In computer science, pointer analysis, or points-to analysis, is a static code analysis technique that establishes which pointers, or heap references, can point to which variables, or storage locations. It is often a component of more complex analyses such as escape analysis. A closely related technique is shape analysis.
This is the most common colloquial use of the term. A secondary use has pointer analysis be the collective name for both points-to analysis, defined as above, and alias analysis. Points-to and alias analysis are closely related but not always equivalent problems.
Consider the following C program:
int *id(int* p) { return p; } void main(void) { int x; int y; int *u = id(&x); int *v = id(&y); }
A pointer analysis computes a mapping from pointer expressions to a set of allocation sites of objects they may point to. For the above program, an idealized, fully precise analysis would compute the following results:
&x
main::x
&y
main::y
u
v
p
(Where X::Y represents the stack allocation holding the local variable Y in the function X.)
X::Y
Y
X
However, a context-insensitive analysis such as Andersen's or Steensgaard's algorithm would lose precision when analyzing the calls to id, and compute the following result:
id
As a form of static analysis, fully precise pointer analysis can be shown to be undecidable.[1] Most approaches are sound, but range widely in performance and precision. Many design decisions impact both the precision and performance of an analysis; often (but not always) lower precision yields higher performance. These choices include:[2][3]
malloc
Pointer analysis algorithms are used to convert collected raw pointer usages (assignments of one pointer to another or assigning a pointer to point to another one) to a useful graph of what each pointer can point to.[4]
Steensgaard's algorithm and Andersen's algorithm are common context-insensitive, flow-insensitive algorithms for pointer analysis. They are often used in compilers, and have implementations in SVF [5] and LLVM.
Many approaches to flow-insensitive pointer analysis can be understood as forms of abstract interpretation, where heap allocations are abstracted by their allocation site (i.e., a program location).[6]
Many flow-insensitive algorithms are specified in Datalog, including those in the Soot analysis framework for Java.[7]
Context-sensitive, flow-sensitive algorithms achieve higher precision, generally at the cost of some performance, by analyzing each procedure several times, once per context.[8] Most analyses use a "context-string" approach, where contexts consist of a list of entries (common choices of context entry include call sites, allocation sites, and types).[9] To ensure termination (and more generally, scalability), such analyses generally use a k-limiting approach, where the context has a fixed maximum size, and the least recently added elements are removed as needed.[10] Three common variants of context-sensitive, flow-insensitive analysis are:[11]
In call-site sensitivity, the points-to set of each variable (the set of abstract heap allocations each variable could point to) is further qualified by a context consisting of a list of callsites in the program. These contexts abstract the control-flow of the program.
The following program demonstrates how call-site sensitivity can achieve higher precision than a flow-insensitive, context-insensitive analysis.
int *id(int* p) { return p; } void main(void) { int x; int y; int *u = id(&x); // main.3 int *v = id(&y); // main.4 }
For this program, a context-insensitive analysis would (soundly but imprecisely) conclude that p can point to either the allocation holding x or that of y, so u and v may alias, and both could point to either allocation:
A callsite-sensitive analysis would analyze id twice, once for main.3 and once for main.4, and the points-to facts for p would be qualified by the call-site, enabling the analysis to deduce that when main returns, u can only point to the allocation holding x and v can only point to the allocation holding y:
main.3
main.4
[]
[main.3]
[main.4]
In an object sensitive analysis, the points-to set of each variable is qualified by the abstract heap allocation of the receiver object of the method call. Unlike call-site sensitivity, object-sensitivity is non-syntactic or non-local: the context entries are derived during the points-to analysis itself.[12]
Type sensitivity is a variant of object sensitivity where the allocation site of the receiver object is replaced by the class/type containing the method containing the allocation site of the receiver object.[13] This results in strictly fewer contexts than would be used in an object-sensitive analysis, which generally means better performance.