[fpc-devel] Generated code of constant parameters of double type

[Luiz Americo Pereira Camara]

Florian Klaempfl escreveu:
> Luiz Americo Pereira Camara schrieb:
>> Florian Klaempfl escreveu:
>>> Luiz Americo Pereira Camara schrieb:
>>>> My point was that this could be a missing optimization opportunity: 
>>>> pass by reference a 8 bytes parameter when the pointer size is 4. 
>>>
>>> Don't forget that this makes an extra memory access.
>>
>> I will do my question in a simpler way:
>>
>> Using constant parameters (add const keyword in front of an argument) 
>> is beneficial for record types with size > pointer size and for 
>> string types but not for integer and pointer types.
>
> No, this is not necessarily true. The extra indirect can be also slower.

For string types it will avoid incrementing and decrementing the ref 
count. See the test i did at 
http://lazarusroad.blogspot.com/2008/11/effect-of-using-constant-parameter-for.html 
and the code diff at http://www.geocities.com/camara_luiz/coddiff.htm

For record (TPoint) the constant version generate less code. See the 
attached diff.

>
>>
>> Currently is not beneficial for double types and there's no way to 
>> improve that. So when developing an interface that uses double types 
>> does not matter using const or not. Now and in the future. Right?
>
> It might matter for the optimizer in other situations.

I am developing an component that heavily uses double types. So for read 
only arguments is better to use constant parameters?

For the record: Int64 has the same behavior as double regarding this issue.

Luiz
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