The Performance Impact of C++'s `final` Keyword • Hacker News Board

Negitivefrags wrote: post #37

mgaunard wrote: post #2

What final enables is devirtualization in certain cases. The main advantage of devirtualization is that it is necessary for inlining. Inlining has other requirements as well -- LTO pretty much covers it. The article doesn't have sufficient data to tell whether the testcase is built in such a way that any of these optimizations can happen or is beneficial.

See this is why I find this odd. Is there a theory as to how devirtualisation could hurt performance?

It basically never should unless the inliner made a terrible judgement. Devirtualizing in C++ can remove 3 levels of pointer chasing, all of which could be cache misses. Many optimizations in modern compilers require the context of the function to be inlined to make major optimizations, which requires devirtualization. The only downside is I$ pressure, but this is generally not a problem because hot loops are usually tight.