\"10.3 The value/reference tradeoff The reference model of variables is arguably

Question

"10.3 The value/reference tradeoff

The reference model of variables is arguably more elegant than the value model, particularly for object-oriented languages, but generally requires that objects be allocated from the heap, and imposes (in the absence of compiler optimizations) an extra level of indirection on every access. The value model tends to be more efcient, but makes it difcult to control initialization. In languages with a reference model (including Java), an optimization known as escape analysis can sometimes allow the compiler to determine that references to a given object will always be contained within (will never escape) a given method. In this case the object can be allocated in the method’s stack frame, avoiding the overhead of heap allocation and, more signicantly, eventual garbage collection."

Explain how escape analysis, mentioned briey in Sidebar 10.3, could be used to reduce the cost of certain synchronized statementsand methods in Java.

Explanation / Answer

Hint

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.

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