Inlining Virtual Functions

[bearophile]

dsimcha:

> Under what circumstances is this actually true?

At the moment I think that spec is pure theory for DMD, even if it's worded to almost sound as the compiler already works like that :-)


>As far as I can tell, there is no way DMD could know about the whole class hierarchy when generating code, given that in practice modules are compiled separately.<

I think with LDC it will be possible to perform global optimizations among different modules, allowing such optimization too, in some situations (LDC developers may confirm of deny this).

For Java (and this may be true for C# of dotnet too) a virtual machine like HotSpot is able to analyze the call loci and compile and/or inline code dynamically. Sometimes it's able to turn a virtual dispatch into a direct call, sometimes such virtual dispatch is replaced with a branch with two possible direct calls, and sometimes it puts a test that allows a direct call in most cases or a virtual dispatch as fallback for the less common case. There are even more complex cases, but you often don't gain much more. Removing virtual calls speeds up code a little by itself, but then later it allows the compiler to inline methods, and this may speed up code significantly in inner loops.
A possibility for D is runtime-driven optimization: you compiler and run the code, the compiler collects statistics on the virtual call sites (among many other kinds of statistics) and then you can re-compile your code letting the compiler use such run-time information to inline things, etc. Of course your test run must be significant of the final normal usage of your code, otherwise you optimize against the wrong things (while HotSpot runs concurrently of your code, so it surely collects real-world data).
The GCC (and Intel) compiler is already able to perform such collection of run-time statistics, with -fprofile-generate and -fprofile-use, but I don't know if it uses such information to inline ex-virtual calls too.
This too is related:
http://www.digitalmars.com/ctg/trace.html

Bye,
bearophile