Linus with some good observations on garbage collection

["Jérôme M. Berger"]

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
-- 
mailto:jeberger at free.fr
http://jeberger.free.fr
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