Gareth sent me the following two mails in private, but they were ment
for the list:
Am 09.08.2018 um 16:18 schrieb J. Gareth Moreton:
Thanks Sven. Normally I would agree with
"pure" only belonging in the interface
section, but it causes problems when you
try to put a pure function inside the main
program block, as the entire thing is
considered equivalent to implementation.
Also, inline is allowed in the
implementation section, and the two follow
similar rules in regards to their calls
being modified.
Just to clarify, "pure" doesn't change
anything in regards to the parameter types
or how a call with variable arguments is
handled. The raw signature shouldn't
change. It's an optimisation hint. At
least it is in a perfect world!
Am 09.08.2018 um 23:10 schrieb J. Gareth Moreton:
I asked the question because I stumbled across something interesting.
I've been using the following set of functions to see how the compiler
handles things that are a bit out of order (using 'inline' as a
temporary stand-in):
program PureTest;
function PureMax(const a: Double; const b: Double): Double; forward;
procedure TestFunc;
begin
WriteLn(PureMax(2.0, 3.0));
end;
function PureMax(const a: Double; const b: Double): Double; inline;
begin
if (a > b) then
Result := a
else
Result := b;
end;
begin
TestFunc;
end.
****
Turns out, to my surprise, after analysing the nodes and the
disassembly, that "PureMax" is not inlined inside the TestFunc
routine. I haven't tested units yet, but would a similar situation
occur if a function is not defined as inline in the "interface"
section and only the "implementation" section (unless the caller
appears after said function in the source file)?
Either way, I'm leaning towards making the new "pure" directive an
interface-only directive as Sven suggested, as that will make things a
lot easier if a pure function is used as part of a constant
definition. After all, "pure" would have a bit more of a notable
effect than "inline" because it dictates where the function can and
cannot be used.
I hadn't thought that I need to explain that, but apparantly you haven't
yet reached that far in your research of the compiler, so:
- of course I only mean that it's an interface-only modifier if there
*is* an interface section; routines that are - inside a unit - only
declared inside the implementation section are obviously exempt from
this (thus also for the program file)
- again I wrote that I don't mean that "pure" modifies the parameters,
but it modifies the meta data (most importantly checksum) of the
interface section when you add a modifier flag inside the implementation
section. The compiler first handles the interface sections of units and
might then compile interface sections or even implementation sections of
other units depending on the dependencies between the units. If a flag
only appears in the implementation section than this messes up this
whole thing as the other units would need to be recompiled. This is
already a mess with "inline" so we don't need to do that with "pure" as
well, especially as we don't need to be Delphi compatible here.
- yes, it is correct that the compiler's capability to inline depends on
the order of the routine definitions. The compiler generates the code
routine by routine so if the routine's body has not yet been encountered
then it can't be inlined. (When using generic specializations it
triggeres the generation of the specialized routine bodies before
generating the routine to avoid "inline" not working for generics) For
"pure" we can even go so far as to say that a routine body *must* be
available to be able to call it inside another "pure" routine.
Everything else would be an error.
I hope this clears things up a bit.
Regards,
Sven
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