I was wrong.

[Yigal Chripun]

Jb wrote:
> "superdan" <super at dan.org> wrote in message 
> news:g81o61$21bs$1 at digitalmars.com...
>> Jb Wrote:
>>
>>> "superdan" <super at dan.org> wrote in message
>>> news:g81e1l$18js$1 at digitalmars.com...
>>>> downs Wrote:
>>>>> 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.
>>> To speculate localy you still have to lookup the class info, compare it 
>>> to
>>> the speculated object type, and then conditionaly branch. You cant avoid 
>>> a
>>> conditional branch, and as it will share the same pattern as the indirect
>>> jump, and (in most cases) the same prediction mechanism, neither will be
>>> better predicted than the other.
>> no that's wrong. conditional branch is better speculated than indirect 
>> jump.
>> they share no other hardware than the instruction fetching pipe.
> 
> Sorry but you dont know what you are talking about. Indirect jumps get a 
> slot in the branch target buffer (BTB) the same as conditional branches. 
> They both share the same target prediction hardware. It's not only a fact 
> it's also common sense that they would do so.
> 
> http://www.agner.org/optimize/microarchitecture.pdf
> 
> Read the chapter on branch prediction.
> 
> http://www.intel.com/design/processor/manuals/248966.pdf
> 
> Read Chapter 3.4.1.6
> 
> 
>> the mechanism for conditional branch speculation is that stuff is executed 
>> in parallel
>> on both branches inside the pipeline. the branch that makes it is 
>> committed. the other
>>  is retired without getting to write to the register file or memory.
> 
> Actualy AFAIK no current x86 processor *execute* both branches. What does 
> happen is the instruction cache will *prefetch* both branches, so that when 
> the BTB decides which way to go it has the instruction data ready.
> 
> But it will still cost you a whole pipeline flush if the prediction was 
> wrong.
> 
> 
>>> So the point is that you cant make the common path any faster unless you
>>> actualy *inline* the speculated method. In fact if you dont inline you 
>>> are
>>> most probably making the situation worse because you are replacing 
>>> this...
>> you are right here tho for the wrong reasons :) yeah speculation is good 
>> if there are
>> a few good chunky instructions to eat while speculating. if it's a 
>> function call that in
>> turn is just a ret... the results are nothing to write home about. as it's 
>> been shown.
> 
> No.. erm NO... and NOOooOOOO!!!!
> 
> ;-)
> 
> Speculation is good if it allows you to inline the method and chop out a 
> whole bunch of register shuffling, stack setup, and entry / exit code.
> 
> It's got naff all to do with "having shit to do" while you're speculating. 
> Just like it's got naff all to do with conditional branches being faster / 
> better predicted than indirect calls.
> 
> 

I think Superdan is just too old ;)
What he is talking about is probably the way things worked in P4 and
that design was canceled for many good reasons. As Jb says above, No
current x86 processor *execute* both branches.
My Digital Systems professor works at Intel here in Israel where they
design the chips ;) and he taught us the same thing Jb said above.