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703 lines
30 KiB
ReStructuredText
.. _alias_analysis:
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==================================
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LLVM Alias Analysis Infrastructure
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==================================
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.. contents::
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:local:
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Introduction
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============
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Alias Analysis (aka Pointer Analysis) is a class of techniques which attempt to
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determine whether or not two pointers ever can point to the same object in
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memory. There are many different algorithms for alias analysis and many
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different ways of classifying them: flow-sensitive vs. flow-insensitive,
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context-sensitive vs. context-insensitive, field-sensitive
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vs. field-insensitive, unification-based vs. subset-based, etc. Traditionally,
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alias analyses respond to a query with a `Must, May, or No`_ alias response,
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indicating that two pointers always point to the same object, might point to the
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same object, or are known to never point to the same object.
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The LLVM `AliasAnalysis
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<http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html>`__ class is the
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primary interface used by clients and implementations of alias analyses in the
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LLVM system. This class is the common interface between clients of alias
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analysis information and the implementations providing it, and is designed to
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support a wide range of implementations and clients (but currently all clients
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are assumed to be flow-insensitive). In addition to simple alias analysis
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information, this class exposes Mod/Ref information from those implementations
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which can provide it, allowing for powerful analyses and transformations to work
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well together.
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This document contains information necessary to successfully implement this
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interface, use it, and to test both sides. It also explains some of the finer
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points about what exactly results mean. If you feel that something is unclear
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or should be added, please `let me know <mailto:sabre@nondot.org>`_.
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``AliasAnalysis`` Class Overview
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================================
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The `AliasAnalysis <http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html>`__
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class defines the interface that the various alias analysis implementations
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should support. This class exports two important enums: ``AliasResult`` and
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``ModRefResult`` which represent the result of an alias query or a mod/ref
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query, respectively.
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The ``AliasAnalysis`` interface exposes information about memory, represented in
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several different ways. In particular, memory objects are represented as a
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starting address and size, and function calls are represented as the actual
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``call`` or ``invoke`` instructions that performs the call. The
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``AliasAnalysis`` interface also exposes some helper methods which allow you to
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get mod/ref information for arbitrary instructions.
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All ``AliasAnalysis`` interfaces require that in queries involving multiple
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values, values which are not `constants <LangRef.html#constants>`_ are all
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defined within the same function.
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Representation of Pointers
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--------------------------
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Most importantly, the ``AliasAnalysis`` class provides several methods which are
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used to query whether or not two memory objects alias, whether function calls
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can modify or read a memory object, etc. For all of these queries, memory
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objects are represented as a pair of their starting address (a symbolic LLVM
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``Value*``) and a static size.
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Representing memory objects as a starting address and a size is critically
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important for correct Alias Analyses. For example, consider this (silly, but
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possible) C code:
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.. code-block:: c++
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int i;
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char C[2];
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char A[10];
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/* ... */
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for (i = 0; i != 10; ++i) {
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C[0] = A[i]; /* One byte store */
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C[1] = A[9-i]; /* One byte store */
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}
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In this case, the ``basicaa`` pass will disambiguate the stores to ``C[0]`` and
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``C[1]`` because they are accesses to two distinct locations one byte apart, and
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the accesses are each one byte. In this case, the Loop Invariant Code Motion
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(LICM) pass can use store motion to remove the stores from the loop. In
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constrast, the following code:
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.. code-block:: c++
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int i;
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char C[2];
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char A[10];
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/* ... */
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for (i = 0; i != 10; ++i) {
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((short*)C)[0] = A[i]; /* Two byte store! */
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C[1] = A[9-i]; /* One byte store */
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}
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In this case, the two stores to C do alias each other, because the access to the
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``&C[0]`` element is a two byte access. If size information wasn't available in
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the query, even the first case would have to conservatively assume that the
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accesses alias.
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.. _alias:
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The ``alias`` method
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--------------------
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The ``alias`` method is the primary interface used to determine whether or not
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two memory objects alias each other. It takes two memory objects as input and
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returns MustAlias, PartialAlias, MayAlias, or NoAlias as appropriate.
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Like all ``AliasAnalysis`` interfaces, the ``alias`` method requires that either
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the two pointer values be defined within the same function, or at least one of
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the values is a `constant <LangRef.html#constants>`_.
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.. _Must, May, or No:
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Must, May, and No Alias Responses
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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The ``NoAlias`` response may be used when there is never an immediate dependence
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between any memory reference *based* on one pointer and any memory reference
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*based* the other. The most obvious example is when the two pointers point to
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non-overlapping memory ranges. Another is when the two pointers are only ever
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used for reading memory. Another is when the memory is freed and reallocated
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between accesses through one pointer and accesses through the other --- in this
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case, there is a dependence, but it's mediated by the free and reallocation.
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As an exception to this is with the `noalias <LangRef.html#noalias>`_ keyword;
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the "irrelevant" dependencies are ignored.
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The ``MayAlias`` response is used whenever the two pointers might refer to the
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same object.
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The ``PartialAlias`` response is used when the two memory objects are known to
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be overlapping in some way, but do not start at the same address.
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The ``MustAlias`` response may only be returned if the two memory objects are
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guaranteed to always start at exactly the same location. A ``MustAlias``
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response implies that the pointers compare equal.
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The ``getModRefInfo`` methods
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-----------------------------
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The ``getModRefInfo`` methods return information about whether the execution of
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an instruction can read or modify a memory location. Mod/Ref information is
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always conservative: if an instruction **might** read or write a location,
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``ModRef`` is returned.
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The ``AliasAnalysis`` class also provides a ``getModRefInfo`` method for testing
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dependencies between function calls. This method takes two call sites (``CS1``
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& ``CS2``), returns ``NoModRef`` if neither call writes to memory read or
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written by the other, ``Ref`` if ``CS1`` reads memory written by ``CS2``,
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``Mod`` if ``CS1`` writes to memory read or written by ``CS2``, or ``ModRef`` if
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``CS1`` might read or write memory written to by ``CS2``. Note that this
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relation is not commutative.
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Other useful ``AliasAnalysis`` methods
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--------------------------------------
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Several other tidbits of information are often collected by various alias
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analysis implementations and can be put to good use by various clients.
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The ``pointsToConstantMemory`` method
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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The ``pointsToConstantMemory`` method returns true if and only if the analysis
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can prove that the pointer only points to unchanging memory locations
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(functions, constant global variables, and the null pointer). This information
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can be used to refine mod/ref information: it is impossible for an unchanging
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memory location to be modified.
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.. _never access memory or only read memory:
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The ``doesNotAccessMemory`` and ``onlyReadsMemory`` methods
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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These methods are used to provide very simple mod/ref information for function
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calls. The ``doesNotAccessMemory`` method returns true for a function if the
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analysis can prove that the function never reads or writes to memory, or if the
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function only reads from constant memory. Functions with this property are
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side-effect free and only depend on their input arguments, allowing them to be
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eliminated if they form common subexpressions or be hoisted out of loops. Many
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common functions behave this way (e.g., ``sin`` and ``cos``) but many others do
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not (e.g., ``acos``, which modifies the ``errno`` variable).
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The ``onlyReadsMemory`` method returns true for a function if analysis can prove
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that (at most) the function only reads from non-volatile memory. Functions with
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this property are side-effect free, only depending on their input arguments and
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the state of memory when they are called. This property allows calls to these
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functions to be eliminated and moved around, as long as there is no store
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instruction that changes the contents of memory. Note that all functions that
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satisfy the ``doesNotAccessMemory`` method also satisfies ``onlyReadsMemory``.
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Writing a new ``AliasAnalysis`` Implementation
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==============================================
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Writing a new alias analysis implementation for LLVM is quite straight-forward.
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There are already several implementations that you can use for examples, and the
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following information should help fill in any details. For a examples, take a
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look at the `various alias analysis implementations`_ included with LLVM.
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Different Pass styles
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---------------------
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The first step to determining what type of `LLVM pass <WritingAnLLVMPass.html>`_
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you need to use for your Alias Analysis. As is the case with most other
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analyses and transformations, the answer should be fairly obvious from what type
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of problem you are trying to solve:
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#. If you require interprocedural analysis, it should be a ``Pass``.
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#. If you are a function-local analysis, subclass ``FunctionPass``.
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#. If you don't need to look at the program at all, subclass ``ImmutablePass``.
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In addition to the pass that you subclass, you should also inherit from the
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``AliasAnalysis`` interface, of course, and use the ``RegisterAnalysisGroup``
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template to register as an implementation of ``AliasAnalysis``.
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Required initialization calls
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-----------------------------
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Your subclass of ``AliasAnalysis`` is required to invoke two methods on the
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``AliasAnalysis`` base class: ``getAnalysisUsage`` and
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``InitializeAliasAnalysis``. In particular, your implementation of
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``getAnalysisUsage`` should explicitly call into the
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``AliasAnalysis::getAnalysisUsage`` method in addition to doing any declaring
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any pass dependencies your pass has. Thus you should have something like this:
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.. code-block:: c++
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void getAnalysisUsage(AnalysisUsage &AU) const {
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AliasAnalysis::getAnalysisUsage(AU);
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// declare your dependencies here.
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}
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Additionally, your must invoke the ``InitializeAliasAnalysis`` method from your
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analysis run method (``run`` for a ``Pass``, ``runOnFunction`` for a
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``FunctionPass``, or ``InitializePass`` for an ``ImmutablePass``). For example
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(as part of a ``Pass``):
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.. code-block:: c++
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bool run(Module &M) {
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InitializeAliasAnalysis(this);
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// Perform analysis here...
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return false;
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}
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Interfaces which may be specified
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---------------------------------
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All of the `AliasAnalysis
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<http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html>`__ virtual methods
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default to providing `chaining`_ to another alias analysis implementation, which
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ends up returning conservatively correct information (returning "May" Alias and
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"Mod/Ref" for alias and mod/ref queries respectively). Depending on the
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capabilities of the analysis you are implementing, you just override the
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interfaces you can improve.
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.. _chaining:
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.. _chain:
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``AliasAnalysis`` chaining behavior
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-----------------------------------
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With only one special exception (the `no-aa`_ pass) every alias analysis pass
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chains to another alias analysis implementation (for example, the user can
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specify "``-basicaa -ds-aa -licm``" to get the maximum benefit from both alias
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analyses). The alias analysis class automatically takes care of most of this
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for methods that you don't override. For methods that you do override, in code
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paths that return a conservative MayAlias or Mod/Ref result, simply return
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whatever the superclass computes. For example:
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.. code-block:: c++
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AliasAnalysis::AliasResult alias(const Value *V1, unsigned V1Size,
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const Value *V2, unsigned V2Size) {
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if (...)
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return NoAlias;
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...
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// Couldn't determine a must or no-alias result.
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return AliasAnalysis::alias(V1, V1Size, V2, V2Size);
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}
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In addition to analysis queries, you must make sure to unconditionally pass LLVM
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`update notification`_ methods to the superclass as well if you override them,
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which allows all alias analyses in a change to be updated.
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.. _update notification:
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Updating analysis results for transformations
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---------------------------------------------
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Alias analysis information is initially computed for a static snapshot of the
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program, but clients will use this information to make transformations to the
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code. All but the most trivial forms of alias analysis will need to have their
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analysis results updated to reflect the changes made by these transformations.
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The ``AliasAnalysis`` interface exposes four methods which are used to
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communicate program changes from the clients to the analysis implementations.
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Various alias analysis implementations should use these methods to ensure that
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their internal data structures are kept up-to-date as the program changes (for
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example, when an instruction is deleted), and clients of alias analysis must be
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sure to call these interfaces appropriately.
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The ``deleteValue`` method
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^^^^^^^^^^^^^^^^^^^^^^^^^^
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The ``deleteValue`` method is called by transformations when they remove an
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instruction or any other value from the program (including values that do not
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use pointers). Typically alias analyses keep data structures that have entries
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for each value in the program. When this method is called, they should remove
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any entries for the specified value, if they exist.
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The ``copyValue`` method
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^^^^^^^^^^^^^^^^^^^^^^^^
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The ``copyValue`` method is used when a new value is introduced into the
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program. There is no way to introduce a value into the program that did not
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exist before (this doesn't make sense for a safe compiler transformation), so
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this is the only way to introduce a new value. This method indicates that the
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new value has exactly the same properties as the value being copied.
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The ``replaceWithNewValue`` method
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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This method is a simple helper method that is provided to make clients easier to
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use. It is implemented by copying the old analysis information to the new
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value, then deleting the old value. This method cannot be overridden by alias
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analysis implementations.
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The ``addEscapingUse`` method
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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The ``addEscapingUse`` method is used when the uses of a pointer value have
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changed in ways that may invalidate precomputed analysis information.
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Implementations may either use this callback to provide conservative responses
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for points whose uses have change since analysis time, or may recompute some or
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all of their internal state to continue providing accurate responses.
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In general, any new use of a pointer value is considered an escaping use, and
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must be reported through this callback, *except* for the uses below:
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* A ``bitcast`` or ``getelementptr`` of the pointer
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* A ``store`` through the pointer (but not a ``store`` *of* the pointer)
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* A ``load`` through the pointer
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Efficiency Issues
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-----------------
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From the LLVM perspective, the only thing you need to do to provide an efficient
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alias analysis is to make sure that alias analysis **queries** are serviced
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quickly. The actual calculation of the alias analysis results (the "run"
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method) is only performed once, but many (perhaps duplicate) queries may be
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performed. Because of this, try to move as much computation to the run method
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as possible (within reason).
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Limitations
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-----------
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The AliasAnalysis infrastructure has several limitations which make writing a
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new ``AliasAnalysis`` implementation difficult.
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There is no way to override the default alias analysis. It would be very useful
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to be able to do something like "``opt -my-aa -O2``" and have it use ``-my-aa``
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for all passes which need AliasAnalysis, but there is currently no support for
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that, short of changing the source code and recompiling. Similarly, there is
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also no way of setting a chain of analyses as the default.
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There is no way for transform passes to declare that they preserve
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``AliasAnalysis`` implementations. The ``AliasAnalysis`` interface includes
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``deleteValue`` and ``copyValue`` methods which are intended to allow a pass to
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keep an AliasAnalysis consistent, however there's no way for a pass to declare
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in its ``getAnalysisUsage`` that it does so. Some passes attempt to use
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``AU.addPreserved<AliasAnalysis>``, however this doesn't actually have any
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effect.
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``AliasAnalysisCounter`` (``-count-aa``) and ``AliasDebugger`` (``-debug-aa``)
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are implemented as ``ModulePass`` classes, so if your alias analysis uses
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``FunctionPass``, it won't be able to use these utilities. If you try to use
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them, the pass manager will silently route alias analysis queries directly to
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``BasicAliasAnalysis`` instead.
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Similarly, the ``opt -p`` option introduces ``ModulePass`` passes between each
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pass, which prevents the use of ``FunctionPass`` alias analysis passes.
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The ``AliasAnalysis`` API does have functions for notifying implementations when
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values are deleted or copied, however these aren't sufficient. There are many
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other ways that LLVM IR can be modified which could be relevant to
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``AliasAnalysis`` implementations which can not be expressed.
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The ``AliasAnalysisDebugger`` utility seems to suggest that ``AliasAnalysis``
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implementations can expect that they will be informed of any relevant ``Value``
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before it appears in an alias query. However, popular clients such as ``GVN``
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don't support this, and are known to trigger errors when run with the
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``AliasAnalysisDebugger``.
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Due to several of the above limitations, the most obvious use for the
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``AliasAnalysisCounter`` utility, collecting stats on all alias queries in a
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compilation, doesn't work, even if the ``AliasAnalysis`` implementations don't
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use ``FunctionPass``. There's no way to set a default, much less a default
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sequence, and there's no way to preserve it.
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The ``AliasSetTracker`` class (which is used by ``LICM``) makes a
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non-deterministic number of alias queries. This can cause stats collected by
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``AliasAnalysisCounter`` to have fluctuations among identical runs, for
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example. Another consequence is that debugging techniques involving pausing
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execution after a predetermined number of queries can be unreliable.
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Many alias queries can be reformulated in terms of other alias queries. When
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multiple ``AliasAnalysis`` queries are chained together, it would make sense to
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start those queries from the beginning of the chain, with care taken to avoid
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infinite looping, however currently an implementation which wants to do this can
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only start such queries from itself.
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Using alias analysis results
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============================
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There are several different ways to use alias analysis results. In order of
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preference, these are:
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Using the ``MemoryDependenceAnalysis`` Pass
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-------------------------------------------
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The ``memdep`` pass uses alias analysis to provide high-level dependence
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information about memory-using instructions. This will tell you which store
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feeds into a load, for example. It uses caching and other techniques to be
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efficient, and is used by Dead Store Elimination, GVN, and memcpy optimizations.
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.. _AliasSetTracker:
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Using the ``AliasSetTracker`` class
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-----------------------------------
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Many transformations need information about alias **sets** that are active in
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some scope, rather than information about pairwise aliasing. The
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`AliasSetTracker <http://llvm.org/doxygen/classllvm_1_1AliasSetTracker.html>`__
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class is used to efficiently build these Alias Sets from the pairwise alias
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analysis information provided by the ``AliasAnalysis`` interface.
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First you initialize the AliasSetTracker by using the "``add``" methods to add
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information about various potentially aliasing instructions in the scope you are
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interested in. Once all of the alias sets are completed, your pass should
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simply iterate through the constructed alias sets, using the ``AliasSetTracker``
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``begin()``/``end()`` methods.
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The ``AliasSet``\s formed by the ``AliasSetTracker`` are guaranteed to be
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disjoint, calculate mod/ref information and volatility for the set, and keep
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track of whether or not all of the pointers in the set are Must aliases. The
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AliasSetTracker also makes sure that sets are properly folded due to call
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instructions, and can provide a list of pointers in each set.
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As an example user of this, the `Loop Invariant Code Motion
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<doxygen/structLICM.html>`_ pass uses ``AliasSetTracker``\s to calculate alias
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sets for each loop nest. If an ``AliasSet`` in a loop is not modified, then all
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load instructions from that set may be hoisted out of the loop. If any alias
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sets are stored to **and** are must alias sets, then the stores may be sunk
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to outside of the loop, promoting the memory location to a register for the
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duration of the loop nest. Both of these transformations only apply if the
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pointer argument is loop-invariant.
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The AliasSetTracker implementation
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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The AliasSetTracker class is implemented to be as efficient as possible. It
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uses the union-find algorithm to efficiently merge AliasSets when a pointer is
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inserted into the AliasSetTracker that aliases multiple sets. The primary data
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structure is a hash table mapping pointers to the AliasSet they are in.
|
|
|
|
The AliasSetTracker class must maintain a list of all of the LLVM ``Value*``\s
|
|
that are in each AliasSet. Since the hash table already has entries for each
|
|
LLVM ``Value*`` of interest, the AliasesSets thread the linked list through
|
|
these hash-table nodes to avoid having to allocate memory unnecessarily, and to
|
|
make merging alias sets extremely efficient (the linked list merge is constant
|
|
time).
|
|
|
|
You shouldn't need to understand these details if you are just a client of the
|
|
AliasSetTracker, but if you look at the code, hopefully this brief description
|
|
will help make sense of why things are designed the way they are.
|
|
|
|
Using the ``AliasAnalysis`` interface directly
|
|
----------------------------------------------
|
|
|
|
If neither of these utility class are what your pass needs, you should use the
|
|
interfaces exposed by the ``AliasAnalysis`` class directly. Try to use the
|
|
higher-level methods when possible (e.g., use mod/ref information instead of the
|
|
`alias`_ method directly if possible) to get the best precision and efficiency.
|
|
|
|
Existing alias analysis implementations and clients
|
|
===================================================
|
|
|
|
If you're going to be working with the LLVM alias analysis infrastructure, you
|
|
should know what clients and implementations of alias analysis are available.
|
|
In particular, if you are implementing an alias analysis, you should be aware of
|
|
the `the clients`_ that are useful for monitoring and evaluating different
|
|
implementations.
|
|
|
|
.. _various alias analysis implementations:
|
|
|
|
Available ``AliasAnalysis`` implementations
|
|
-------------------------------------------
|
|
|
|
This section lists the various implementations of the ``AliasAnalysis``
|
|
interface. With the exception of the `-no-aa`_ implementation, all of these
|
|
`chain`_ to other alias analysis implementations.
|
|
|
|
.. _no-aa:
|
|
.. _-no-aa:
|
|
|
|
The ``-no-aa`` pass
|
|
^^^^^^^^^^^^^^^^^^^
|
|
|
|
The ``-no-aa`` pass is just like what it sounds: an alias analysis that never
|
|
returns any useful information. This pass can be useful if you think that alias
|
|
analysis is doing something wrong and are trying to narrow down a problem.
|
|
|
|
The ``-basicaa`` pass
|
|
^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
The ``-basicaa`` pass is an aggressive local analysis that *knows* many
|
|
important facts:
|
|
|
|
* Distinct globals, stack allocations, and heap allocations can never alias.
|
|
* Globals, stack allocations, and heap allocations never alias the null pointer.
|
|
* Different fields of a structure do not alias.
|
|
* Indexes into arrays with statically differing subscripts cannot alias.
|
|
* Many common standard C library functions `never access memory or only read
|
|
memory`_.
|
|
* Pointers that obviously point to constant globals "``pointToConstantMemory``".
|
|
* Function calls can not modify or references stack allocations if they never
|
|
escape from the function that allocates them (a common case for automatic
|
|
arrays).
|
|
|
|
The ``-globalsmodref-aa`` pass
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
This pass implements a simple context-sensitive mod/ref and alias analysis for
|
|
internal global variables that don't "have their address taken". If a global
|
|
does not have its address taken, the pass knows that no pointers alias the
|
|
global. This pass also keeps track of functions that it knows never access
|
|
memory or never read memory. This allows certain optimizations (e.g. GVN) to
|
|
eliminate call instructions entirely.
|
|
|
|
The real power of this pass is that it provides context-sensitive mod/ref
|
|
information for call instructions. This allows the optimizer to know that calls
|
|
to a function do not clobber or read the value of the global, allowing loads and
|
|
stores to be eliminated.
|
|
|
|
.. note::
|
|
|
|
This pass is somewhat limited in its scope (only support non-address taken
|
|
globals), but is very quick analysis.
|
|
|
|
The ``-steens-aa`` pass
|
|
^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
The ``-steens-aa`` pass implements a variation on the well-known "Steensgaard's
|
|
algorithm" for interprocedural alias analysis. Steensgaard's algorithm is a
|
|
unification-based, flow-insensitive, context-insensitive, and field-insensitive
|
|
alias analysis that is also very scalable (effectively linear time).
|
|
|
|
The LLVM ``-steens-aa`` pass implements a "speculatively field-**sensitive**"
|
|
version of Steensgaard's algorithm using the Data Structure Analysis framework.
|
|
This gives it substantially more precision than the standard algorithm while
|
|
maintaining excellent analysis scalability.
|
|
|
|
.. note::
|
|
|
|
``-steens-aa`` is available in the optional "poolalloc" module. It is not part
|
|
of the LLVM core.
|
|
|
|
The ``-ds-aa`` pass
|
|
^^^^^^^^^^^^^^^^^^^
|
|
|
|
The ``-ds-aa`` pass implements the full Data Structure Analysis algorithm. Data
|
|
Structure Analysis is a modular unification-based, flow-insensitive,
|
|
context-**sensitive**, and speculatively field-**sensitive** alias
|
|
analysis that is also quite scalable, usually at ``O(n * log(n))``.
|
|
|
|
This algorithm is capable of responding to a full variety of alias analysis
|
|
queries, and can provide context-sensitive mod/ref information as well. The
|
|
only major facility not implemented so far is support for must-alias
|
|
information.
|
|
|
|
.. note::
|
|
|
|
``-ds-aa`` is available in the optional "poolalloc" module. It is not part of
|
|
the LLVM core.
|
|
|
|
The ``-scev-aa`` pass
|
|
^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
The ``-scev-aa`` pass implements AliasAnalysis queries by translating them into
|
|
ScalarEvolution queries. This gives it a more complete understanding of
|
|
``getelementptr`` instructions and loop induction variables than other alias
|
|
analyses have.
|
|
|
|
Alias analysis driven transformations
|
|
-------------------------------------
|
|
|
|
LLVM includes several alias-analysis driven transformations which can be used
|
|
with any of the implementations above.
|
|
|
|
The ``-adce`` pass
|
|
^^^^^^^^^^^^^^^^^^
|
|
|
|
The ``-adce`` pass, which implements Aggressive Dead Code Elimination uses the
|
|
``AliasAnalysis`` interface to delete calls to functions that do not have
|
|
side-effects and are not used.
|
|
|
|
The ``-licm`` pass
|
|
^^^^^^^^^^^^^^^^^^
|
|
|
|
The ``-licm`` pass implements various Loop Invariant Code Motion related
|
|
transformations. It uses the ``AliasAnalysis`` interface for several different
|
|
transformations:
|
|
|
|
* It uses mod/ref information to hoist or sink load instructions out of loops if
|
|
there are no instructions in the loop that modifies the memory loaded.
|
|
|
|
* It uses mod/ref information to hoist function calls out of loops that do not
|
|
write to memory and are loop-invariant.
|
|
|
|
* If uses alias information to promote memory objects that are loaded and stored
|
|
to in loops to live in a register instead. It can do this if there are no may
|
|
aliases to the loaded/stored memory location.
|
|
|
|
The ``-argpromotion`` pass
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
The ``-argpromotion`` pass promotes by-reference arguments to be passed in
|
|
by-value instead. In particular, if pointer arguments are only loaded from it
|
|
passes in the value loaded instead of the address to the function. This pass
|
|
uses alias information to make sure that the value loaded from the argument
|
|
pointer is not modified between the entry of the function and any load of the
|
|
pointer.
|
|
|
|
The ``-gvn``, ``-memcpyopt``, and ``-dse`` passes
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
These passes use AliasAnalysis information to reason about loads and stores.
|
|
|
|
.. _the clients:
|
|
|
|
Clients for debugging and evaluation of implementations
|
|
-------------------------------------------------------
|
|
|
|
These passes are useful for evaluating the various alias analysis
|
|
implementations. You can use them with commands like:
|
|
|
|
.. code-block:: bash
|
|
|
|
% opt -ds-aa -aa-eval foo.bc -disable-output -stats
|
|
|
|
The ``-print-alias-sets`` pass
|
|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
The ``-print-alias-sets`` pass is exposed as part of the ``opt`` tool to print
|
|
out the Alias Sets formed by the `AliasSetTracker`_ class. This is useful if
|
|
you're using the ``AliasSetTracker`` class. To use it, use something like:
|
|
|
|
.. code-block:: bash
|
|
|
|
% opt -ds-aa -print-alias-sets -disable-output
|
|
|
|
The ``-count-aa`` pass
|
|
^^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
The ``-count-aa`` pass is useful to see how many queries a particular pass is
|
|
making and what responses are returned by the alias analysis. As an example:
|
|
|
|
.. code-block:: bash
|
|
|
|
% opt -basicaa -count-aa -ds-aa -count-aa -licm
|
|
|
|
will print out how many queries (and what responses are returned) by the
|
|
``-licm`` pass (of the ``-ds-aa`` pass) and how many queries are made of the
|
|
``-basicaa`` pass by the ``-ds-aa`` pass. This can be useful when debugging a
|
|
transformation or an alias analysis implementation.
|
|
|
|
The ``-aa-eval`` pass
|
|
^^^^^^^^^^^^^^^^^^^^^
|
|
|
|
The ``-aa-eval`` pass simply iterates through all pairs of pointers in a
|
|
function and asks an alias analysis whether or not the pointers alias. This
|
|
gives an indication of the precision of the alias analysis. Statistics are
|
|
printed indicating the percent of no/may/must aliases found (a more precise
|
|
algorithm will have a lower number of may aliases).
|
|
|
|
Memory Dependence Analysis
|
|
==========================
|
|
|
|
If you're just looking to be a client of alias analysis information, consider
|
|
using the Memory Dependence Analysis interface instead. MemDep is a lazy,
|
|
caching layer on top of alias analysis that is able to answer the question of
|
|
what preceding memory operations a given instruction depends on, either at an
|
|
intra- or inter-block level. Because of its laziness and caching policy, using
|
|
MemDep can be a significant performance win over accessing alias analysis
|
|
directly.
|