Monday, 24 July 2017

Java Intrinsic magic

I got question on atomicinteger-java-7-vs-java-8 blog about implementation of unsafe.getAndAddInt function



It looks like JDK8 is still using CAS version.
If you just look at the code in IDE then you are correct , but JVM is intelligent hotspot compiler.

Code goes through various optimization before it is executed, optimization list is huge and it will required blog series to just touch on it.

In this blog i will discuss about Intrinsic function

As per WikiPedia 


In compiler theory, an intrinsic function is a function available for use in a given programming language whose implementation is handled specially by the compiler. Typically, it substitutes a sequence of automatically generated instructions for the original function call, similar to an inline function. Unlike an inline function though, the compiler has an intimate knowledge of the intrinsic function and can therefore better integrate it and optimize it for the situation. This is also called builtin function in many languages.
Compilers that implement intrinsic functions generally enable them only when the user has requested optimization, falling back to a default implementation provided by the language runtime environment otherwise.


So in other words, some function are just like native function, it is different from native function written by us.


Compiler makes decision to use native/special function if is available other wise if will fallback to default implementation.

Unsafe.getAndAddInt is special function that gets advantage of optimization, but normal code is
still required for case when optimization is not requested or not available

To verify this we have to look at Assembly code generated.

how-to-print-dissasembly-from-jit-code article has all the details you need for this.

You can also download dll/lib for windows/linux from github

For below code snippet, optimization kicks in

Assembly code for above code looks like below ( look at line number 7, it is xadd)



Java maintains list of all intrinsic functions in code @ vmSymbols.hpp

Hotspot compiler is full of magic:-)

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