Escape analysis

In compiler optimization, escape analysis is a method for determining the dynamic scope of pointers - where in the program a pointer can be accessed. It is related to pointer analysis and shape analysis. When a variable (or an object) is allocated in a subroutine, a pointer to the variable can escape to other threads of execution, or to calling subroutines. If an implementation uses tail call optimization (usually required for functional languages), objects may also be seen as escaping to called subroutines. If a language supports first-class continuations (as do Scheme and Standard ML of New Jersey), portions of the call stack may also escape. If a subroutine allocates an object and returns a pointer to it, the object can be accessed from undetermined places in the program - the pointer has "escaped". Pointers can also escape if they are stored in global variables or other data structures that, in turn, escape the current procedure. Escape analysis determines all the places where a pointer can be stored and whether the lifetime of the pointer can be proven to be restricted only to the current procedure and/or thread. A compiler can use the results of escape analysis as a basis for optimizations: Converting heap allocations to stack allocations. If an object is allocated in a subroutine, and a pointer to the object never escapes, the object may be a candidate for stack allocation instead of heap allocation. In garbage-collected languages this can reduce how often the collector needs to run. Synchronization elision. If an object is found to be accessible from one thread only, operations on the object can be performed without synchronization. Breaking up objects or scalar replacement. An object may be found to be accessed in ways that do not require the object to exist as a sequential memory structure. This may allow parts (or all) of the object to be stored in CPU registers instead of in memory. In object-oriented programming languages, dynamic compilers are particularly good candidates for performing escape analysis.