Optimization by Java Compiler

Question

Recently, I was reading this article.

According to that article, Java Compiler i.e. javac does not perform any optimization while generating bytecode. Is it really true? If so, then can it be implemented as an intermediate code generator to remove redundancy and generate optimal code?

Answer 1

javac will only do a very little optimization, if any.

The point is that the JIT compiler does most of the optimization - and it works best if it has a lot of information, some of which may be lost if javac performed optimization too. If javac performed some sort of loop unrolling, it would be harder for the JIT to do that itself in a general way - and it has more information about which optimizations will actually work, as it knows the target platform.

Answer 2

I stopped reading when I got to this section:

More importantly, the javac compiler does not perform simple optimizations like loop unrolling, algebraic simplification, strength reduction, and others. To get these benefits and other simple optimizations, the programmer must perform them on the Java source code and not rely on the javac compiler to perform them.

Firstly, doing loop unrolling on Java source code is hardly ever a good idea. The reason javac doesn't do much in the way of optimization is that it's done by the JIT compiler in the JVM, which can make much better decisions that the compiler could, because it can see exactly which code is getting run the most.