A lot of programmers believe that compilers are magic black boxes in which you put your messy code in and get a nice optimized binary out. The hallway philosophers will often start a debate on which language features or compiler flags to use in order to capture the full power of the compiler’s magic. If you have ever seen the GCC codebase, you would really believe it must be doing some magical optimizations coming from another planet.

Nevertheless, if you analyze the compiler’s output you will found out that the compilers are not really that great at optimizing your code. Not because people writing them wouldn’t know how to generate efficient instructions, but simply because the compilers can only reason about a very small part of the problem space 1.

In order to understand why magic compiler optimizations are not going to speed up your software, we have to go back in time when the dinosaurs roamed the earth and the processors were still extremely slow…

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