When can the C++ compiler devirtualize a call?

MSVC (Microsoft's C++ compiler) had an pretty advanced inter-procedural (LTO) way of doing devirtualization, but it was so buggy and slow that it eventually got disabled. It was trying to prove that a pointer can only target a certain class all the time (or maybe a couple), but things get really messy with typical C/C++ code and even worse once you have DLLs which may inject new derived classes.

Nice post, I had never thought about those tricky ways of proving leafness outside of the obvious "final".

Re "When we know the dynamic type", I made a similar assertion on HN years ago, and of course it turned out that there's a weird wrinkle:

If the code in your snippet is expanded to:

    Derived d;
    Base *p = &d;
    any_external_func(); // Added
    p->f();

where any_external_func() is defined in some other translation unit (and, I'm now fairly sure, Derived's ctor is also defined in another translation unit, or it transitively calls something that is), this would seem not to affect anything -- but in fact the compiler must not assume that p's dynamic type is still Derived by the final line. Why? Because the following insane sequence of events might have happened:

1. d's ctor registers its this pointer in some global table.

2. Using this table, any_external_func() calls d's dtor and then overwrites it in place with a fresh instance of Base using placement new, which replaces everything, including its vtable, meaning that p->f() should call Base's version, not Derived's.

(It might be UB to call placement new on static or automatic storage holding an in-lifetime object, I don't know. If so, the above construction is moot -- but the closely analogous situation where we instead dynamically allocate dp = new Derived() still goes through, and is nearly as surprising.)

Conclusion: devirtualization optimization is so fragile, so that it's better to avoid using virtual calls in performance-critical code to be sure, that no virtual call happens.

For comparison, Java will devirtualize calls if the call site typically calls only a single type. To the extent that writing a byte to a ByteBuffer, which looks like five or six virtual calls if you follow the java source code, actually ends up being a single assembly store instruction.

I ran into a fun crash a year or so ago in the interaction of clang’s profile guided speculative devirtualization and identical code folding (ICF) done by BOLT on the binary.

Clang relied on checking the address of a function pointer in the vtable to validate the class was the type it expected, but it wasn’t necessarily the function that is currently being called. But due to ICF two different subclasses with two different functions shared the same address, so the code made incorrect assumptions about the type. Then it promptly segfaulted.

I also wonder what about devirtualization of dyn traits in Rust. Sure, impl traits like foo<T: AsRef>(bar: T) or foo(bar: impl AsRef) is for sure devirtualized, but foo(bar: &dyn AsRef) or most likely foo(bar: Box<dyn AsRef>), far as I remember, isn't always devirtualized. Sometimes it does, sometimes it doesn't. I wonder if it is MIR that did it, or just completely handed off to LLVM for detection.

Maybe just me, but doesn't most C++ code being written today not use inheritance very much, and so make virtual dispatch moot?

> final method

A pedantic Pete would mention C++ has member functions.

Blog post is from 2021, should update the title to include that.

Interesting study though and I wonder if it has at all improved since.

I’ve been programming C++ on and off for over 20 years and have had my moments where I’ve checked on godbolt to make sure classes got devirtualised.

This year, I’ve finally taken the plunge to properly learn Rust (I’ve used it for little things over the years, but never for anything particularly extensive) and one thing that jumped out at me is that you don’t need to think about it, because Rust makes it explicit: everything is statically known unless you explicitly ask for it to be virtual.

[edit: since it wasn’t clear, I mean polymorphism in rust is static by default while in c++ static polymorphism requires relying on the compiler or using templates, otherwise polymorphism is via virtual]

It’s was a little annoying at first because some things don’t just work automatically, but once I got used to it, it was wonderful to never have to think about when the compiler might do something. You also don’t need dynamism most of the time.

I still like tinkering in C++, but I do find you need to know too much about compiler heuristics.