Linus with some good observations on garbage collection

[Don]

Jérôme M. Berger wrote:
> Bruno Medeiros wrote:
>> On 29/04/2011 17:30, Andrei Alexandrescu wrote:
>>> On 4/29/11 10:40 AM, Bruno Medeiros wrote:
>>>> On 23/04/2011 15:45, Andrei Alexandrescu wrote:
>>>>> On 4/23/11 8:57 AM, dsimcha wrote:
>>>>>> BTW, since when does the ternary operator work with functions, as
>>>>>> opposed to variables?
>>>> And that's since the C days btw. The function call is just an operation,
>>>> the callee an operand, so any expression can be there.
>>>>
>>>>> They are converted to pointers to functions. Not something I'd
>>>>> recommend
>>>>> because it makes the call slower.
>>>>>
>>>>> Andrei
>>>> Hum, I didn't know that. Why does it make the call slower (other than
>>>> the actual ternary operation), is it because it has to load the function
>>>> address from a register/variable, instead of being a constant value?
>>>> And/or not being able to inline the call?
>>>> I mean, the first aspect seems like a very minor impact in performance,
>>>> almost negligible.
>>> It's an indirect call instead of a direct call.
>>>
>>> Andrei
>> Well, yes, that's kinda of what I was thinking already (although I
>> referred to more low-level, assembler terms).
>> But my question remains, why is an indirect function call slower than a
>> direct one, and is that difference significant? (excluding of course the
>> possibilities of inlining and other static analysis that direct
>> invocations offer) There is no extra overhead other than loading the
>> function address from a register/variable, right?
>>
> 	AIUI, with a direct call the CPU can anticipate the call (because
> it knows the target address) and start filling the pipeline
> immediately. OTOH, with an indirect call the CPU cannot know the
> target until it reads the register, which means that the pipeline
> will empty itself before the call is executed. With the pipeline
> length of some modern CPUs (Pentium 4 being amongst the worst), this
> can result in very large slowdowns.
> 
> 		Jerome

That was true ten years ago, but not any more. Modern CPUs can do branch 
prediction of indirect jumps.