Escape analysis
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In programming language compiler optimization theory, escape analysis is a method for determining the dynamic scope of pointers. It is related to pointer analysis and shape analysis (software).
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 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.
[edit] Optimizations
A compiler can use the results of escape analysis as 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.
- 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.
[edit] Practical considerations
In object-oriented programming languages dynamic compilers are particularly good candidates for performing escape analysis. In traditional static compilation method overriding can make escape analysis impossible, as any called method might be overridden by a version that allows a pointer to escape. Dynamic compilers can perform escape analysis using the available information on overloading, and re-do the analysis when relevant methods are overridden by dynamic code loading.
Escape analysis has been a target of interest in particular due to the Java programming language. Java's combination of heap-only object allocation, built-in threading, and the Sun HotSpot dynamic compiler creates a candidate platform for escape analysis related optimizations (see Escape analysis in Java). Escape analysis is implemented in Java Standard Edition 6.
