Why are JIT-ed languages still slower and less memory efficient than native C/C++?

Question

Interpreters do a lot of extra work, so it is understandable that they end up significantly slower than native machine code. But languages such as C# or Java have JIT compilers, which supposedly compile to platform native machine code.

And yet, according to benchmarks that seem legit enough, in most of the cases are still 2-4x times slower than C/C++? Of course, I mean compared to equally optimized C/C++ code. I am well aware of the optimization benefits of JIT compilation and their ability to produce code that is faster than poorly optimized C+C++.

And after all that noise about how good the Java memory allocation is, why such a horrendous memory usage? 2x to 50x, on average about 30x times more memory is being used across that particular benchmark suite, which is nothing to sneeze at...

NOTE that I don't want to start a WAR, I am asking about the technical details which define those performance and efficiency figures.

Answer 1

Some reasons for differences;

• JIT compilers mostly compile quickly and skip some optimizations that take longer to find.

• VM's often enforce safety and this slows execution. E.g. Array access is always bounds checked in .Net unless guaranteed within the correct range

• Using SSE (great for performance if applicable) is easy from C++ and hard from current VM's

• Performance gets more priority in C++ over other aspects when compared to VM's

• VM's often keep unused memory a while before returning to the OS seeming to 'use' more memory.

• Some VM's make objects of value types like int/ulong.. adding object memory overhead

• Some VM's auto-Align data structures a lot wasting memory (for performance gains)

• Some VM's implement a boolean as int (4 bytes), showing little focus on memoryconservation.