A leap year check in three instructions

Daniel Berlin's https://news.ycombinator.com/item?id=9397169 does kind of disagree, saying, "If GCC didn't beat an expert at optimizing interpreter loops, it was because they didn't file a bug and give us code to optimize," but his actual example is the CPython interpreter loop, which is light-years from the kind of hand-optimized assembly interpreter Mike Pall's post is talking about, and moreover it wasn't feeding an interpreter loop to GCC but rather replacing interpretation with run-time compilation. Mostly what he disagrees about is the same thing Regehr disagrees about: whether there's enough code in the category of "not worth hand-optimizing but still runs often enough to matter", not whether you can beat a compiler by hand-optimizing your code. On the contrary, he brings up whole categories of code where compilers can't hope to compete with hand-optimization, such as numerical algorithms where optimization requires sacrificing numerical stability. mpweiher's comment in response discusses other scenarios where compilers can't hope to compete, like systems-level optimization.

It's worth reading the comments by haberman and Mike Pall in the HN thread there where they correct Berlin about LuaJIT, and kjksf also points out a number of widely-used libraries that got 2–4× speedups over optimized C by hand-optimizing the assembly: libjpeg-turbo, Skia, and ffmpeg. It'd be interesting to see if the intervening 10 years have changed the situation, because GCC and LLVM have improved in that time, but I doubt they've improved by even 50%, much less 300%.