[fpc-devel] Standard generic classes

[J. Gareth Moreton]

Still, the theory's there just in case... if the arguments are constant 
(but unknown) and the pure function is called multiple times, then it 
can be optimised to only be called once.

Gareth aka. Kit

On 06/06/2019 01:08, J. Gareth Moreton wrote:
>
> Ah, fair enough.  I didn't think to study the for-loop that closely 
> yet since I haven't really found a need to check its code.  Sometimes 
> I feel silly blurting out my ideas like this, but at least you stopped 
> me from doing a fool's errand.  Thanks.
>
> Gareth aka. Kit
>
> On 05/06/2019 22:38, Sven Barth via fpc-devel wrote:
>> J. Gareth Moreton <gareth at moreton-family.com 
>> <mailto:gareth at moreton-family.com>> schrieb am Mi., 5. Juni 2019, 18:06:
>>
>>
>>     *for *X := 0 *to *Min(BUFFER_SIZE, DataLeft) *- 1 do*
>>     *begin*
>>       { Do some work that doesn't modify DataLeft }
>>     *end*;
>>
>>     Say that Min is given the *pure *directive, BUFFER_SIZE is a
>>     *const *and DataLeft is a property or some non-local variable. 
>>     Since no volatile intrinsic is used to safeguard multi-threading
>>     issues, and DataLeft isn't modified by the for-loop, DataLeft can
>>     be promoted to the stack or a register.  Even though its exact
>>     value is not known, DataLeft is determined to be constant within
>>     the confines of the for-loop, hence since both arguments in
>>     "Min(BUFFER_SIZE, DataLeft)" are constants, the result must also
>>     be constant, since it's a pure function, therefore the function
>>     only has to be called once and its result be stored on the stack,
>>     recalled during each iteration of the for-loop.  At least that's
>>     the theory.  Of course, you can just store the result yourself in
>>     a local variable and use that as part of the for-loop, but it's
>>     just a thought where pure functions could be used that doesn't
>>     involve computing their result at compile-time.
>>
>>
>> With properties or global variables you can't guarantee that DataLeft 
>> is constant as the for loop might call some routine that more or less 
>> indirectly might change what DataLeft returns.
>> That said: the end-expression of a for-loop is specified to be 
>> evaluated only once anyway (in contrast to C-like languages).
>>
>> Regards,
>> Sven
>>
>>
>> _______________________________________________
>> fpc-devel maillist  -fpc-devel at lists.freepascal.org
>> http://lists.freepascal.org/cgi-bin/mailman/listinfo/fpc-devel
>
> <https://www.avast.com/sig-email?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=emailclient> 
> 	Virus-free. www.avast.com 
> <https://www.avast.com/sig-email?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=emailclient> 
>
>
> <#DAB4FAD8-2DD7-40BB-A1B8-4E2AA1F9FDF2>
>
> _______________________________________________
> fpc-devel maillist  -  fpc-devel at lists.freepascal.org
> http://lists.freepascal.org/cgi-bin/mailman/listinfo/fpc-devel


---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.freepascal.org/pipermail/fpc-devel/attachments/20190606/bb47c393/attachment.html>