llvm/docs/WritingAnLLVMPass.rst

========================================
Writing an LLVM Pass (legacy PM version)
========================================

.. program:: opt

.. contents::
    :local:

Introduction --- What is a pass?
================================

.. warning::
  This document deals with the legacy pass manager. LLVM uses the new pass
  manager for the optimization pipeline (the codegen pipeline
  still uses the legacy pass manager), which has its own way of defining
  passes. For more details, see :doc:`WritingAnLLVMNewPMPass` and
  :doc:`NewPassManager`.

The LLVM Pass Framework is an important part of the LLVM system, because LLVM
passes are where most of the interesting parts of the compiler exist.  Passes
perform the transformations and optimizations that make up the compiler, they
build the analysis results that are used by these transformations, and they
are, above all, a structuring technique for compiler code.

All LLVM passes are subclasses of the `Pass
<https://llvm.org/doxygen/classllvm_1_1Pass.html>`_ class, which implement
functionality by overriding virtual methods inherited from ``Pass``.  Depending
on how your pass works, you should inherit from the :ref:`ModulePass
<writing-an-llvm-pass-ModulePass>` , :ref:`CallGraphSCCPass
<writing-an-llvm-pass-CallGraphSCCPass>`, :ref:`FunctionPass
<writing-an-llvm-pass-FunctionPass>` , or :ref:`LoopPass
<writing-an-llvm-pass-LoopPass>`, or :ref:`RegionPass
<writing-an-llvm-pass-RegionPass>` classes, which gives the system more
information about what your pass does, and how it can be combined with other
passes.  One of the main features of the LLVM Pass Framework is that it
schedules passes to run in an efficient way based on the constraints that your
pass meets (which are indicated by which class they derive from).

.. _writing-an-llvm-pass-pass-classes:

Pass classes and requirements
=============================

One of the first things that you should do when designing a new pass is to
decide what class you should subclass for your pass. Here we talk about the
classes available, from the most general to the most specific.

When choosing a superclass for your ``Pass``, you should choose the **most
specific** class possible, while still being able to meet the requirements
listed.  This gives the LLVM Pass Infrastructure information necessary to
optimize how passes are run, so that the resultant compiler isn't unnecessarily
slow.

The ``ImmutablePass`` class
---------------------------

The most plain and boring type of pass is the "`ImmutablePass
<https://llvm.org/doxygen/classllvm_1_1ImmutablePass.html>`_" class.  This pass
type is used for passes that do not have to be run, do not change state, and
never need to be updated.  This is not a normal type of transformation or
analysis, but can provide information about the current compiler configuration.

Although this pass class is very infrequently used, it is important for
providing information about the current target machine being compiled for, and
other static information that can affect the various transformations.

``ImmutablePass``\ es never invalidate other transformations, are never
invalidated, and are never "run".

.. _writing-an-llvm-pass-ModulePass:

The ``ModulePass`` class
------------------------

The `ModulePass <https://llvm.org/doxygen/classllvm_1_1ModulePass.html>`_ class
is the most general of all superclasses that you can use.  Deriving from
``ModulePass`` indicates that your pass uses the entire program as a unit,
referring to function bodies in no predictable order, or adding and removing
functions.  Because nothing is known about the behavior of ``ModulePass``
subclasses, no optimization can be done for their execution.

A module pass can use function level passes (e.g. dominators) using the
``getAnalysis`` interface ``getAnalysis<DominatorTree>(llvm::Function *)`` to
provide the function to retrieve analysis result for, if the function pass does
not require any module or immutable passes.  Note that this can only be done
for functions for which the analysis ran, e.g. in the case of dominators you
should only ask for the ``DominatorTree`` for function definitions, not
declarations.

To write a correct ``ModulePass`` subclass, derive from ``ModulePass`` and
override the ``runOnModule`` method with the following signature:

The ``runOnModule`` method
^^^^^^^^^^^^^^^^^^^^^^^^^^

.. code-block:: c++

  virtual bool runOnModule(Module &M) = 0;

The ``runOnModule`` method performs the interesting work of the pass.  It
should return ``true`` if the module was modified by the transformation and
``false`` otherwise.

.. _writing-an-llvm-pass-CallGraphSCCPass:

The ``CallGraphSCCPass`` class
------------------------------

The `CallGraphSCCPass
<https://llvm.org/doxygen/classllvm_1_1CallGraphSCCPass.html>`_ is used by
passes that need to traverse the program bottom-up on the call graph (callees
before callers).  Deriving from ``CallGraphSCCPass`` provides some mechanics
for building and traversing the ``CallGraph``, but also allows the system to
optimize execution of ``CallGraphSCCPass``\ es.  If your pass meets the
requirements outlined below, and doesn't meet the requirements of a
:ref:`FunctionPass <writing-an-llvm-pass-FunctionPass>`, you should derive from
``CallGraphSCCPass``.

``TODO``: explain briefly what SCC, Tarjan's algo, and B-U mean.

To be explicit, CallGraphSCCPass subclasses are:

#. ... *not allowed* to inspect or modify any ``Function``\ s other than those
   in the current SCC and the direct callers and direct callees of the SCC.
#. ... *required* to preserve the current ``CallGraph`` object, updating it to
   reflect any changes made to the program.
#. ... *not allowed* to add or remove SCC's from the current Module, though
   they may change the contents of an SCC.
#. ... *allowed* to add or remove global variables from the current Module.
#. ... *allowed* to maintain state across invocations of :ref:`runOnSCC
   <writing-an-llvm-pass-runOnSCC>` (including global data).

Implementing a ``CallGraphSCCPass`` is slightly tricky in some cases because it
has to handle SCCs with more than one node in it.  All of the virtual methods
described below should return ``true`` if they modified the program, or
``false`` if they didn't.

The ``doInitialization(CallGraph &)`` method
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

.. code-block:: c++

  virtual bool doInitialization(CallGraph &CG);

The ``doInitialization`` method is allowed to do most of the things that
``CallGraphSCCPass``\ es are not allowed to do.  They can add and remove
functions, get pointers to functions, etc.  The ``doInitialization`` method is
designed to do simple initialization type of stuff that does not depend on the
SCCs being processed.  The ``doInitialization`` method call is not scheduled to
overlap with any other pass executions (thus it should be very fast).

.. _writing-an-llvm-pass-runOnSCC:

The ``runOnSCC`` method
^^^^^^^^^^^^^^^^^^^^^^^

.. code-block:: c++

  virtual bool runOnSCC(CallGraphSCC &SCC) = 0;

The ``runOnSCC`` method performs the interesting work of the pass, and should
return ``true`` if the module was modified by the transformation, ``false``
otherwise.

The ``doFinalization(CallGraph &)`` method
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

.. code-block:: c++

  virtual bool doFinalization(CallGraph &CG);

The ``doFinalization`` method is an infrequently used method that is called
when the pass framework has finished calling :ref:`runOnSCC
<writing-an-llvm-pass-runOnSCC>` for every SCC in the program being compiled.

.. _writing-an-llvm-pass-FunctionPass:

The ``FunctionPass`` class
--------------------------

In contrast to ``ModulePass`` subclasses, `FunctionPass
<https://llvm.org/doxygen/classllvm_1_1Pass.html>`_ subclasses do have a
predictable, local behavior that can be expected by the system.  All
``FunctionPass`` execute on each function in the program independent of all of
the other functions in the program.  ``FunctionPass``\ es do not require that
they are executed in a particular order, and ``FunctionPass``\ es do not modify
external functions.

To be explicit, ``FunctionPass`` subclasses are not allowed to:

#. Inspect or modify a ``Function`` other than the one currently being processed.
#. Add or remove ``Function``\ s from the current ``Module``.
#. Add or remove global variables from the current ``Module``.
#. Maintain state across invocations of :ref:`runOnFunction
   <writing-an-llvm-pass-runOnFunction>` (including global data).

Implementing a ``FunctionPass`` is usually straightforward. ``FunctionPass``\
es may override three virtual methods to do their work.  All of these methods
should return ``true`` if they modified the program, or ``false`` if they
didn't.

.. _writing-an-llvm-pass-doInitialization-mod:

The ``doInitialization(Module &)`` method
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

.. code-block:: c++

  virtual bool doInitialization(Module &M);

The ``doInitialization`` method is allowed to do most of the things that
``FunctionPass``\ es are not allowed to do.  They can add and remove functions,
get pointers to functions, etc.  The ``doInitialization`` method is designed to
do simple initialization type of stuff that does not depend on the functions
being processed.  The ``doInitialization`` method call is not scheduled to
overlap with any other pass executions (thus it should be very fast).

A good example of how this method should be used is the `LowerAllocations
<https://llvm.org/doxygen/LowerAllocations_8cpp-source.html>`_ pass.  This pass
converts ``malloc`` and ``free`` instructions into platform dependent
``malloc()`` and ``free()`` function calls.  It uses the ``doInitialization``
method to get a reference to the ``malloc`` and ``free`` functions that it
needs, adding prototypes to the module if necessary.

.. _writing-an-llvm-pass-runOnFunction:

The ``runOnFunction`` method
^^^^^^^^^^^^^^^^^^^^^^^^^^^^

.. code-block:: c++

  virtual bool runOnFunction(Function &F) = 0;

The ``runOnFunction`` method must be implemented by your subclass to do the
transformation or analysis work of your pass.  As usual, a ``true`` value
should be returned if the function is modified.

.. _writing-an-llvm-pass-doFinalization-mod:

The ``doFinalization(Module &)`` method
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

.. code-block:: c++

  virtual bool doFinalization(Module &M);

The ``doFinalization`` method is an infrequently used method that is called
when the pass framework has finished calling :ref:`runOnFunction
<writing-an-llvm-pass-runOnFunction>` for every function in the program being
compiled.

.. _writing-an-llvm-pass-LoopPass:

The ``LoopPass`` class
----------------------

All ``LoopPass`` execute on each :ref:`loop <loop-terminology>` in the function
independent of all of the other loops in the function.  ``LoopPass`` processes
loops in loop nest order such that outer most loop is processed last.

``LoopPass`` subclasses are allowed to update loop nest using ``LPPassManager``
interface.  Implementing a loop pass is usually straightforward.
``LoopPass``\ es may override three virtual methods to do their work.  All
these methods should return ``true`` if they modified the program, or ``false``
if they didn't.

A ``LoopPass`` subclass which is intended to run as part of the main loop pass
pipeline needs to preserve all of the same *function* analyses that the other
loop passes in its pipeline require. To make that easier,
a ``getLoopAnalysisUsage`` function is provided by ``LoopUtils.h``. It can be
called within the subclass's ``getAnalysisUsage`` override to get consistent
and correct behavior. Analogously, ``INITIALIZE_PASS_DEPENDENCY(LoopPass)``
will initialize this set of function analyses.

The ``doInitialization(Loop *, LPPassManager &)`` method
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

.. code-block:: c++

  virtual bool doInitialization(Loop *, LPPassManager &LPM);

The ``doInitialization`` method is designed to do simple initialization type of
stuff that does not depend on the functions being processed.  The
``doInitialization`` method call is not scheduled to overlap with any other
pass executions (thus it should be very fast).  ``LPPassManager`` interface
should be used to access ``Function`` or ``Module`` level analysis information.

.. _writing-an-llvm-pass-runOnLoop:

The ``runOnLoop`` method
^^^^^^^^^^^^^^^^^^^^^^^^

.. code-block:: c++

  virtual bool runOnLoop(Loop *, LPPassManager &LPM) = 0;

The ``runOnLoop`` method must be implemented by your subclass to do the
transformation or analysis work of your pass.  As usual, a ``true`` value
should be returned if the function is modified.  ``LPPassManager`` interface
should be used to update loop nest.

The ``doFinalization()`` method
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

.. code-block:: c++

  virtual bool doFinalization();

The ``doFinalization`` method is an infrequently used method that is called
when the pass framework has finished calling :ref:`runOnLoop
<writing-an-llvm-pass-runOnLoop>` for every loop in the program being compiled.

.. _writing-an-llvm-pass-RegionPass:

The ``RegionPass`` class
------------------------

``RegionPass`` is similar to :ref:`LoopPass <writing-an-llvm-pass-LoopPass>`,
but executes on each single entry single exit region in the function.
``RegionPass`` processes regions in nested order such that the outer most
region is processed last.

``RegionPass`` subclasses are allowed to update the region tree by using the
``RGPassManager`` interface.  You may override three virtual methods of
``RegionPass`` to implement your own region pass.  All these methods should
return ``true`` if they modified the program, or ``false`` if they did not.

The ``doInitialization(Region *, RGPassManager &)`` method
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

.. code-block:: c++

  virtual bool doInitialization(Region *, RGPassManager &RGM);

The ``doInitialization`` method is designed to do simple initialization type of
stuff that does not depend on the functions being processed.  The
``doInitialization`` method call is not scheduled to overlap with any other
pass executions (thus it should be very fast).  ``RPPassManager`` interface
should be used to access ``Function`` or ``Module`` level analysis information.

.. _writing-an-llvm-pass-runOnRegion:

The ``runOnRegion`` method
^^^^^^^^^^^^^^^^^^^^^^^^^^

.. code-block:: c++

  virtual bool runOnRegion(Region *, RGPassManager &RGM) = 0;

The ``runOnRegion`` method must be implemented by your subclass to do the
transformation or analysis work of your pass.  As usual, a true value should be
returned if the region is modified.  ``RGPassManager`` interface should be used to
update region tree.

The ``doFinalization()`` method
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

.. code-block:: c++

  virtual bool doFinalization();

The ``doFinalization`` method is an infrequently used method that is called
when the pass framework has finished calling :ref:`runOnRegion
<writing-an-llvm-pass-runOnRegion>` for every region in the program being
compiled.


The ``MachineFunctionPass`` class
---------------------------------

A ``MachineFunctionPass`` is a part of the LLVM code generator that executes on
the machine-dependent representation of each LLVM function in the program.

Code generator passes are registered and initialized specially by
``TargetMachine::addPassesToEmitFile`` and similar routines, so they cannot
generally be run from the :program:`opt` or :program:`bugpoint` commands.

A ``MachineFunctionPass`` is also a ``FunctionPass``, so all the restrictions
that apply to a ``FunctionPass`` also apply to it.  ``MachineFunctionPass``\ es
also have additional restrictions.  In particular, ``MachineFunctionPass``\ es
are not allowed to do any of the following:

#. Modify or create any LLVM IR ``Instruction``\ s, ``BasicBlock``\ s,
   ``Argument``\ s, ``Function``\ s, ``GlobalVariable``\ s,
   ``GlobalAlias``\ es, or ``Module``\ s.
#. Modify a ``MachineFunction`` other than the one currently being processed.
#. Maintain state across invocations of :ref:`runOnMachineFunction
   <writing-an-llvm-pass-runOnMachineFunction>` (including global data).

.. _writing-an-llvm-pass-runOnMachineFunction:

The ``runOnMachineFunction(MachineFunction &MF)`` method
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

.. code-block:: c++

  virtual bool runOnMachineFunction(MachineFunction &MF) = 0;

``runOnMachineFunction`` can be considered the main entry point of a
``MachineFunctionPass``; that is, you should override this method to do the
work of your ``MachineFunctionPass``.

The ``runOnMachineFunction`` method is called on every ``MachineFunction`` in a
``Module``, so that the ``MachineFunctionPass`` may perform optimizations on
the machine-dependent representation of the function.  If you want to get at
the LLVM ``Function`` for the ``MachineFunction`` you're working on, use
``MachineFunction``'s ``getFunction()`` accessor method --- but remember, you
may not modify the LLVM ``Function`` or its contents from a
``MachineFunctionPass``.

.. _writing-an-llvm-pass-registration:

Pass registration
-----------------

Passes are registered with the ``RegisterPass`` template.  The template
parameter is the name of the pass that is to be used on the command line to
specify that the pass should be added to a program. The first argument is the
name of the pass, which is to be used for the :option:`-help` output of
programs, as well as for debug output generated by the `--debug-pass` option.

If you want your pass to be easily dumpable, you should implement the virtual
print method:

The ``print`` method
^^^^^^^^^^^^^^^^^^^^

.. code-block:: c++

  virtual void print(llvm::raw_ostream &O, const Module *M) const;

The ``print`` method must be implemented by "analyses" in order to print a
human readable version of the analysis results.  This is useful for debugging
an analysis itself, as well as for other people to figure out how an analysis
works.  Use the opt ``-analyze`` argument to invoke this method.

The ``llvm::raw_ostream`` parameter specifies the stream to write the results
on, and the ``Module`` parameter gives a pointer to the top level module of the
program that has been analyzed.  Note however that this pointer may be ``NULL``
in certain circumstances (such as calling the ``Pass::dump()`` from a
debugger), so it should only be used to enhance debug output, it should not be
depended on.

.. _writing-an-llvm-pass-interaction:

Specifying interactions between passes
--------------------------------------

One of the main responsibilities of the ``PassManager`` is to make sure that
passes interact with each other correctly.  Because ``PassManager`` tries to
:ref:`optimize the execution of passes <writing-an-llvm-pass-passmanager>` it
must know how the passes interact with each other and what dependencies exist
between the various passes.  To track this, each pass can declare the set of
passes that are required to be executed before the current pass, and the passes
which are invalidated by the current pass.

Typically this functionality is used to require that analysis results are
computed before your pass is run.  Running arbitrary transformation passes can
invalidate the computed analysis results, which is what the invalidation set
specifies.  If a pass does not implement the :ref:`getAnalysisUsage
<writing-an-llvm-pass-getAnalysisUsage>` method, it defaults to not having any
prerequisite passes, and invalidating **all** other passes.

.. _writing-an-llvm-pass-getAnalysisUsage:

The ``getAnalysisUsage`` method
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

.. code-block:: c++

  virtual void getAnalysisUsage(AnalysisUsage &Info) const;

By implementing the ``getAnalysisUsage`` method, the required and invalidated
sets may be specified for your transformation.  The implementation should fill
in the `AnalysisUsage
<https://llvm.org/doxygen/classllvm_1_1AnalysisUsage.html>`_ object with
information about which passes are required and not invalidated.  To do this, a
pass may call any of the following methods on the ``AnalysisUsage`` object:

The ``AnalysisUsage::addRequired<>`` and ``AnalysisUsage::addRequiredTransitive<>`` methods
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

If your pass requires a previous pass to be executed (an analysis for example),
it can use one of these methods to arrange for it to be run before your pass.
LLVM has many different types of analyses and passes that can be required,
spanning the range from ``DominatorSet`` to ``BreakCriticalEdges``.  Requiring
``BreakCriticalEdges``, for example, guarantees that there will be no critical
edges in the CFG when your pass has been run.

Some analyses chain to other analyses to do their job.  For example, an
`AliasAnalysis <AliasAnalysis.html>`_ implementation is required to :ref:`chain
<aliasanalysis-chaining>` to other alias analysis passes.  In cases where
analyses chain, the ``addRequiredTransitive`` method should be used instead of
the ``addRequired`` method.  This informs the ``PassManager`` that the
transitively required pass should be alive as long as the requiring pass is.

The ``AnalysisUsage::addPreserved<>`` method
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

One of the jobs of the ``PassManager`` is to optimize how and when analyses are
run.  In particular, it attempts to avoid recomputing data unless it needs to.
For this reason, passes are allowed to declare that they preserve (i.e., they
don't invalidate) an existing analysis if it's available.  For example, a
simple constant folding pass would not modify the CFG, so it can't possibly
affect the results of dominator analysis.  By default, all passes are assumed
to invalidate all others.

The ``AnalysisUsage`` class provides several methods which are useful in
certain circumstances that are related to ``addPreserved``.  In particular, the
``setPreservesAll`` method can be called to indicate that the pass does not
modify the LLVM program at all (which is true for analyses), and the
``setPreservesCFG`` method can be used by transformations that change
instructions in the program but do not modify the CFG or terminator
instructions.

``addPreserved`` is particularly useful for transformations like
``BreakCriticalEdges``.  This pass knows how to update a small set of loop and
dominator related analyses if they exist, so it can preserve them, despite the
fact that it hacks on the CFG.

Example implementations of ``getAnalysisUsage``
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

.. code-block:: c++

  // This example modifies the program, but does not modify the CFG
  void LICM::getAnalysisUsage(AnalysisUsage &AU) const {
    AU.setPreservesCFG();
    AU.addRequired<LoopInfoWrapperPass>();
  }

.. _writing-an-llvm-pass-getAnalysis:

The ``getAnalysis<>`` and ``getAnalysisIfAvailable<>`` methods
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

The ``Pass::getAnalysis<>`` method is automatically inherited by your class,
providing you with access to the passes that you declared that you required
with the :ref:`getAnalysisUsage <writing-an-llvm-pass-getAnalysisUsage>`
method.  It takes a single template argument that specifies which pass class
you want, and returns a reference to that pass.  For example:

.. code-block:: c++

  bool LICM::runOnFunction(Function &F) {
    LoopInfo &LI = getAnalysis<LoopInfoWrapperPass>().getLoopInfo();
    //...
  }

This method call returns a reference to the pass desired.  You may get a
runtime assertion failure if you attempt to get an analysis that you did not
declare as required in your :ref:`getAnalysisUsage
<writing-an-llvm-pass-getAnalysisUsage>` implementation.  This method can be
called by your ``run*`` method implementation, or by any other local method
invoked by your ``run*`` method.

A module level pass can use function level analysis info using this interface.
For example:

.. code-block:: c++

  bool ModuleLevelPass::runOnModule(Module &M) {
    //...
    DominatorTree &DT = getAnalysis<DominatorTree>(Func);
    //...
  }

In above example, ``runOnFunction`` for ``DominatorTree`` is called by pass
manager before returning a reference to the desired pass.

If your pass is capable of updating analyses if they exist (e.g.,
``BreakCriticalEdges``, as described above), you can use the
``getAnalysisIfAvailable`` method, which returns a pointer to the analysis if
it is active.  For example:

.. code-block:: c++

  if (DominatorSet *DS = getAnalysisIfAvailable<DominatorSet>()) {
    // A DominatorSet is active.  This code will update it.
  }

Pass Statistics
===============

The `Statistic <https://llvm.org/doxygen/Statistic_8h_source.html>`_ class is
designed to be an easy way to expose various success metrics from passes.
These statistics are printed at the end of a run, when the :option:`-stats`
command line option is enabled on the command line.  See the :ref:`Statistics
section <Statistic>` in the Programmer's Manual for details.

.. _writing-an-llvm-pass-passmanager:

What PassManager does
---------------------

The `PassManager <https://llvm.org/doxygen/PassManager_8h_source.html>`_ `class
<https://llvm.org/doxygen/classllvm_1_1PassManager.html>`_ takes a list of
passes, ensures their :ref:`prerequisites <writing-an-llvm-pass-interaction>`
are set up correctly, and then schedules passes to run efficiently.  All of the
LLVM tools that run passes use the PassManager for execution of these passes.

The PassManager does two main things to try to reduce the execution time of a
series of passes:

#. **Share analysis results.**  The ``PassManager`` attempts to avoid
   recomputing analysis results as much as possible.  This means keeping track
   of which analyses are available already, which analyses get invalidated, and
   which analyses are needed to be run for a pass.  An important part of work
   is that the ``PassManager`` tracks the exact lifetime of all analysis
   results, allowing it to :ref:`free memory
   <writing-an-llvm-pass-releaseMemory>` allocated to holding analysis results
   as soon as they are no longer needed.

#. **Pipeline the execution of passes on the program.**  The ``PassManager``
   attempts to get better cache and memory usage behavior out of a series of
   passes by pipelining the passes together.  This means that, given a series
   of consecutive :ref:`FunctionPass <writing-an-llvm-pass-FunctionPass>`, it
   will execute all of the :ref:`FunctionPass
   <writing-an-llvm-pass-FunctionPass>` on the first function, then all of the
   :ref:`FunctionPasses <writing-an-llvm-pass-FunctionPass>` on the second
   function, etc... until the entire program has been run through the passes.

   This improves the cache behavior of the compiler, because it is only
   touching the LLVM program representation for a single function at a time,
   instead of traversing the entire program.  It reduces the memory consumption
   of compiler, because, for example, only one `DominatorSet
   <https://llvm.org/doxygen/classllvm_1_1DominatorSet.html>`_ needs to be
   calculated at a time.

The effectiveness of the ``PassManager`` is influenced directly by how much
information it has about the behaviors of the passes it is scheduling.  For
example, the "preserved" set is intentionally conservative in the face of an
unimplemented :ref:`getAnalysisUsage <writing-an-llvm-pass-getAnalysisUsage>`
method.  Not implementing when it should be implemented will have the effect of
not allowing any analysis results to live across the execution of your pass.

The ``PassManager`` class exposes a ``--debug-pass`` command line options that
is useful for debugging pass execution, seeing how things work, and diagnosing
when you should be preserving more analyses than you currently are.  (To get
information about all of the variants of the ``--debug-pass`` option, just type
"``llc -help-hidden``").

By using the --debug-pass=Structure option, for example, we can see inspect the
default optimization pipelines, e.g. (the output has been trimmed):

.. code-block:: console

  $ llc -mtriple=arm64-- -O3 -debug-pass=Structure file.ll > /dev/null
  (...)
  ModulePass Manager
  Pre-ISel Intrinsic Lowering
  FunctionPass Manager
    Expand large div/rem
    Expand fp
    Expand Atomic instructions
  SVE intrinsics optimizations
    FunctionPass Manager
      Dominator Tree Construction
  FunctionPass Manager
    Simplify the CFG
    Dominator Tree Construction
    Natural Loop Information
    Canonicalize natural loops
  (...)

.. _writing-an-llvm-pass-releaseMemory:

The ``releaseMemory`` method
^^^^^^^^^^^^^^^^^^^^^^^^^^^^

.. code-block:: c++

  virtual void releaseMemory();

The ``PassManager`` automatically determines when to compute analysis results,
and how long to keep them around for.  Because the lifetime of the pass object
itself is effectively the entire duration of the compilation process, we need
some way to free analysis results when they are no longer useful.  The
``releaseMemory`` virtual method is the way to do this.

If you are writing an analysis or any other pass that retains a significant
amount of state (for use by another pass which "requires" your pass and uses
the :ref:`getAnalysis <writing-an-llvm-pass-getAnalysis>` method) you should
implement ``releaseMemory`` to, well, release the memory allocated to maintain
this internal state.  This method is called after the ``run*`` method for the
class, before the next call of ``run*`` in your pass.

Registering dynamically loaded passes
=====================================

*Size matters* when constructing production quality tools using LLVM, both for
the purposes of distribution, and for regulating the resident code size when
running on the target system.  Therefore, it becomes desirable to selectively
use some passes, while omitting others and maintain the flexibility to change
configurations later on.  You want to be able to do all this, and, provide
feedback to the user.  This is where pass registration comes into play.

The fundamental mechanisms for pass registration are the
``MachinePassRegistry`` class and subclasses of ``MachinePassRegistryNode``.

An instance of ``MachinePassRegistry`` is used to maintain a list of
``MachinePassRegistryNode`` objects.  This instance maintains the list and
communicates additions and deletions to the command line interface.

An instance of ``MachinePassRegistryNode`` subclass is used to maintain
information provided about a particular pass.  This information includes the
command line name, the command help string and the address of the function used
to create an instance of the pass.  A global static constructor of one of these
instances *registers* with a corresponding ``MachinePassRegistry``, the static
destructor *unregisters*.  Thus a pass that is statically linked in the tool
will be registered at start up.  A dynamically loaded pass will register on
load and unregister at unload.

Using existing registries
-------------------------

There are predefined registries to track instruction scheduling
(``RegisterScheduler``) and register allocation (``RegisterRegAlloc``) machine
passes.  Here we will describe how to *register* a register allocator machine
pass.

Implement your register allocator machine pass.  In your register allocator
``.cpp`` file add the following include:

.. code-block:: c++

  #include "llvm/CodeGen/RegAllocRegistry.h"

Also in your register allocator ``.cpp`` file, define a creator function in the
form:

.. code-block:: c++

  FunctionPass *createMyRegisterAllocator() {
    return new MyRegisterAllocator();
  }

Note that the signature of this function should match the type of
``RegisterRegAlloc::FunctionPassCtor``.  In the same file add the "installing"
declaration, in the form:

.. code-block:: c++

  static RegisterRegAlloc myRegAlloc("myregalloc",
                                     "my register allocator help string",
                                     createMyRegisterAllocator);

Note the two spaces prior to the help string produces a tidy result on the
:option:`-help` query.

.. code-block:: console

  $ llc -help
    ...
    -regalloc                    - Register allocator to use (default=linearscan)
      =linearscan                -   linear scan register allocator
      =local                     -   local register allocator
      =simple                    -   simple register allocator
      =myregalloc                -   my register allocator help string
    ...

And that's it.  The user is now free to use ``-regalloc=myregalloc`` as an
option.  Registering instruction schedulers is similar except use the
``RegisterScheduler`` class.  Note that the
``RegisterScheduler::FunctionPassCtor`` is significantly different from
``RegisterRegAlloc::FunctionPassCtor``.

To force the load/linking of your register allocator into the
:program:`llc`/:program:`lli` tools, add your creator function's global
declaration to ``Passes.h`` and add a "pseudo" call line to
``llvm/Codegen/LinkAllCodegenComponents.h``.

Creating new registries
-----------------------

The easiest way to get started is to clone one of the existing registries; we
recommend ``llvm/CodeGen/RegAllocRegistry.h``.  The key things to modify are
the class name and the ``FunctionPassCtor`` type.

Then you need to declare the registry.  Example: if your pass registry is
``RegisterMyPasses`` then define:

.. code-block:: c++

  MachinePassRegistry<RegisterMyPasses::FunctionPassCtor> RegisterMyPasses::Registry;

And finally, declare the command line option for your passes.  Example:

.. code-block:: c++

  cl::opt<RegisterMyPasses::FunctionPassCtor, false,
          RegisterPassParser<RegisterMyPasses> >
  MyPassOpt("mypass",
            cl::init(&createDefaultMyPass),
            cl::desc("my pass option help"));

Here the command option is "``mypass``", with ``createDefaultMyPass`` as the
default creator.

Using GDB with dynamically loaded passes
----------------------------------------

Unfortunately, using GDB with dynamically loaded passes is not as easy as it
should be.  First of all, you can't set a breakpoint in a shared object that
has not been loaded yet, and second of all there are problems with inlined
functions in shared objects.  Here are some suggestions to debugging your pass
with GDB.

For sake of discussion, I'm going to assume that you are debugging a
transformation invoked by :program:`opt`, although nothing described here
depends on that.

Setting a breakpoint in your pass
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

First thing you do is start gdb on the opt process:

.. code-block:: console

  $ gdb opt
  GNU gdb 5.0
  Copyright 2000 Free Software Foundation, Inc.
  GDB is free software, covered by the GNU General Public License, and you are
  welcome to change it and/or distribute copies of it under certain conditions.
  Type "show copying" to see the conditions.
  There is absolutely no warranty for GDB.  Type "show warranty" for details.
  This GDB was configured as "sparc-sun-solaris2.6"...
  (gdb)

Note that :program:`opt` has a lot of debugging information in it, so it takes
time to load.  Be patient.  Since we cannot set a breakpoint in our pass yet
(the shared object isn't loaded until runtime), we must execute the process,
and have it stop before it invokes our pass, but after it has loaded the shared
object.  The most foolproof way of doing this is to set a breakpoint in
``PassManager::run`` and then run the process with the arguments you want:

.. code-block:: console

  $ (gdb) break llvm::PassManager::run
  Breakpoint 1 at 0x2413bc: file Pass.cpp, line 70.
  (gdb) run test.bc -load $(LLVMTOP)/llvm/Debug+Asserts/lib/[libname].so -[passoption]
  Starting program: opt test.bc -load $(LLVMTOP)/llvm/Debug+Asserts/lib/[libname].so -[passoption]
  Breakpoint 1, PassManager::run (this=0xffbef174, M=@0x70b298) at Pass.cpp:70
  70      bool PassManager::run(Module &M) { return PM->run(M); }
  (gdb)

Once the :program:`opt` stops in the ``PassManager::run`` method you are now
free to set breakpoints in your pass so that you can trace through execution or
do other standard debugging stuff.

Miscellaneous Problems
^^^^^^^^^^^^^^^^^^^^^^

Once you have the basics down, there are a couple of problems that GDB has,
some with solutions, some without.

* Inline functions have bogus stack information.  In general, GDB does a pretty
  good job getting stack traces and stepping through inline functions.  When a
  pass is dynamically loaded however, it somehow completely loses this
  capability.  The only solution I know of is to de-inline a function (move it
  from the body of a class to a ``.cpp`` file).

* Restarting the program breaks breakpoints.  After following the information
  above, you have succeeded in getting some breakpoints planted in your pass.
  Next thing you know, you restart the program (i.e., you type "``run``" again),
  and you start getting errors about breakpoints being unsettable.  The only
  way I have found to "fix" this problem is to delete the breakpoints that are
  already set in your pass, run the program, and re-set the breakpoints once
  execution stops in ``PassManager::run``.

Hopefully these tips will help with common case debugging situations.  If you'd
like to contribute some tips of your own, just contact `Chris
<mailto:sabre@nondot.org>`_.