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Runtime optimization of static languages: JIT for C++?

Asked 2009-04-23T04:42:50.217
22

Is anyone using JIT tricks to improve the runtime performance of statically compiled languages such as C++? It seems like hotspot analysis and branch prediction based on observations made during runtime could improve the performance of any code, but maybe there's some fundamental strategic reason why making such observations and implementing changes during runtime are only possible in virtual machines. I distinctly recall overhearing C++ compiler writers mutter "you can do that for programs written in C++ too" while listening to dynamic language enthusiasts talk about collecting statistics and rearranging code, but my web searches for evidence to support this memory have come up dry.

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3 Answers

10

I did that kind of optimization quite a lot in the last years. It was for a graphic rendering API that I've implemented. Since the API defined several thousand different drawing modes as general purpose function was way to slow.

I ended up writing my own little Jit-compiler for a domain specific language (very close to asm, but with some high level control structures and local variables thrown in).

The performance improvement I got was between factor 10 and 60 (depended on the complexity of the compiled code), so the extra work paid off big time.

On the PC I would not start to write my own jit-compiler but use either LIBJIT or LLVM for the jit-compilation. It wasn't possible in my case due to the fact that I was working on a non mainstream embedded processor that is not supported by LIBJIT/LLVM, so I had to invent my own.

answered 2009-04-23T10:28:58.620
5

Microsoft Visual Studio calls this "profile guided optimization"; you can learn more about it at MSDN. Basically, you run the program a bunch of times with a profiler attached to record its hotspots and other performance characteristics, and then you can feed the profiler's output into the compiler to get appropriate optimizations.

answered 2009-04-23T04:49:56.157
2

Reasonable question - but with a doubtful premise.

As in Nils' answer, sometimes "optimization" means "low-level optimization", which is a nice subject in its own right.

However, it is based on the concept of a "hot-spot", which has nowhere near the relevance it is commonly given.

Definition: a hot-spot is a small region of code where a process's program counter spends a large percentage of its time.

If there is a hot-spot, such as a tight inner loop occupying a lot of time, it is worth trying to optimize at the low level, if it is in code that you control (i.e. not in a third-party library).

Now suppose that inner loop contains a call to a function, any function. Now the program counter is not likely to be found there, because it is more likely to be in the function. So while the code may be wasteful, it is no longer a hot-spot.

There are many common ways to make software slow, of which hot-spots are one. However, in my experience, that is the only one of which most programmers are aware, and the only one to which low-level optimization applies.

See this.

answered 2009-04-24T18:31:00.330

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