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POLTO, Devirtualization, and Virtual Tables
 

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    <p>Comparing virtual functions in C++ and virtual tables in C, do compilers in general (and for sufficiently large projects) do as good a job at devirtualization?</p> <p>Naively, it seems like virtual functions in C++ have slightly more semantics, thus may be easier to devirtualize.</p> <p><em>Update:</em> Mooing Duck mentioned inlining devirtualized functions. A quick check shows missed optimizations with virtual tables:</p> <pre><code>struct vtab { int (*f)(); }; struct obj { struct vtab *vtab; int data; }; int f() { return 5; } int main() { struct vtab vtab = {f}; struct obj obj = {&amp;vtab, 10}; printf("%d\n", obj.vtab-&gt;f()); } </code></pre> <p>My GCC will not inline f, although it is called directly, i.e., devirtualized. The equivalent in C++,</p> <pre><code>class A { public: virtual int f() = 0; }; class B { public: int f() {return 5;} }; int main() { B b; printf("%d\n", b.f()); } </code></pre> <p>does even inline f. So there's a first difference between C and C++, although I don't think that the added semantics in the C++ version are relevant in this case.</p> <p><em>Update 2:</em> In order to devirtualize in <em>C</em>, the compiler has to prove that the function pointer in the virtual table has a certain value. In order to devirtualize in <em>C++</em>, the compiler has to prove that the object is an instance of a particular class. It would seem that the proof is harder in the first case. However, virtual tables are typically modified in only very few places, and most importantly: just because it looks harder, doesn't mean that compilers aren't as good in it (for otherwise you might argue that xoring is generally faster than adding two integers).</p>
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    1. COjust to clarify, are you asking if function pointers whose values are "known" at compile time are reduced to direct calls in with a good optimizing C compiler?
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    2. COYes. In particular in real code, where it may be difficult for the compiler to deduce what it knows.
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    3. COWho even cares. Correctness is king as long as the code works why do you care about such a small macro optimization. The benefits of this kind of things are always going to be swamped by other factors that you can't even think about.
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      1. USMartin York
 

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