Inlining – The Ultimate Optimisation
xania.org24 points by PaulHoule 4 days ago
24 points by PaulHoule 4 days ago
Is there a name for duplicating function calls such that different optimizations for the same function can be compiled, but they are not fully duplicated at every call site?
I think that is called specialization (https://www.linkedin.com/posts/compilers-lab_compiler-progra...).
Even if the compiler doesn’t explicitly do it, it can happen when doing subsequent optimization steps after inlining such as constant folding and dead code elimination.
Specialization is one of the reasons my call trees are just a little bit deeper than what one would expect given my loud but moderate stance on function splitting. Uncle Bob is nuts for espousing one line functions. But the answer to Bob being a lunatic is not two page functions. I think you can say a lot in five to six lines, and not overshoot meaningful names into word salad because you’ve run out of ideas. That’s still small enough for branch prediction, inlining, and specialization to kick in per call site, particularly if some callers follow one conditional branch and the others favors the other.
If I understand what you're asking for correctly, function cloning.
If you have f(x, y) and the compiler realizes the function optimizes nicely when y == 2 it can create a clone of f with a fixed argument y == 2, optimize that, and rewrite the appropriate call sites to call the clone.
There was a weird period in JavaScript’s history where the threshold for inlining was rather fixed and counted comments as part of the function weight. So there was code that would go faster if you deleted the comments.
I believe it was counting AST nodes rather than bytes, otherwise that would have also created problems for descriptive function names as well and that would have been what we heard about instead of comments.
force inline and related attributes are critical to get the right decision made consistently.
There's also flatten; unfortunately no equivalent with MSVC.
That's the reason why polymorphism is sometimes described as slow. It's not really slow... But it prevents inlining and therefore always is a function call as opposed to sometimes no function call. It's not the polymorphism is slow. It's that alternatives can sometimes compile to zero
Pedantic, but I assume you're referring to virtual methods?
Ad hoc polymorphism (C++ templates) and parametric polymorphism (Rust) can be inlined. Although those examples are slow to compile, because they must be specialized for each set of generic arguments.
On the other hand, if the compiler can prove at compile-time what type the object must have at run-time, it can eliminate the dynamic dispatch and effectively re-enable inlining.