4 ms·

The jit can optimize away virtual function calls. ParentClass g = ....might be a subclass... for (int i=0;i<100000;i++) { g.func(i); } The

The jit can optimize away virtual function calls. ParentClass g = ....might be a subclass... for (int i=0;i<100000;i++) { g.func(i); } The JIT can optimize away the vtable lookups to find func, and sometimes inline the code. Ok, maybe you could do this in C++ too. ParentClass[] g = ....might be a subclass... for (int i=0;i<100000;i++) { g[i].func(i); } Suppose 99% of g's are the same class. The JIT can optimize away most of the virtual function lookups (particularly if this is one of your hotspots). I.e. the code becomes: ParentClass[] g = ....might be a subclass... for (int i=0;i<100000;i++) { if (g.class == COMMONCLASS) { inlined_func(i); else { g[i].func(i); } } In the common case, a simple pointer comparison + inlined code is a lot faster than a function dereferene.

richardwhiuk 10y ago
You could, I guess, annotate C++ child classes as likely, and then perform this optimization. That'd be an extension to the standard of course.
vitalyd 10y ago
Devirtualization is mostly an issue for Java since everything is virtual by default and the language doesn't have support for compile time monomorphization. While C++ code does use virtuals, it's nowhere near the amount as Java - there are language constructs to avoid that and move the dispatch selection to compile time.
ahh 10y ago
GCC in fact performs this optimization.
- yummyfajitas 10y ago
  GCC can get runtime information at compile time? That's a truly advanced compiler.
  - ahh 10y ago
    It can in fact make predictions in a number of circumstances. The most common case is FDO (https://gcc.gnu.org/wiki/AutoFDO https://gcc.gnu.org/wiki/AutoFDO) - with FDO coverage GCC can easily demonstrate a particular vcall is ~always a particular type and emit exactly the code you describe. (To be clear, this isn't just in theory, but is actually happening in a wide variety of binaries I use.) This is perhaps not fully "ahead of time", granted, but it's extremely easy to deploy and highly effective, and entirely accessible to C++.
  - srean 10y ago
    Snark not warranted, profile guidance is a thing. But more likely many of those virtual functions will not be a virtual function in idiomatic C++ for performance oriented design. I have seen that I can get away with compile time polymorphism many a time. For the rest there is compiler devirtualization and profiles.

richardwhiuk 10y ago

You could, I guess, annotate C++ child classes as likely, and then perform this optimization. That'd be an extension to the standard of course.

vitalyd 10y ago

Devirtualization is mostly an issue for Java since everything is virtual by default and the language doesn't have support for compile time monomorphization. While C++ code does use virtuals, it's nowhere near the amount as Java - there are language constructs to avoid that and move the dispatch selection to compile time.

ahh 10y ago

GCC in fact performs this optimization.

yummyfajitas 10y ago
GCC can get runtime information at compile time? That's a truly advanced compiler.
- ahh 10y ago
  It can in fact make predictions in a number of circumstances. The most common case is FDO (https://gcc.gnu.org/wiki/AutoFDO https://gcc.gnu.org/wiki/AutoFDO) - with FDO coverage GCC can easily demonstrate a particular vcall is ~always a particular type and emit exactly the code you describe. (To be clear, this isn't just in theory, but is actually happening in a wide variety of binaries I use.) This is perhaps not fully "ahead of time", granted, but it's extremely easy to deploy and highly effective, and entirely accessible to C++.
- srean 10y ago
  Snark not warranted, profile guidance is a thing. But more likely many of those virtual functions will not be a virtual function in idiomatic C++ for performance oriented design. I have seen that I can get away with compile time polymorphism many a time. For the rest there is compiler devirtualization and profiles.