Feature Request: nontrivial functions and vtable optimizations

[Don]

Craig Black wrote:
> "downs" <default_357-line at yahoo.de> wrote in message 
> news:g7ribh$1vii$1 at digitalmars.com...
>> The overhead of vtable calls is well known.
>>
>> For this reason, it may be worthwhile to include an optimization that 
>> essentially checks if a certain class is only inherited from a few times, 
>> like, <= three, in the visible program, and if so, replace vtable calls 
>> with a tree of classinfo pointer comparisons and direct function calls.
>>
>> When I did this manually in some hobbyist path tracing code, I gained 
>> significant speed-ups, even though I didn't collect any hard numbers.
>>
>> So, for instance, the equivalent D code would look like so:
>>
>> class A { ... } class B : A { ... } class C : A { ... }
>>
>> A a = genSomeObj();
>> // a.test();
>> if (a.classinfo == typeid(B)) (cast(B)cast(void*) a).test(); // call as if 
>> final
>> else if (a.classinfo == typeid(C)) (cast(C)cast(void*) a).test(); // dito
>> else a.test(); // fallback
>>
>> Ideas?
> 
> You are right.  Function pointer invokation tends to slow down processing 
> because it doesn't cooperate well with pipelining and branch prediction.

I don't think this is true any more.
It was true in processors older than the Pentium M and AMD K10. On 
recent CPUs, indirect jumps and calls are predicted as well as 
conditional branches are. If you're seeing this kind of the speed 
difference, it's something else (probably, more code is being executed).