Can someone explain what inlined means here? It was my assumption that the compiler will automatically inline functions and you don't need to do it explicitly. Unless it means something else in this context
It means not using explicit functions, just writing the same code as little inline blocks inside the main function because it allows you to see all the things that would be hidden if all the code wasn't immediately visible.
To the other point though, the quality of compiler inlining heuristics is a bit of a white lie. The compiler doesn't make optimal choices, but very few people care enough to notice the difference. V8 used a strategy of considering the entire source code function length (including comments) in inlining decisions for many years, despite the obvious drawbacks.
Well there's also compiler directives other than `inline`, like msvc's `__inline` and `__forceinline` (which probably also have an equivalent in gcc or clang), so personally I don't think you need to make the tradeoff between readability and reusability while avoiding function calls. Not to mention C++ constevals and C-style macros, though consteval didn't exist in 2007
__forceinline is purely a suggestion to the compiler, not a requirement. Carmack's point isn't about optimizing the costs of function calls though. It's about the benefits to code quality by having everything locally visible to the developer.
It's an interesting view because I find neatly compartmentalized functions easier to read and less error prone, though he does point out that copying chunks of code such as vector operations can lead to bugs when you forget to change some variable. I guess it depends on the function. Something like
Vector c = dotProduct(a, b);
is readable enough and doesn't warrant inlining, I think. There's nothing about `dotProduct` that I would expect to have any side effects, especially if its prototype looks like:
Vector dotProduct(Vector const& a, Vector const& b) const;
