Precise GC

[Manu]

On 13 April 2012 15:53, Kagamin <spam at here.lot> wrote:

> On Sunday, 8 April 2012 at 12:02:10 UTC, Alex Rønne Petersen wrote:
>
>> This sounds important to me. If it is also possible to do the work with
>>> generated tables, and not calling thousands of indirect functions in
>>> someone's implementation, it would be nice to reserve that possibility.
>>> Indirect function calls in hot loops make me very nervous for non-x86
>>> machines.
>>>
>>
>> Yes, I agree here. The last thing we need is a huge amount of
>> kinda-sorta-virtual function calls on ARM, MIPS, etc. It may work fine on
>> x86, but anywhere else, it's really not what you want in a GC.
>>
>
> What's the problem with virtual calls on ARM?
>

No other processors have branch prediction units anywhere near
the sophistication of modern x86. Any call through a function pointer
stalls the pipeline, pipelines are getting longer all the time, and PPC has
even more associated costs/hazards.
Most processors can only perform trivial binary branch prediction around an
'if'.
It also places burden on the icache (unable to prefetch), and of course the
dcache, both of which are much less sophisticated than x86 aswell.
Compiler can't do anything with code locality (improves icache usage),
since the target is unknown at compile time... there are also pipeline
stalls introduced by the sequence of indirect pointer lookups preceding any
virtual call.
Virtuals are possibly the worst hazard to modern CPU's, and the hardest to
detect/profile, since their cost is evenly spread throughout the entire
code base, you can never gauge their true impact on your performance. You
also can't easily measure the affect of icache misses on your code, suffice
to say, you will have MANY more in virtual-heavy code.

While I'm at it. 'final:' and 'virtual' keyword please ;)
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