184 lines
8.5 KiB
ReStructuredText
184 lines
8.5 KiB
ReStructuredText
============
|
|
Region Store
|
|
============
|
|
The analyzer "Store" represents the contents of memory regions. It is an opaque
|
|
functional data structure stored in each ``ProgramState``; the only class that
|
|
can modify the store is its associated StoreManager.
|
|
|
|
Currently (Feb. 2013), the only StoreManager implementation being used is
|
|
``RegionStoreManager``. This store records bindings to memory regions using a
|
|
"base region + offset" key. (This allows ``*p`` and ``p[0]`` to map to the same
|
|
location, among other benefits.)
|
|
|
|
Regions are grouped into "clusters", which roughly correspond to "regions with
|
|
the same base region". This allows certain operations to be more efficient,
|
|
such as invalidation.
|
|
|
|
Regions that do not have a known offset use a special "symbolic" offset. These
|
|
keys store both the original region, and the "concrete offset region" -- the
|
|
last region whose offset is entirely concrete. (For example, in the expression
|
|
``foo.bar[1][i].baz``, the concrete offset region is the array ``foo.bar[1]``,
|
|
since that has a known offset from the start of the top-level ``foo`` struct.)
|
|
|
|
|
|
Binding Invalidation
|
|
--------------------
|
|
|
|
Supporting both concrete and symbolic offsets makes things a bit tricky. Here's
|
|
an example:
|
|
|
|
.. code-block:: cpp
|
|
|
|
foo[0] = 0;
|
|
foo[1] = 1;
|
|
foo[i] = i;
|
|
|
|
After the third assignment, nothing can be said about the value of ``foo[0]``,
|
|
because ``foo[i]`` may have overwritten it! Thus, *binding to a region with a
|
|
symbolic offset invalidates the entire concrete offset region.* We know
|
|
``foo[i]`` is somewhere within ``foo``, so we don't have to invalidate
|
|
anything else, but we do have to be conservative about all other bindings within
|
|
``foo``.
|
|
|
|
Continuing the example:
|
|
|
|
.. code-block:: cpp
|
|
|
|
foo[i] = i;
|
|
foo[0] = 0;
|
|
|
|
After this latest assignment, nothing can be said about the value of ``foo[i]``,
|
|
because ``foo[0]`` may have overwritten it! *Binding to a region R with a
|
|
concrete offset invalidates any symbolic offset bindings whose concrete offset
|
|
region is a super-region **or** sub-region of R.* All we know about ``foo[i]``
|
|
is that it is somewhere within ``foo``, so changing *anything* within ``foo``
|
|
might change ``foo[i]``, and changing *all* of ``foo`` (or its base region) will
|
|
*definitely* change ``foo[i]``.
|
|
|
|
This logic could be improved by using the current constraints on ``i``, at the
|
|
cost of speed. The latter case could also be improved by matching region kinds,
|
|
i.e. changing ``foo[0].a`` is unlikely to affect ``foo[i].b``, no matter what
|
|
``i`` is.
|
|
|
|
For more detail, read through ``RegionStoreManager::removeSubRegionBindings`` in
|
|
RegionStore.cpp.
|
|
|
|
|
|
ObjCIvarRegions
|
|
---------------
|
|
|
|
Objective-C instance variables require a bit of special handling. Like struct
|
|
fields, they are not base regions, and when their parent object region is
|
|
invalidated, all the instance variables must be invalidated as well. However,
|
|
they have no concrete compile-time offsets (in the modern, "non-fragile"
|
|
runtime), and so cannot easily be represented as an offset from the start of
|
|
the object in the analyzer. Moreover, this means that invalidating a single
|
|
instance variable should *not* invalidate the rest of the object, since unlike
|
|
struct fields or array elements there is no way to perform pointer arithmetic
|
|
to access another instance variable.
|
|
|
|
Consequently, although the base region of an ObjCIvarRegion is the entire
|
|
object, RegionStore offsets are computed from the start of the instance
|
|
variable. Thus it is not valid to assume that all bindings with non-symbolic
|
|
offsets start from the base region!
|
|
|
|
|
|
Region Invalidation
|
|
-------------------
|
|
|
|
Unlike binding invalidation, region invalidation occurs when the entire
|
|
contents of a region may have changed---say, because it has been passed to a
|
|
function the analyzer can model, like memcpy, or because its address has
|
|
escaped, usually as an argument to an opaque function call. In these cases we
|
|
need to throw away not just all bindings within the region itself, but within
|
|
its entire cluster, since neighboring regions may be accessed via pointer
|
|
arithmetic.
|
|
|
|
Region invalidation typically does even more than this, however. Because it
|
|
usually represents the complete escape of a region from the analyzer's model,
|
|
its *contents* must also be transitively invalidated. (For example, if a region
|
|
``p`` of type ``int **`` is invalidated, the contents of ``*p`` and ``**p`` may
|
|
have changed as well.) The algorithm that traverses this transitive closure of
|
|
accessible regions is known as ClusterAnalysis, and is also used for finding
|
|
all live bindings in the store (in order to throw away the dead ones). The name
|
|
"ClusterAnalysis" predates the cluster-based organization of bindings, but
|
|
refers to the same concept: during invalidation and liveness analysis, all
|
|
bindings within a cluster must be treated in the same way for a conservative
|
|
model of program behavior.
|
|
|
|
|
|
Default Bindings
|
|
----------------
|
|
|
|
Most bindings in RegionStore are simple scalar values -- integers and pointers.
|
|
These are known as "Direct" bindings. However, RegionStore supports a second
|
|
type of binding called a "Default" binding. These are used to provide values to
|
|
all the elements of an aggregate type (struct or array) without having to
|
|
explicitly specify a binding for each individual element.
|
|
|
|
When there is no Direct binding for a particular region, the store manager
|
|
looks at each super-region in turn to see if there is a Default binding. If so,
|
|
this value is used as the value of the original region. The search ends when
|
|
the base region is reached, at which point the RegionStore will pick an
|
|
appropriate default value for the region (usually a symbolic value, but
|
|
sometimes zero, for static data, or "uninitialized", for stack variables).
|
|
|
|
.. code-block:: cpp
|
|
|
|
int manyInts[10];
|
|
manyInts[1] = 42; // Creates a Direct binding for manyInts[1].
|
|
print(manyInts[1]); // Retrieves the Direct binding for manyInts[1];
|
|
print(manyInts[0]); // There is no Direct binding for manyInts[0].
|
|
// Is there a Default binding for the entire array?
|
|
// There is not, but it is a stack variable, so we use
|
|
// "uninitialized" as the default value (and emit a
|
|
// diagnostic!).
|
|
|
|
NOTE: The fact that bindings are stored as a base region plus an offset limits
|
|
the Default Binding strategy, because in C aggregates can contain other
|
|
aggregates. In the current implementation of RegionStore, there is no way to
|
|
distinguish a Default binding for an entire aggregate from a Default binding
|
|
for the sub-aggregate at offset 0.
|
|
|
|
|
|
Lazy Bindings (LazyCompoundVal)
|
|
-------------------------------
|
|
|
|
RegionStore implements an optimization for copying aggregates (structs and
|
|
arrays) called "lazy bindings", implemented using a special SVal called
|
|
LazyCompoundVal. When the store is asked for the "binding" for an entire
|
|
aggregate (i.e. for an lvalue-to-rvalue conversion), it returns a
|
|
LazyCompoundVal instead. When this value is then stored into a variable, it is
|
|
bound as a Default value. This makes copying arrays and structs much cheaper
|
|
than if they had required memberwise access.
|
|
|
|
Under the hood, a LazyCompoundVal is implemented as a uniqued pair of (region,
|
|
store), representing "the value of the region during this 'snapshot' of the
|
|
store". This has important implications for any sort of liveness or
|
|
reachability analysis, which must take the bindings in the old store into
|
|
account.
|
|
|
|
Retrieving a value from a lazy binding happens in the same way as any other
|
|
Default binding: since there is no direct binding, the store manager falls back
|
|
to super-regions to look for an appropriate default binding. LazyCompoundVal
|
|
differs from a normal default binding, however, in that it contains several
|
|
different values, instead of one value that will appear several times. Because
|
|
of this, the store manager has to reconstruct the subregion chain on top of the
|
|
LazyCompoundVal region, and look up *that* region in the previous store.
|
|
|
|
Here's a concrete example:
|
|
|
|
.. code-block:: cpp
|
|
|
|
CGPoint p;
|
|
p.x = 42; // A Direct binding is made to the FieldRegion 'p.x'.
|
|
CGPoint p2 = p; // A LazyCompoundVal is created for 'p', along with a
|
|
// snapshot of the current store state. This value is then
|
|
// used as a Default binding for the VarRegion 'p2'.
|
|
return p2.x; // The binding for FieldRegion 'p2.x' is requested.
|
|
// There is no Direct binding, so we look for a Default
|
|
// binding to 'p2' and find the LCV.
|
|
// Because it's a LCV, we look at our requested region
|
|
// and see that it's the '.x' field. We ask for the value
|
|
// of 'p.x' within the snapshot, and get back 42.
|