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Incompiler optimization,escape analysis is a method for determining the dynamic scope ofpointers – where in the program a pointer can be accessed. It is related topointer analysis andshape analysis.
When a variable (or an object) is allocated in asubroutine, apointer to the variable canescape to otherthreads of execution, or to calling subroutines. If an implementation usestail call optimization (usually required forfunctional languages), objects may also be seen as escaping tocalled subroutines. If a language supports first-classcontinuations (as doScheme andStandard ML of New Jersey), portions of thecall 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:[1]
Inobject-oriented programming languages,dynamic compilers are particularly good candidates for performing escape analysis. In traditionalstatic 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.[1]
The popularity of theJava programming language has made escape analysis a target of interest. Java's combination of heap-only object allocation, built-in threading, the SunHotSpot dynamic compiler, andOpenJ9'sjust-in-time compiler (JIT) creates a candidate platform for escape analysis related optimizations (seeEscape analysis in Java). Escape analysis is implemented in Java Standard Edition 6. Some JVMs support a stronger variant of escape analysis calledpartial escape analysis that makes scalar replacement of an allocated object possible even if the object escapes in some paths of a function.[4]
classMain{publicstaticvoidmain(String[]args){example();}publicstaticvoidexample(){Foofoo=newFoo();//allocBarbar=newBar();//allocbar.setFoo(foo);}}classFoo{}classBar{privateFoofoo;publicvoidsetFoo(Foofoo){this.foo=foo;}}
In this example, two objects are created (commented with alloc), and one of them is given as an argument to a method of another. The methodsetFoo() stores a reference to a received Foo object. If the Bar object was on the heap then the reference to Foo would escape. But in this case a compiler can determine, with escape analysis, that the Bar object itself does not escape the invocation ofexample(). As a result, the reference to Foo cannot escape either, and the compiler can safely allocate both objects on the stack.
In the following example, the vectorp does not escape intog, so it can be allocated on the stack and then removed from the stack before callingg.
(define(fx)(let((p(make-vector10000)))(fill-vector-with-good-stuffp)(g(vector-refp7023))))
If, however, we had
(define(fx)(let((p(make-vector10000)))(fill-vector-with-good-stuffp)(gp)))
then eitherp would need to be allocated on the heap or (ifg is known to the compiler whenf is compiled, and behaves well) allocated on the stack in such a fashion that it can remain in place wheng is called.
If continuations are used to implement exception-like control structures, escape analysis can often detect this to avoid having to actually allocate a continuation and copy the call stack into it. For example, in
;;Reads scheme objects entered by the user. If all of them are numbers,;;returns a list containing all of them in order. If the user enters one that;;is not a number, immediately returns #f.(define(getnumlist)(call/cc(lambda(continuation)(define(get-numbers)(let((next-object(read)))(cond((eof-object?next-object)'())((number?next-object)(consnext-object(get-numbers)))(else(continuation#f)))))(get-numbers))))
escape analysis will determine that the continuation captured bycall/cc doesn't escape, so no continuation structure needs to be allocated, and invoking the continuation by callingcontinuation can be implemented by unwinding the stack.