I'd like to see (good) examples of optimizations performed by compilers (static and JIT). Why?
- To learn what we don't have to optimize ourselves (often leading to better code)
- To be amazed
closed as too broad by MichaelT, gnat, GlenH7, Bart van Ingen Schenau, Dan Pichelman Nov 18 '13 at 22:25
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Someone once tried to examine optimizers in C code. They ran this code through various C compilers with different levels of optimization:
First the compiler noticed that
The loop is empty, so we can remove that, too:
A classical example of removing unused code.
When I wrote the compiler for my implementation of ECMAScript I noticed that I could not perform recursion to a depth that was greater than 3000 calls. To me this was unacceptable so after a bit of experimentation I came up with the idea of replacing the return value with the arguments that were passed recursively. With this in place recursion became a viable option when writing code for my implementation.
I don't believe that you would ever see this technique used in a professional context. Given the current code a strong relationship is implied
Here is a proper example of tail-call optimization:
Reflected CIL as C#
Notice how the compiler is simply reusing the same stack frame in a while loop. This is very similar to what I implemented but without the unnecessary friendliness between the compiler and the run-time.