Feature Request: nontrivial functions and vtable optimizations

[davidl]

在 Tue, 12 Aug 2008 22:46:21 +0800，Craig Black <cblack at ara.com> 写道:

> "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 understand why this is so hard for some programmers to accept.
>
> See Microsoft Visual C++ profile guided optimizations (PGO) "Virtual Call
> Speculation". Using PGO, the most commonly called virtual functions are
> optimized so that they do not require a function pointer invokation.
>
> Rather than relying on PGO, I would prefer the ability to assign a
> priorities to virtual functions in the code.  This would indicate to the
> compiler what virtual functions will be called the most.
>
> -Craig
>
>

But it's so anti-cleanness hack.

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