[fpc-devel] XML node dump feature

[J. Gareth Moreton]

Initially, F probably wouldn't be optimised because 'arg' is not 
constant.  However, if F were marked as inline (not pure) and its actual 
parameter is a constant (say, 5, for the sake of argument), then there 
is definitely potential for optimisation, since it would be expanded to 
the following:

s := 0;
for i := 0 to 100 do
   s := s + PureFunc(5);
Result := s;

In this case, PureFunc(5) would be replaced with its pre-calculated 
result, and some node analysis could potentially replace the entire 
for-loop with s := s + x, where x is equal to 101 * PureFunc(5), since 
it's now dealing with simple numbers.

Now for the case where F is not pure or inline, it may take a bit more 
thought in allowing the compiler to detect the mathematical equivalence 
and replace the for-loop with s := s + 101 * PureFunc(arg), but it's 
certainly doable.  Going back to the original subject title, this is one 
reason why I pushed to get the XML node dump feature implemented, 
because it provides valuable research data for the development of such 
optimisations.  Such an optimisation may have to be -O4 though because 
of a side-effect with range check errors - if the error is handled, s 
might contain a different value to what is expected (e.g. if s was 
initially $7FFFFFF0 and PureFunc(arg) = 1, then assuming s isn't written 
to before the error is raised, it will still contain $7FFFFFF0 rather 
than $7FFFFFFF that would occur if the for-loop ran a few times).

Long story short, it took me a brief moment to see how the optimisation 
would work until I thought about it a bit more, but if PureFunc is 
indeed pure, then that could certainly be a development goal.

Thank you very much for that!

Gareth aka. Kit


On 25/06/2019 09:19, denisgolovan wrote:
>> Can I have an example of "hoisting pure functions out of loops" that you
>> speak of?
> Sure.
> I mean something like following (completely artificial example):
>
> function PureFunc1: ...
> function F(arg: Integer): Integer;
> var s:Integer;
> begin
>    s:=0;
>    for i:=0 to 100 do
>       s:=s+PureFunc(arg);
>    Result:=s;
> end;
>
> Of course, the real value of pure functions for optimization oppotunities is some combination of generics and closures, but that's completely another story...
>

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