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1============2Region Store3============4The analyzer "Store" represents the contents of memory regions. It is an opaque5functional data structure stored in each ``ProgramState``; the only class that6can modify the store is its associated StoreManager.7 8Currently (Feb. 2013), the only StoreManager implementation being used is9``RegionStoreManager``. This store records bindings to memory regions using a10"base region + offset" key. (This allows ``*p`` and ``p[0]`` to map to the same11location, among other benefits.)12 13Regions are grouped into "clusters", which roughly correspond to "regions with14the same base region". This allows certain operations to be more efficient,15such as invalidation.16 17Regions that do not have a known offset use a special "symbolic" offset. These18keys store both the original region, and the "concrete offset region" -- the19last region whose offset is entirely concrete. (For example, in the expression20``foo.bar[1][i].baz``, the concrete offset region is the array ``foo.bar[1]``,21since that has a known offset from the start of the top-level ``foo`` struct.)22 23 24Binding Invalidation25--------------------26 27Supporting both concrete and symbolic offsets makes things a bit tricky. Here's28an example:29 30.. code-block:: cpp31 32 foo[0] = 0;33 foo[1] = 1;34 foo[i] = i;35 36After the third assignment, nothing can be said about the value of ``foo[0]``,37because ``foo[i]`` may have overwritten it! Thus, *binding to a region with a38symbolic offset invalidates the entire concrete offset region.* We know39``foo[i]`` is somewhere within ``foo``, so we don't have to invalidate40anything else, but we do have to be conservative about all other bindings within41``foo``.42 43Continuing the example:44 45.. code-block:: cpp46 47 foo[i] = i;48 foo[0] = 0;49 50After this latest assignment, nothing can be said about the value of ``foo[i]``,51because ``foo[0]`` may have overwritten it! *Binding to a region R with a52concrete offset invalidates any symbolic offset bindings whose concrete offset53region is a super-region **or** sub-region of R.* All we know about ``foo[i]``54is that it is somewhere within ``foo``, so changing *anything* within ``foo``55might change ``foo[i]``, and changing *all* of ``foo`` (or its base region) will56*definitely* change ``foo[i]``.57 58This logic could be improved by using the current constraints on ``i``, at the59cost of speed. The latter case could also be improved by matching region kinds,60i.e. changing ``foo[0].a`` is unlikely to affect ``foo[i].b``, no matter what61``i`` is.62 63For more detail, read through ``RegionStoreManager::removeSubRegionBindings`` in64RegionStore.cpp.65 66 67ObjCIvarRegions68---------------69 70Objective-C instance variables require a bit of special handling. Like struct71fields, they are not base regions, and when their parent object region is72invalidated, all the instance variables must be invalidated as well. However,73they have no concrete compile-time offsets (in the modern, "non-fragile"74runtime), and so cannot easily be represented as an offset from the start of75the object in the analyzer. Moreover, this means that invalidating a single76instance variable should *not* invalidate the rest of the object, since unlike77struct fields or array elements there is no way to perform pointer arithmetic78to access another instance variable.79 80Consequently, although the base region of an ObjCIvarRegion is the entire81object, RegionStore offsets are computed from the start of the instance82variable. Thus it is not valid to assume that all bindings with non-symbolic83offsets start from the base region!84 85 86Region Invalidation87-------------------88 89Unlike binding invalidation, region invalidation occurs when the entire90contents of a region may have changed---say, because it has been passed to a91function the analyzer can model, like memcpy, or because its address has92escaped, usually as an argument to an opaque function call. In these cases we93need to throw away not just all bindings within the region itself, but within94its entire cluster, since neighboring regions may be accessed via pointer95arithmetic.96 97Region invalidation typically does even more than this, however. Because it98usually represents the complete escape of a region from the analyzer's model,99its *contents* must also be transitively invalidated. (For example, if a region100``p`` of type ``int **`` is invalidated, the contents of ``*p`` and ``**p`` may101have changed as well.) The algorithm that traverses this transitive closure of102accessible regions is known as ClusterAnalysis, and is also used for finding103all live bindings in the store (in order to throw away the dead ones). The name104"ClusterAnalysis" predates the cluster-based organization of bindings, but105refers to the same concept: during invalidation and liveness analysis, all106bindings within a cluster must be treated in the same way for a conservative107model of program behavior.108 109 110Default Bindings111----------------112 113Most bindings in RegionStore are simple scalar values -- integers and pointers.114These are known as "Direct" bindings. However, RegionStore supports a second115type of binding called a "Default" binding. These are used to provide values to116all the elements of an aggregate type (struct or array) without having to117explicitly specify a binding for each individual element.118 119When there is no Direct binding for a particular region, the store manager120looks at each super-region in turn to see if there is a Default binding. If so,121this value is used as the value of the original region. The search ends when122the base region is reached, at which point the RegionStore will pick an123appropriate default value for the region (usually a symbolic value, but124sometimes zero, for static data, or "uninitialized", for stack variables).125 126.. code-block:: cpp127 128 int manyInts[10];129 manyInts[1] = 42; // Creates a Direct binding for manyInts[1].130 print(manyInts[1]); // Retrieves the Direct binding for manyInts[1];131 print(manyInts[0]); // There is no Direct binding for manyInts[0].132 // Is there a Default binding for the entire array?133 // There is not, but it is a stack variable, so we use134 // "uninitialized" as the default value (and emit a135 // diagnostic!).136 137NOTE: The fact that bindings are stored as a base region plus an offset limits138the Default Binding strategy, because in C aggregates can contain other139aggregates. In the current implementation of RegionStore, there is no way to140distinguish a Default binding for an entire aggregate from a Default binding141for the sub-aggregate at offset 0.142 143 144Lazy Bindings (LazyCompoundVal)145-------------------------------146 147RegionStore implements an optimization for copying aggregates (structs and148arrays) called "lazy bindings", implemented using a special SVal called149LazyCompoundVal. When the store is asked for the "binding" for an entire150aggregate (i.e. for an lvalue-to-rvalue conversion), it returns a151LazyCompoundVal instead. When this value is then stored into a variable, it is152bound as a Default value. This makes copying arrays and structs much cheaper153than if they had required memberwise access.154 155Under the hood, a LazyCompoundVal is implemented as a uniqued pair of (region,156store), representing "the value of the region during this 'snapshot' of the157store". This has important implications for any sort of liveness or158reachability analysis, which must take the bindings in the old store into159account.160 161Retrieving a value from a lazy binding happens in the same way as any other162Default binding: since there is no direct binding, the store manager falls back163to super-regions to look for an appropriate default binding. LazyCompoundVal164differs from a normal default binding, however, in that it contains several165different values, instead of one value that will appear several times. Because166of this, the store manager has to reconstruct the subregion chain on top of the167LazyCompoundVal region, and look up *that* region in the previous store.168 169Here's a concrete example:170 171.. code-block:: cpp172 173 CGPoint p;174 p.x = 42; // A Direct binding is made to the FieldRegion 'p.x'.175 CGPoint p2 = p; // A LazyCompoundVal is created for 'p', along with a176 // snapshot of the current store state. This value is then177 // used as a Default binding for the VarRegion 'p2'.178 return p2.x; // The binding for FieldRegion 'p2.x' is requested.179 // There is no Direct binding, so we look for a Default180 // binding to 'p2' and find the LCV.181 // Because it's a LCV, we look at our requested region182 // and see that it's the '.x' field. We ask for the value183 // of 'p.x' within the snapshot, and get back 42.184