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0 pointsscott_w14y ago0 comments

The issue isn't programmers writing fib(20) repeatedly, it's that fib(20) may be called repeatedly by the system during the natural course of operation.

A language with the constraints of Haskell allows the JIT to recognise that fib(20) is being called a lot, or will be called again, and simply inline that function call itself. That is completely impossible for a C compiler to achieve unless the developer writes: if (arg == 20) { return X; }

That has two problems: a) The developer has to profile and look for specific behaviour in the system b) The optimisation may change depending on the state of the world e.g. the system may pick a random constant every day to compute the Fibonacci sequence for.

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riobard14y ago

I think you are confusing the “meta-programming” and “run-time optimization” as described by the original article.

The JIT optimization you mentioned is actually his point 3, while I was addressing his point 2, “meta-programing” during compile-time. True, he mentioned in languages like Lisp where compile-time and runtime can be interleaved it would be a feasible solution, but he was focusing on statically-compiled features more in line with C++ templates, which I doubt would bring anything closer to what you were talking about.

C doesn't provide runtime profiling and optimization, but if performance is really critical, offline profiling to find hotspots and replacing them with pre-computed constants is a must. On average, the complexity and additional overhead brought by the JIT techniques probably won't offset the performance gain it buys compared to carefully profiled and optimized C.

PS. Why downvoting the grandparent comment?

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