[fpc-devel] Standard generic classes

[J. Gareth Moreton]

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
>
>
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