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