developed out-of-tree and then merged later.
+As of early 2020, the only such merged project is MLIR, but flang will likely
+be merged in a similar way soon.
+
+Basic operation
+===============
+
+See https://git-scm.com/docs/git-bisect for a good overview. In summary:
+
+  .. code-block:: bash
+
+     git bisect start
+     git bisect bad master
+     git bisect good f00ba
+
+git will check out a revision in between. Try to reproduce your problem at
+that revision, and run ``git bisect good`` or ``git bisect bad``.
+
+If you can't repro at the current commit (maybe the build is broken), run
+``git bisect skip`` and git will pick a nearby alternate commit.
+
+(To abort a bisect, run ``git bisect reset``, and if git complains about not
+being able to reset, do the usual ``git checkout -f master; git reset --hard
+origin/master`` dance and try again).
+
+``git bisect run``
+==================
+
+A single bisect step often requires first building clang, and then compiling
+a large code base with just-built clang. This can take a long time, so it's
+good if it can happen completely automatically. ``git bisect run`` can do
+this for you if you write a run script that reproduces the problem
+automatically. Writing the script can take 10-20 minutes, but it's almost
+always worth it -- you can do something else while the bisect runs (such
+as writing this document).
+
+Here's an example run script. It assumes that you're in ``llvm-project`` and
+that you have a sibling ``llvm-build-project`` build directory where you
+configured CMake to use Ninja. You have a file ``repro.c`` in the current
+directory that makes clang crash at trunk, but it worked fine at revision
+``f00ba``.
+
+  .. code-block:: bash
+
+     # Build clang. If the build fails, `exit 125` causes this
+     # revision to be skipped
+     ninja -C ../llvm-build-project clang || exit 125
+
+     ../llvm-build-project/bin/clang repro.c
+
+To make sure your run script works, it's a good idea to run ``./run.sh`` by
+hand and tweak the script until it works, then run ``git bisect good`` or
+``git bisect bad`` manually once based on the result of the script 
+(check ``echo $?`` after your script ran), and only then run ``git bisect run
+./run.sh``. Don't forget to mark your run script as executable -- ``git bisect
+run`` doesn't check for that, it just assumes the run script failed each time.
+
+Once your run script works, run ``git bisect run ./run.sh`` and a few hours
+later you'll know which commit caused the regression.
+
+(This is a very simple run script. Often, you want to use just-built clang
+to build a different project and then run a built executable of that project
+in the run script.)
+
+Bisecting across multiple roots
+===============================
+
+Here's how LLVM's history currently looks:
+
+  .. code-block::
+
+     A-o-o-......-o-D-o-o-HEAD
+                   /
+       B-o-...-o-C-
+ 
+``A`` is the first commit in LLVM ever, ``97724f18c79c``.
+
+``B`` is the first commit in MLIR, ``aed0d21a62db``.  ``D`` is the merge commit
+that merged MLIR into the main LLVM repository, ``0f0d0ed1c78f``. ``C`` is the
+last commit in MLIR before it got merged, ``0f0d0ed1c78f^2``. (The ``^n``
+modifier selects the n'th parent of a merge commit.)
+
+``git bisect`` goes through all parent revisions. Due to the way MLIR was
+merged, at every revision at ``C`` or earlier, *only* the ``mlir/`` directory
+exists, and nothing else does. This confused both ``git bisect`` and most
+run scripts.
+
+As of early 2020, there is no flag to ``git bisect`` to tell it to not
+descend into all reachable commits. Ideally, we'd want to tell it to only
+follow the first parent of ``D``.
+
+The best workaround is to pass a list of directories to ``git bisect``:
+If you know the bug is due to a change in llvm, clang, or compiler-rt, use
+``git bisect start -- clang llvm compiler-rt``. That way, the commits in
+``mlir`` are never evaluated.
+
+Alternatively, ``git bisect skip aed0d21a6 aed0d21a6..0f0d0ed1c78f`` explicitly
+skips all commits on that branch. It takes 1.5 minutes to run on a fast
+machine, and makes ``git bisect log`` output unreadable. (``aed0d21a6`` is
+listed twice because git ranges exclude the revision listed on the left,
+so it needs to be ignored explicitly.)
+
+Putting something like ``test -e clang || exit 125`` at the top of your
+``run.sh`` script does *not* work: git seems to get confused about the
+directories appearing and disappearing and soon errors out with XXX
+
+More Resources
+==============
+
+https://git-scm.com/book/en/v2/Git-Tools-Revision-Selection