[fpc-pascal] Are there any drawbacks to "reference to"?

Sven Barth pascaldragon at googlemail.com
Mon Jun 20 07:44:53 CEST 2022

Am 20.06.2022 um 03:37 schrieb Hairy Pixels:
>> On Jun 19, 2022, at 10:04 PM, Sven Barth <pascaldragon at googlemail.com> wrote:
>> As you can see the allocation only happens once and not all the time.
>> What might be worse however is the optimization behavior as in this example the compiler wouldn't optimize the counter variable into a regvar with enabled optimizations (however in case of a nested function the compiler wouldn't do this either if the function isn't inlined).
> I’m saying if the OUTER method is in a tight loop (Foo in this example) you’re in trouble because it now contains an allocation which is unexpected.

In that case we're in the same territory as always: ensure that you know 
what your code does. It's the same reason why adding character by 
character to a managed String is slower then allocating the string once 
and then setting the characters. And I think it's very seldom that 
someone uses a function reference that does not leave the scope of the 
surrounding function.

>   In fact if this happened within a class wouldn’t it capture and initialize all the classes fields also or does it know to only use the fields which are encountered in the function body?
If you use a function reference inside a method and access one of the 
class' fields then it's only Self that's captured.
> Either way the reference never leaves the calling scope and thus is wasted and should be optimized away to something which exists on the stack, like “is nested”.
If the compiler can proove that the function reference never leaves the 
scope (that means no assignment to global variables, out/var parameters, 
the Result variable, passing on to some other function or conversion to 
pointer (and I'm sure there are others)) and the function reference 
isn't used as an interface then and only then the compiler could reduce 
this to something else. But that's an optimization for another day and 
in my opinion such a rarlely used usecase that it's simply not worth it.


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