I was wrong.

[downs]

superdan wrote:
> downs Wrote:
> 
>> superdan wrote:
>>> downs Wrote:
>>>
>>>> To clear this up, I've been running a benchmark.
>>>>
>>>> module test91;
>>>>
>>>> import tools.time, std.stdio, tools.base, tools.mersenne;
>>>>
>>>> class A { void test() { } }
>>>> class B : A { final override void test() { } }
>>>> class C : A { final override void test() { } }
>>>>
>>>> A a, b, c;
>>>> static this() { a = new A; b = new B; c = new C; }
>>>>
>>>> A gen() {
>>>>   if (randf() < 1f/3f) return a;
>>>>   else if (randf() < 0.5) return b;
>>>>   else return c;
>>>> }
>>>>
>>>> void main() {
>>>>   const count = 1024*1024;
>>>>   for (int z = 0; z < 4; ++z) {
>>>>     writefln("Naive: ", time({
>>>>       for (int i = 0; i < count; ++i) gen().test();
>>>>     }()));
>>>>     writefln("Speculative for B: ", time({
>>>>       for (int i = 0; i < count; ++i) {
>>>>         auto t = gen();
>>>>         if (t.classinfo is typeid(B)) (cast(B)cast(void*)t).test();
>>>>         else t.test();
>>>>       }
>>>>     }()));
>>>>     writefln("Speculative for B/C: ", time({
>>>>       for (int i = 0; i < count; ++i) {
>>>>         auto t = gen();
>>>>         if (t.classinfo is typeid(B)) (cast(B)cast(void*)t).test();
>>>>         else if (t.classinfo is typeid(C)) (cast(C)cast(void*)t).test();
>>>>         else t.test();
>>>>       }
>>>>     }()));
>>>>   }
>>>> }
>>>>
>>>>
>>>> And as it turns out, virtual method calls were at least fast enough to not make any sort of difference in my calls.
>>>>
>>>> Here's the output of my little proggy in the last iteration:
>>>>
>>>> Naive: 560958
>>>> Speculative for B: 574602
>>>> Speculative for B/C: 572429
>>>>
>>>> If anything, naive is often a little faster.
>>>>
>>>> This kind of completely confuses my established knowledge on the matter. Looks like recent CPUs' branch predictions really are as good as people claim.
>>>>
>>>> Sorry for the confusion.
>>> you are looking with a binocular at a coin a mile away and tryin' to figure quarter or nickel. never gonna work. most likely ur benchmark is buried in randf timing.
>>>
>>> make iteration cost next to nothing. put objects in a medium size vector. then iterate many times over it.
>> I know. But if it doesn't matter in that case, it most likely won't matter in practial situations.
>>
>> Nonetheless, here are some better timings, including a faster (dirtier) randf() function, and a null pass that only generates the object.
>>
>> Null: 729908
>> Naive: 1615314
>> Speculative for B: 1692860
>> Speculative for B/C: 1664040
> 
> the whole premise of speculation is it oils the common path. you have uniform probabilities. what you gain on speculating for B you lose in the extra test when misspeculating on others. so to really test speculation make B like 90% of cases and retry.

As you wish.

Here's the timings for 10% A, then 81% for B, 9% for C.

Null: 629760
Naive: 1383764
Speculative for B: 1397784
Speculative for B/C: 1418163