Why Strings as Classes?

[JAnderson]

Jb wrote:
> "Walter Bright" <newshound1 at digitalmars.com> wrote in message 
> news:g90iia$2jc4$3 at digitalmars.com...
>> Yigal Chripun wrote:
>>> a) people here said that a virtual call will make it slow. How much
>>> slow? how much of an overhead is it on modern hardware considering also
>>> that this is a place where hardware manufacturers spend time on
>>> optimizations?
>> Virtual function calls have been a problem for hardware optimization. 
>> Direct function calls can be speculatively executed, but not virtual ones, 
>> because the hardware cannot predict where it will go. This means virtual 
>> calls can be much slower than direct function calls.
> 
> Modern x86 branch prediction treats indirect calls the same as conditional 
> branches. They get a slot in the branch target buffer, so they do get 
> speculatively executed. And if correctly predicted it's only a couple of 
> cycles more costly direct calls.
> 
> See the thread "Feature Request: nontrivial functions and vtable 
> optimizations" about 2 weeks ago.
> 
> I cited the technical docs and a few doubters ran benchmarks, which proved 
> that virtual methods are not as evil as many people think. In fact they are 
> no more evil than a conditional branch.
> 
> 
> 
> 
> 

That's x86 hardware.  Try something like the ps3.  That systems has 
little or no cache.  They have to jump to the vtable which is in a 
totally different location from the class.  Note I'm not in the camp 
that things they should never be used on these ystems however I think 
you should use them smartly and profile profile profile.

One technique I've used in C++ to help improve things a little is to 
switch the vtable with one that's in the same location or close to the 
class.  The wrapper function looked something like this:

class A {...}

A a = new LocalVirualTable<A>(); //ie LocalVirualTable is a bolt-in template

However, performance only improved in cases where I could flush the 
cache.  It many cases it was slightly worse on a x86 so you had to try 
it, profile and see it it had a positive or negative effect in each 
case.  I imagine when you've got hundreds of these classes its simply 
more memory to process, so on a high cache system it can be inversely 
beneficial.  I never tried it on a ps3 so it might be more effective there.

-Joel