261 lines
11 KiB
HTML
261 lines
11 KiB
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
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"http://www.w3.org/TR/html4/strict.dtd">
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<html>
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<head>
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<title>Static Analyzer Design Document: Memory Regions</title>
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</head>
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<body>
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<h1>Static Analyzer Design Document: Memory Regions</h1>
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<h3>Authors</h3>
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<p>Ted Kremenek, <tt>kremenek at apple</tt><br>
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Zhongxing Xu, <tt>xuzhongzhing at gmail</tt></p>
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<h2 id="intro">Introduction</h2>
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<p>The path-sensitive analysis engine in libAnalysis employs an extensible API
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for abstractly modeling the memory of an analyzed program. This API employs the
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concept of "memory regions" to abstractly model chunks of program memory such as
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program variables and dynamically allocated memory such as those returned from
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'malloc' and 'alloca'. Regions are hierarchical, with subregions modeling
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subtyping relationships, field and array offsets into larger chunks of memory,
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and so on.</p>
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<p>The region API consists of two components:</p>
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<ul> <li>A taxonomy and representation of regions themselves within the analyzer
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engine. The primary definitions and interfaces are described in <tt><a
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href="http://clang.llvm.org/doxygen/MemRegion_8h-source.html">MemRegion.h</a></tt>.
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At the root of the region hierarchy is the class <tt>MemRegion</tt> with
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specific subclasses refining the region concept for variables, heap allocated
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memory, and so forth.</li> <li>The modeling of binding of values to regions. For
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example, modeling the value stored to a local variable <tt>x</tt> consists of
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recording the binding between the region for <tt>x</tt> (which represents the
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raw memory associated with <tt>x</tt>) and the value stored to <tt>x</tt>. This
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binding relationship is captured with the notion of "symbolic
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stores."</li> </ul>
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<p>Symbolic stores, which can be thought of as representing the relation
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<tt>regions -> values</tt>, are implemented by subclasses of the
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<tt>StoreManager</tt> class (<tt><a
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href="http://clang.llvm.org/doxygen/Store_8h-source.html">Store.h</a></tt>). A
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particular StoreManager implementation has complete flexibility concerning the
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following:
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<ul>
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<li><em>How</em> to model the binding between regions and values</li>
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<li><em>What</em> bindings are recorded
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</ul>
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<p>Together, both points allow different StoreManagers to tradeoff between
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different levels of analysis precision and scalability concerning the reasoning
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of program memory. Meanwhile, the core path-sensitive engine makes no
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assumptions about either points, and queries a StoreManager about the bindings
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to a memory region through a generic interface that all StoreManagers share. If
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a particular StoreManager cannot reason about the potential bindings of a given
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memory region (e.g., '<tt>BasicStoreManager</tt>' does not reason about fields
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of structures) then the StoreManager can simply return 'unknown' (represented by
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'<tt>UnknownVal</tt>') for a particular region-binding. This separation of
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concerns not only isolates the core analysis engine from the details of
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reasoning about program memory but also facilities the option of a client of the
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path-sensitive engine to easily swap in different StoreManager implementations
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that internally reason about program memory in very different ways.</p>
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<p>The rest of this document is divided into two parts. We first discuss region
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taxonomy and the semantics of regions. We then discuss the StoreManager
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interface, and details of how the currently available StoreManager classes
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implement region bindings.</p>
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<h2 id="regions">Memory Regions and Region Taxonomy</h2>
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<h3>Pointers</h3>
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<p>Before talking about the memory regions, we would talk about the pointers
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since memory regions are essentially used to represent pointer values.</p>
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<p>The pointer is a type of values. Pointer values have two semantic aspects.
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One is its physical value, which is an address or location. The other is the
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type of the memory object residing in the address.</p>
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<p>Memory regions are designed to abstract these two properties of the pointer.
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The physical value of a pointer is represented by MemRegion pointers. The rvalue
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type of the region corresponds to the type of the pointee object.</p>
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<p>One complication is that we could have different view regions on the same
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memory chunk. They represent the same memory location, but have different
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abstract location, i.e., MemRegion pointers. Thus we need to canonicalize the
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abstract locations to get a unique abstract location for one physical
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location.</p>
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<p>Furthermore, these different view regions may or may not represent memory
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objects of different types. Some different types are semantically the same,
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for example, 'struct s' and 'my_type' are the same type.</p>
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<pre>
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struct s;
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typedef struct s my_type;
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</pre>
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<p>But <tt>char</tt> and <tt>int</tt> are not the same type in the code below:</p>
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<pre>
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void *p;
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int *q = (int*) p;
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char *r = (char*) p;
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</pre>
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<p>Thus we need to canonicalize the MemRegion which is used in binding and
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retrieving.</p>
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<h3>Regions</h3>
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<p>Region is the entity used to model pointer values. A Region has the following
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properties:</p>
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<ul>
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<li>Kind</li>
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<li>ObjectType: the type of the object residing on the region.</li>
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<li>LocationType: the type of the pointer value that the region corresponds to.
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Usually this is the pointer to the ObjectType. But sometimes we want to cache
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this type explicitly, for example, for a CodeTextRegion.</li>
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<li>StartLocation</li>
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<li>EndLocation</li>
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</ul>
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<h3>Symbolic Regions</h3>
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<p>A symbolic region is a map of the concept of symbolic values into the domain
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of regions. It is the way that we represent symbolic pointers. Whenever a
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symbolic pointer value is needed, a symbolic region is created to represent
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it.</p>
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<p>A symbolic region has no type. It wraps a SymbolData. But sometimes we have
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type information associated with a symbolic region. For this case, a
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TypedViewRegion is created to layer the type information on top of the symbolic
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region. The reason we do not carry type information with the symbolic region is
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that the symbolic regions can have no type. To be consistent, we don't let them
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to carry type information.</p>
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<p>Like a symbolic pointer, a symbolic region may be NULL, has unknown extent,
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and represents a generic chunk of memory.</p>
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<p><em><b>NOTE</b>: We plan not to use loc::SymbolVal in RegionStore and remove it
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gradually.</em></p>
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<p>Symbolic regions get their rvalue types through the following ways:</p>
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<ul>
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<li>Through the parameter or global variable that points to it, e.g.:
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<pre>
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void f(struct s* p) {
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...
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}
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</pre>
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<p>The symbolic region pointed to by <tt>p</tt> has type <tt>struct
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s</tt>.</p></li>
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<li>Through explicit or implicit casts, e.g.:
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<pre>
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void f(void* p) {
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struct s* q = (struct s*) p;
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...
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}
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</pre>
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</li>
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</ul>
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<p>We attach the type information to the symbolic region lazily. For the first
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case above, we create the <tt>TypedViewRegion</tt> only when the pointer is
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actually used to access the pointee memory object, that is when the element or
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field region is created. For the cast case, the <tt>TypedViewRegion</tt> is
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created when visiting the <tt>CastExpr</tt>.</p>
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<p>The reason for doing lazy typing is that symbolic regions are sometimes only
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used to do location comparison.</p>
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<h3>Pointer Casts</h3>
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<p>Pointer casts allow people to impose different 'views' onto a chunk of
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memory.</p>
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<p>Usually we have two kinds of casts. One kind of casts cast down with in the
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type hierarchy. It imposes more specific views onto more generic memory regions.
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The other kind of casts cast up with in the type hierarchy. It strips away more
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specific views on top of the more generic memory regions.</p>
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<p>We simulate the down casts by layering another <tt>TypedViewRegion</tt> on
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top of the original region. We simulate the up casts by striping away the top
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<tt>TypedViewRegion</tt>. Down casts is usually simple. For up casts, if the
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there is no <tt>TypedViewRegion</tt> to be stripped, we return the original
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region. If the underlying region is of the different type than the cast-to type,
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we flag an error state.</p>
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<p>For toll-free bridging casts, we return the original region.</p>
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<p>We can set up a partial order for pointer types, with the most general type
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<tt>void*</tt> at the top. The partial order forms a tree with <tt>void*</tt> as
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its root node.</p>
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<p>Every <tt>MemRegion</tt> has a root position in the type tree. For example,
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the pointee region of <tt>void *p</tt> has its root position at the root node of
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the tree. <tt>VarRegion</tt> of <tt>int x</tt> has its root position at the 'int
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type' node.</p>
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<p><tt>TypedViewRegion</tt> is used to move the region down or up in the tree.
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Moving down in the tree adds a <tt>TypedViewRegion</tt>. Moving up in the tree
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removes a <Tt>TypedViewRegion</tt>.</p>
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<p>Do we want to allow moving up beyond the root position? This happens
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when:</p> <pre> int x; void *p = &x; </pre>
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<p>The region of <tt>x</tt> has its root position at 'int*' node. the cast to
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void* moves that region up to the 'void*' node. I propose to not allow such
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casts, and assign the region of <tt>x</tt> for <tt>p</tt>.</p>
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<p>Another non-ideal case is that people might cast to a non-generic pointer
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from another non-generic pointer instead of first casting it back to the generic
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pointer. Direct handling of this case would result in multiple layers of
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TypedViewRegions. This enforces an incorrect semantic view to the region,
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because we can only have one typed view on a region at a time. To avoid this
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inconsistency, before casting the region, we strip the TypedViewRegion, then do
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the cast. In summary, we only allow one layer of TypedViewRegion.</p>
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<h3>Region Bindings</h3>
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<p>The following region kinds are boundable: VarRegion, CompoundLiteralRegion,
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StringRegion, ElementRegion, FieldRegion, and ObjCIvarRegion.</p>
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<p>When binding regions, we perform canonicalization on element regions and field
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regions. This is because we can have different views on the same region, some
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of which are essentially the same view with different sugar type names.</p>
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<p>To canonicalize a region, we get the canonical types for all TypedViewRegions
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along the way up to the root region, and make new TypedViewRegions with those
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canonical types.</p>
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<p>For Objective-C and C++, perhaps another canonicalization rule should be
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added: for FieldRegion, the least derived class that has the field is used as
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the type of the super region of the FieldRegion.</p>
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<p>All bindings and retrievings are done on the canonicalized regions.</p>
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<p>Canonicalization is transparent outside the region store manager, and more
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specifically, unaware outside the Bind() and Retrieve() method. We don't need to
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consider region canonicalization when doing pointer cast.</p>
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<h3>Constraint Manager</h3>
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<p>The constraint manager reasons about the abstract location of memory objects.
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We can have different views on a region, but none of these views changes the
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location of that object. Thus we should get the same abstract location for those
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regions.</p>
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</body>
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</html>
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