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The cost of dynamic vs. static dispatch in C++ (2013)

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Re: The cost of dynamic vs. static dispatch in C++ (2013)

#31

Post by [deleted] »

MichaelGG wrote: post #19
twoodfin wrote: post #16

As the article demonstrates, in anything other than trivial microbenchmarks it's the inability to inline across a virtual call that will cost you. Inlining is the single most fundamental optimization for a modern C++ compiler. It enables the 0-cost abstractions for which C++ is rightly famous.

Why shouldn't it be possible to inline virtual calls? The CLR folks were complaining about this, too. But it seems rather straightforward to figure out the actual implementation that's commonly called and inline it, perhaps with a guard for uncommon cases. And I wouldn't be surprised if many apps end up with only a single actual implementation, so the entire virtual overhead can just be dropped.

[deleted]

Re: The cost of dynamic vs. static dispatch in C++ (2013)

#32

Post by bit_razor »

Rusky wrote: post #30
kazinator wrote: post #27

Earlier quoted context omitted.

If everything else can be controlled, then virtual calls are being used unnecessarily. The fact that an indirect calling mechanism which has to first use indirection to retrieve the address to be called (and possibly do more work, like fixing up a pointer) is slower than a static call is completely unsurprising. If virtual calls are used essentially, then it's a canoe-versus-bicycle comparison, because everything els…

Regardless, it's still useful to actually test the performance of virtual vs static calls- how much slower are they? in which situations can the compiler devirtualize them? Or should we only ever benchmark the entire software stack at once, even though people do in fact sometimes use CRTP and virtual functions in the same situations?

My beef with this article is that it misleads you believe that need CRTP is the only way to eliminate vtable lookups on interface calls. It left me wondering how CRTP compared to invoking DynamicImplementation::tick() directly. The article is actually quite great otherwise.
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