Static Analyzer: 'Regions'
--------------------------

INTRODUCTION

  The path-sensitive analysis engine in libAnalysis employs an extensible API
  for abstractly modeling the memory of an analyzed program. This API employs
  the concept of "memory regions" to abstractly model chunks of program memory
  such as program variables and dynamically allocated memory such as those
  returned from 'malloc' and 'alloca'. Regions are hierarchical, with subregions
  modeling subtyping relationships, field and array offsets into larger chunks
  of memory, and so on.

  The region API consists of two components. The first is the taxonomy and
  representation of regions themselves within the analyzer engine. The primary
  definitions and interfaces are described in
  'include/clang/Analysis/PathSensitive/MemRegion.h'. At the root of the region
  hierarchy is the class 'MemRegion' with specific subclasses refining the
  region concept for variables, heap allocated memory, and so forth.

  The second component in the region API is the modeling of the binding of
  values to regions. For example, modeling the value stored to a local variable
  'x' consists of recording the binding between the region for 'x' (which
  represents the raw memory associated with 'x') and the value stored to 'x'.
  This binding relationship is captured with the notion of "symbolic stores."

  Symbolic stores, which can be thought of as representing the relation 'regions
  -> values', are implemented by subclasses of the StoreManager class (Store.h).
  A particular StoreManager implementation has complete flexibility concerning
  (a) *how* to model the binding between regions and values and (b) *what*
  bindings are recorded. Together, both points allow different StoreManagers to
  tradeoff between different levels of analysis precision and scalability
  concerning the reasoning of program memory. Meanwhile, the core path-sensitive
  engine makes no assumptions about (a) or (b), and queries a StoreManager about
  the bindings to a memory region through a generic interface that all
  StoreManagers share. If a particular StoreManager cannot reason about the
  potential bindings of a given memory region (e.g., 'BasicStoreManager' does
  not reason about fields of structures) then the StoreManager can simply return
  'unknown' (represented by 'UnknownVal') for a particular region-binding. This
  separation of concerns not only isolates the core analysis engine from the
  details of reasoning about program memory but also facilities the option of a
  client of the path-sensitive engine to easily swap in different StoreManager
  implementations that internally reason about program memory in very different
  ways.

  The rest of this document is divided into two parts. We first discuss region
  taxonomy and the semantics of regions. We then discuss the StoreManager
  interface, and details of how the currently available StoreManager classes
  implement region bindings.

MEMORY REGIONS and REGION TAXONOMY

  POINTERS

  Before talking about the memory regions, we would talk about the pointers
  since memory regions are essentially used to represent pointer values.

  The pointer is a type of values. Pointer values have two semantic aspects. One
  is its physical value, which is an address or location. The other is the type
  of the memory object residing in the address.

  Memory regions are designed to abstract these two properties of the
  pointer. The physical value of a pointer is represented by MemRegion
  pointers. The rvalue type of the region corresponds to the type of the pointee
  object.

  One complication is that we could have different view regions on the same
  memory chunk. They represent the same memory location, but have different
  abstract location, i.e., MemRegion pointers. Thus we need to canonicalize
  the abstract locations to get a unique abstract location for one physical
  location.

  Furthermore, these different view regions may or may not represent memory
  objects of different types. Some different types are semantically the same,
  for example, 'struct s' and 'my_type' are the same type.
    struct s;
    typedef struct s my_type;

  But 'char' and 'int' are not the same type in the code below:
    void *p;
    int *q = (int*) p;
    char *r = (char*) p;

  Thus we need to canonicalize the MemRegion which is used in binding and
  retrieving.

  SYMBOLIC REGIONS

  A symbolic region is a map of the concept of symbolic values into the domain
  of regions. It is the way that we represent symbolic pointers. Whenever a
  symbolic pointer value is needed, a symbolic region is created to represent
  it.

  A symbolic region has no type. It wraps a SymbolData. But sometimes we have
  type information associated with a symbolic region. For this case, a
  TypedViewRegion is created to layer the type information on top of the
  symbolic region. The reason we do not carry type information with the symbolic
  region is that the symbolic regions can have no type. To be consistent, we
  don't let them to carry type information.

  Like a symbolic pointer, a symbolic region may be NULL, has unknown extent,
  and represents a generic chunk of memory.

  NOTE: We plan not to use loc::SymbolVal in RegionStore and remove it
  gradually.

  Symbolic regions get their rvalue types through the following ways:
  * through the parameter or global variable that points to it, e.g.:

    void f(struct s* p) {
      ...
    }

    The symbolic region pointed to by 'p' has type 'struct s'.

  * through explicit or implicit casts, e.g.:
    void f(void* p) {
      struct s* q = (struct s*) p;
      ...
    }

  We attach the type information to the symbolic region lazily. For the first
  case above, we create the TypedViewRegion only when the pointer is actually
  used to access the pointee memory object, that is when the element or field
  region is created. For the cast case, the TypedViewRegion is created when
  visiting the CastExpr.

  The reason for doing lazy typing is that symbolic regions are sometimes only
  used to do location comparison.

Pointer Casts

  Pointer casts allow people to impose different 'views' onto a chunk of memory.
  
  Usually we have two kinds of casts. One kind of casts cast down with in the
  type hierarchy. It imposes more specific views onto more generic memory
  regions. The other kind of casts cast up with in the type hierarchy. It strips
  away more specific views on top of the more generic memory regions.
  
  We simulate the down casts by layering another TypedViewRegion on top of the
  original region. We simulate the up casts by striping away the top
  TypedViewRegion. Down casts is usually simple. For up casts, if the there is
  no TypedViewRegion to be stripped, we return the original region. If the
  underlying region is of the different type than the cast-to type, we flag an
  error state.
  
  For toll-free bridging casts, we return the original region.

  We can set up a lattice for pointer types, with the most general type 'void*'
  at the top. The lattice enforces a partial order among types.

  Every MemRegion has a root position in the type lattice. For example, the
  pointee region of 'void *p' has its root position at the top of the lattice.
  VarRegion of 'int x' has its root position at the 'int type' node.

  TypedViewRegion is used to move the region down or up in the lattice. Moving
  down in the lattice adds a TypedViewRegion. Moving up in the lattice removes a
  TypedViewRegion.

  Do we want to allow moving up beyond the root position? This happens when:
    int x;
    void *p = &x;
  
  The region of 'x' has its root position at 'int*' node. the cast to void*
  moves that region up to the 'void*' node. I propose to not allow such casts,
  and assign the region of 'x' for 'p'.

Region Bindings

  The following region kinds are boundable: VarRegion, CompoundLiteralRegion,
  StringRegion, ElementRegion, FieldRegion, and ObjCIvarRegion.

  When binding regions, we perform canonicalization on element regions and field
  regions. This is because we can have different views on the same region, some
  of which are essentially the same view with different sugar type names.

  To canonicalize a region, we get the canonical types for all TypedViewRegions
  along the way up to the root region, and make new TypedViewRegions with those
  canonical types.

  For ObjC and C++, perhaps another canonicalization rule should be added: for
  FieldRegion, the least derived class that has the field is used as the type
  of the super region of the FieldRegion.
  
  All bindings and retrievings are done on the canonicalized regions.
  
  Canonicalization is transparent outside the region store manager, and more
  specifically, unaware outside the Bind() and Retrieve() method. We don't need
  to consider region canonicalization when doing pointer cast.

Constraint Manager

  The constraint manager reasons about the abstract location of memory
  objects. We can have different views on a region, but none of these views
  changes the location of that object. Thus we should get the same abstract
  location for those regions.
