-.. _alias_analysis:
-
==================================
LLVM Alias Analysis Infrastructure
==================================
get mod/ref information for arbitrary instructions.
All ``AliasAnalysis`` interfaces require that in queries involving multiple
-values, values which are not `constants <LangRef.html#constants>`_ are all
+values, values which are not :ref:`constants <constants>` are all
defined within the same function.
Representation of Pointers
Like all ``AliasAnalysis`` interfaces, the ``alias`` method requires that either
the two pointer values be defined within the same function, or at least one of
-the values is a `constant <LangRef.html#constants>`_.
+the values is a :ref:`constant <constants>`.
.. _Must, May, or No:
between accesses through one pointer and accesses through the other --- in this
case, there is a dependence, but it's mediated by the free and reallocation.
-As an exception to this is with the `noalias <LangRef.html#noalias>`_ keyword;
+As an exception to this is with the :ref:`noalias <noalias>` keyword;
the "irrelevant" dependencies are ignored.
The ``MayAlias`` response is used whenever the two pointers might refer to the
the state of memory when they are called. This property allows calls to these
functions to be eliminated and moved around, as long as there is no store
instruction that changes the contents of memory. Note that all functions that
-satisfy the ``doesNotAccessMemory`` method also satisfies ``onlyReadsMemory``.
+satisfy the ``doesNotAccessMemory`` method also satisfy ``onlyReadsMemory``.
Writing a new ``AliasAnalysis`` Implementation
==============================================
Different Pass styles
---------------------
-The first step to determining what type of `LLVM pass <WritingAnLLVMPass.html>`_
+The first step to determining what type of :doc:`LLVM pass <WritingAnLLVMPass>`
you need to use for your Alias Analysis. As is the case with most other
analyses and transformations, the answer should be fairly obvious from what type
of problem you are trying to solve:
.. code-block:: c++
- void getAnalysisUsage(AnalysisUsage &AU) const {
+ void getAnalysisUsage(AnalysisUsage &AU) const {
AliasAnalysis::getAnalysisUsage(AU);
// declare your dependencies here.
}
return false;
}
+Required methods to override
+----------------------------
+
+You must override the ``getAdjustedAnalysisPointer`` method on all subclasses
+of ``AliasAnalysis``. An example implementation of this method would look like:
+
+.. code-block:: c++
+
+ void *getAdjustedAnalysisPointer(const void* ID) override {
+ if (ID == &AliasAnalysis::ID)
+ return (AliasAnalysis*)this;
+ return this;
+ }
+
Interfaces which may be specified
---------------------------------
All of the `AliasAnalysis
<http://llvm.org/doxygen/classllvm_1_1AliasAnalysis.html>`__ virtual methods
-default to providing `chaining`_ to another alias analysis implementation, which
-ends up returning conservatively correct information (returning "May" Alias and
-"Mod/Ref" for alias and mod/ref queries respectively). Depending on the
-capabilities of the analysis you are implementing, you just override the
-interfaces you can improve.
+default to providing :ref:`chaining <aliasanalysis-chaining>` to another alias
+analysis implementation, which ends up returning conservatively correct
+information (returning "May" Alias and "Mod/Ref" for alias and mod/ref queries
+respectively). Depending on the capabilities of the analysis you are
+implementing, you just override the interfaces you can improve.
-.. _chaining:
-.. _chain:
+.. _aliasanalysis-chaining:
``AliasAnalysis`` chaining behavior
-----------------------------------
-With only one special exception (the `no-aa`_ pass) every alias analysis pass
-chains to another alias analysis implementation (for example, the user can
-specify "``-basicaa -ds-aa -licm``" to get the maximum benefit from both alias
-analyses). The alias analysis class automatically takes care of most of this
-for methods that you don't override. For methods that you do override, in code
-paths that return a conservative MayAlias or Mod/Ref result, simply return
-whatever the superclass computes. For example:
+With only one special exception (the :ref:`-no-aa <aliasanalysis-no-aa>` pass)
+every alias analysis pass chains to another alias analysis implementation (for
+example, the user can specify "``-basicaa -ds-aa -licm``" to get the maximum
+benefit from both alias analyses). The alias analysis class automatically
+takes care of most of this for methods that you don't override. For methods
+that you do override, in code paths that return a conservative MayAlias or
+Mod/Ref result, simply return whatever the superclass computes. For example:
.. code-block:: c++
- AliasAnalysis::AliasResult alias(const Value *V1, unsigned V1Size,
- const Value *V2, unsigned V2Size) {
+ AliasResult alias(const Value *V1, unsigned V1Size,
+ const Value *V2, unsigned V2Size) {
if (...)
return NoAlias;
...
``AU.addPreserved<AliasAnalysis>``, however this doesn't actually have any
effect.
-``AliasAnalysisCounter`` (``-count-aa``) and ``AliasDebugger`` (``-debug-aa``)
-are implemented as ``ModulePass`` classes, so if your alias analysis uses
-``FunctionPass``, it won't be able to use these utilities. If you try to use
-them, the pass manager will silently route alias analysis queries directly to
-``BasicAliasAnalysis`` instead.
+``AliasAnalysisCounter`` (``-count-aa``) are implemented as ``ModulePass``
+classes, so if your alias analysis uses ``FunctionPass``, it won't be able to
+use these utilities. If you try to use them, the pass manager will silently
+route alias analysis queries directly to ``BasicAliasAnalysis`` instead.
Similarly, the ``opt -p`` option introduces ``ModulePass`` passes between each
pass, which prevents the use of ``FunctionPass`` alias analysis passes.
-------------------------------------------
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.
+interface. With the exception of the :ref:`-no-aa <aliasanalysis-no-aa>`
+implementation, all of these :ref:`chain <aliasanalysis-chaining>` to other
+alias analysis implementations.
-.. _no-aa:
-.. _-no-aa:
+.. _aliasanalysis-no-aa:
The ``-no-aa`` pass
^^^^^^^^^^^^^^^^^^^
* 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
+* It 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.
projects / oota-llvm.git / blobdiff