mirror of
https://github.com/c64scene-ar/llvm-6502.git
synced 2024-08-25 16:29:20 +00:00
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@159199 91177308-0d34-0410-b5e6-96231b3b80d8
219 lines
10 KiB
ReStructuredText
219 lines
10 KiB
ReStructuredText
.. _bugpoint:
|
|
|
|
====================================
|
|
LLVM bugpoint tool: design and usage
|
|
====================================
|
|
|
|
.. contents::
|
|
:local:
|
|
|
|
Description
|
|
===========
|
|
|
|
``bugpoint`` narrows down the source of problems in LLVM tools and passes. It
|
|
can be used to debug three types of failures: optimizer crashes, miscompilations
|
|
by optimizers, or bad native code generation (including problems in the static
|
|
and JIT compilers). It aims to reduce large test cases to small, useful ones.
|
|
For example, if ``opt`` crashes while optimizing a file, it will identify the
|
|
optimization (or combination of optimizations) that causes the crash, and reduce
|
|
the file down to a small example which triggers the crash.
|
|
|
|
For detailed case scenarios, such as debugging ``opt``, or one of the LLVM code
|
|
generators, see `How To Submit a Bug Report document <HowToSubmitABug.html>`_.
|
|
|
|
Design Philosophy
|
|
=================
|
|
|
|
``bugpoint`` is designed to be a useful tool without requiring any hooks into
|
|
the LLVM infrastructure at all. It works with any and all LLVM passes and code
|
|
generators, and does not need to "know" how they work. Because of this, it may
|
|
appear to do stupid things or miss obvious simplifications. ``bugpoint`` is
|
|
also designed to trade off programmer time for computer time in the
|
|
compiler-debugging process; consequently, it may take a long period of
|
|
(unattended) time to reduce a test case, but we feel it is still worth it. Note
|
|
that ``bugpoint`` is generally very quick unless debugging a miscompilation
|
|
where each test of the program (which requires executing it) takes a long time.
|
|
|
|
Automatic Debugger Selection
|
|
----------------------------
|
|
|
|
``bugpoint`` reads each ``.bc`` or ``.ll`` file specified on the command line
|
|
and links them together into a single module, called the test program. If any
|
|
LLVM passes are specified on the command line, it runs these passes on the test
|
|
program. If any of the passes crash, or if they produce malformed output (which
|
|
causes the verifier to abort), ``bugpoint`` starts the `crash debugger`_.
|
|
|
|
Otherwise, if the ``-output`` option was not specified, ``bugpoint`` runs the
|
|
test program with the "safe" backend (which is assumed to generate good code) to
|
|
generate a reference output. Once ``bugpoint`` has a reference output for the
|
|
test program, it tries executing it with the selected code generator. If the
|
|
selected code generator crashes, ``bugpoint`` starts the `crash debugger`_ on
|
|
the code generator. Otherwise, if the resulting output differs from the
|
|
reference output, it assumes the difference resulted from a code generator
|
|
failure, and starts the `code generator debugger`_.
|
|
|
|
Finally, if the output of the selected code generator matches the reference
|
|
output, ``bugpoint`` runs the test program after all of the LLVM passes have
|
|
been applied to it. If its output differs from the reference output, it assumes
|
|
the difference resulted from a failure in one of the LLVM passes, and enters the
|
|
`miscompilation debugger`_. Otherwise, there is no problem ``bugpoint`` can
|
|
debug.
|
|
|
|
.. _crash debugger:
|
|
|
|
Crash debugger
|
|
--------------
|
|
|
|
If an optimizer or code generator crashes, ``bugpoint`` will try as hard as it
|
|
can to reduce the list of passes (for optimizer crashes) and the size of the
|
|
test program. First, ``bugpoint`` figures out which combination of optimizer
|
|
passes triggers the bug. This is useful when debugging a problem exposed by
|
|
``opt``, for example, because it runs over 38 passes.
|
|
|
|
Next, ``bugpoint`` tries removing functions from the test program, to reduce its
|
|
size. Usually it is able to reduce a test program to a single function, when
|
|
debugging intraprocedural optimizations. Once the number of functions has been
|
|
reduced, it attempts to delete various edges in the control flow graph, to
|
|
reduce the size of the function as much as possible. Finally, ``bugpoint``
|
|
deletes any individual LLVM instructions whose absence does not eliminate the
|
|
failure. At the end, ``bugpoint`` should tell you what passes crash, give you a
|
|
bitcode file, and give you instructions on how to reproduce the failure with
|
|
``opt`` or ``llc``.
|
|
|
|
.. _code generator debugger:
|
|
|
|
Code generator debugger
|
|
-----------------------
|
|
|
|
The code generator debugger attempts to narrow down the amount of code that is
|
|
being miscompiled by the selected code generator. To do this, it takes the test
|
|
program and partitions it into two pieces: one piece which it compiles with the
|
|
"safe" backend (into a shared object), and one piece which it runs with either
|
|
the JIT or the static LLC compiler. It uses several techniques to reduce the
|
|
amount of code pushed through the LLVM code generator, to reduce the potential
|
|
scope of the problem. After it is finished, it emits two bitcode files (called
|
|
"test" [to be compiled with the code generator] and "safe" [to be compiled with
|
|
the "safe" backend], respectively), and instructions for reproducing the
|
|
problem. The code generator debugger assumes that the "safe" backend produces
|
|
good code.
|
|
|
|
.. _miscompilation debugger:
|
|
|
|
Miscompilation debugger
|
|
-----------------------
|
|
|
|
The miscompilation debugger works similarly to the code generator debugger. It
|
|
works by splitting the test program into two pieces, running the optimizations
|
|
specified on one piece, linking the two pieces back together, and then executing
|
|
the result. It attempts to narrow down the list of passes to the one (or few)
|
|
which are causing the miscompilation, then reduce the portion of the test
|
|
program which is being miscompiled. The miscompilation debugger assumes that
|
|
the selected code generator is working properly.
|
|
|
|
Advice for using bugpoint
|
|
=========================
|
|
|
|
``bugpoint`` can be a remarkably useful tool, but it sometimes works in
|
|
non-obvious ways. Here are some hints and tips:
|
|
|
|
* In the code generator and miscompilation debuggers, ``bugpoint`` only works
|
|
with programs that have deterministic output. Thus, if the program outputs
|
|
``argv[0]``, the date, time, or any other "random" data, ``bugpoint`` may
|
|
misinterpret differences in these data, when output, as the result of a
|
|
miscompilation. Programs should be temporarily modified to disable outputs
|
|
that are likely to vary from run to run.
|
|
|
|
* In the code generator and miscompilation debuggers, debugging will go faster
|
|
if you manually modify the program or its inputs to reduce the runtime, but
|
|
still exhibit the problem.
|
|
|
|
* ``bugpoint`` is extremely useful when working on a new optimization: it helps
|
|
track down regressions quickly. To avoid having to relink ``bugpoint`` every
|
|
time you change your optimization however, have ``bugpoint`` dynamically load
|
|
your optimization with the ``-load`` option.
|
|
|
|
* ``bugpoint`` can generate a lot of output and run for a long period of time.
|
|
It is often useful to capture the output of the program to file. For example,
|
|
in the C shell, you can run:
|
|
|
|
.. code-block:: bash
|
|
|
|
bugpoint ... |& tee bugpoint.log
|
|
|
|
to get a copy of ``bugpoint``'s output in the file ``bugpoint.log``, as well
|
|
as on your terminal.
|
|
|
|
* ``bugpoint`` cannot debug problems with the LLVM linker. If ``bugpoint``
|
|
crashes before you see its "All input ok" message, you might try ``llvm-link
|
|
-v`` on the same set of input files. If that also crashes, you may be
|
|
experiencing a linker bug.
|
|
|
|
* ``bugpoint`` is useful for proactively finding bugs in LLVM. Invoking
|
|
``bugpoint`` with the ``-find-bugs`` option will cause the list of specified
|
|
optimizations to be randomized and applied to the program. This process will
|
|
repeat until a bug is found or the user kills ``bugpoint``.
|
|
|
|
What to do when bugpoint isn't enough
|
|
=====================================
|
|
|
|
Sometimes, ``bugpoint`` is not enough. In particular, InstCombine and
|
|
TargetLowering both have visitor structured code with lots of potential
|
|
transformations. If the process of using bugpoint has left you with still too
|
|
much code to figure out and the problem seems to be in instcombine, the
|
|
following steps may help. These same techniques are useful with TargetLowering
|
|
as well.
|
|
|
|
Turn on ``-debug-only=instcombine`` and see which transformations within
|
|
instcombine are firing by selecting out lines with "``IC``" in them.
|
|
|
|
At this point, you have a decision to make. Is the number of transformations
|
|
small enough to step through them using a debugger? If so, then try that.
|
|
|
|
If there are too many transformations, then a source modification approach may
|
|
be helpful. In this approach, you can modify the source code of instcombine to
|
|
disable just those transformations that are being performed on your test input
|
|
and perform a binary search over the set of transformations. One set of places
|
|
to modify are the "``visit*``" methods of ``InstCombiner`` (*e.g.*
|
|
``visitICmpInst``) by adding a "``return false``" as the first line of the
|
|
method.
|
|
|
|
If that still doesn't remove enough, then change the caller of
|
|
``InstCombiner::DoOneIteration``, ``InstCombiner::runOnFunction`` to limit the
|
|
number of iterations.
|
|
|
|
You may also find it useful to use "``-stats``" now to see what parts of
|
|
instcombine are firing. This can guide where to put additional reporting code.
|
|
|
|
At this point, if the amount of transformations is still too large, then
|
|
inserting code to limit whether or not to execute the body of the code in the
|
|
visit function can be helpful. Add a static counter which is incremented on
|
|
every invocation of the function. Then add code which simply returns false on
|
|
desired ranges. For example:
|
|
|
|
.. code-block:: c++
|
|
|
|
|
|
static int calledCount = 0;
|
|
calledCount++;
|
|
DEBUG(if (calledCount < 212) return false);
|
|
DEBUG(if (calledCount > 217) return false);
|
|
DEBUG(if (calledCount == 213) return false);
|
|
DEBUG(if (calledCount == 214) return false);
|
|
DEBUG(if (calledCount == 215) return false);
|
|
DEBUG(if (calledCount == 216) return false);
|
|
DEBUG(dbgs() << "visitXOR calledCount: " << calledCount << "\n");
|
|
DEBUG(dbgs() << "I: "; I->dump());
|
|
|
|
could be added to ``visitXOR`` to limit ``visitXor`` to being applied only to
|
|
calls 212 and 217. This is from an actual test case and raises an important
|
|
point---a simple binary search may not be sufficient, as transformations that
|
|
interact may require isolating more than one call. In TargetLowering, use
|
|
``return SDNode();`` instead of ``return false;``.
|
|
|
|
Now that that the number of transformations is down to a manageable number, try
|
|
examining the output to see if you can figure out which transformations are
|
|
being done. If that can be figured out, then do the usual debugging. If which
|
|
code corresponds to the transformation being performed isn't obvious, set a
|
|
breakpoint after the call count based disabling and step through the code.
|
|
Alternatively, you can use "``printf``" style debugging to report waypoints.
|