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 <gar...@moreton-family.com
<mailto:gar...@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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