[fpc-devel] Compile time functions

[J. Gareth Moreton]

An interesting read. If you can collapse an inline function into a single
assignment in most situations, then brilliant! Definitely keep that optimisation in.

While there are many similarities and prerequisites between an inline function
and a pure function, and many simple mathematical functions could easily carry
both modifiers, they are not exactly the same.  Pure functions cannot be
evaluated at compile time if one of the arguments is a variable (or at the very
least, not deterministic), but it might be inlined, while a function with
recursion can't be reliably inlined, but might still be pure (e.g. factorial,
assuming you don't rewrite it to use a for-loop), and not all recursive functions
can be rewritten to use simpler loops (e.g. the infamous Ackermann function).  It
is true though... proper constant propagation would cover most situations, even
when an inline function calls another inline function such as Max, although there
might still be issues in exceptional conditions (e.g. division by zero or trying
to calculate tan(pi/2), for example).

The other point with a "pure" modifier is that, assuming the compiler determines
that the function is indeed pure, it would permit their assignment to a constant.
 True, you could replace the constant with the function call whenever it appears,
but that's not exactly good coding practice. And that's sort of the other reason
for the "pure" modifier.  If you release a library of functions that a
third-party then looks at, with no background knowledge of its inner workings,
the appearance of the "pure" modifier would quickly clue them in in regards to
the functions' intent.

The final issue is that while a function might be pure, you might not want to
inline it because of its complexity, especially if it's called multiple times
with variable arguments. The simplest example I can think of is the two-argument
binomial coefficient:

function Binomial(n, r: Cardinal): Cardinal; pure;
begin
  if (r > n) then
    Result := 0
  else if (r = n) then
    Result := 1 { Faster than calculating the factorials }
  else
    Result := Factorial(n) div (Factorial(r) * Factorial(n - r));
end;

The alternative versions either require recursion, or for exceptionally large
values of n and r, calculating the integral that defines the gamma function.

Gareth aka. Kit


On Sat 21/07/18 18:51 , Sven Barth via fpc-devel fpc-devel at lists.freepascal.org sent:
> Martok  schrieb am Sa., 21. Juli 2018, 17:12:
> Am 21.07.2018 um 02:08 schrieb Sven Barth via fpc-devel:
> > The main problem is that not all functions that would be eligible for
> your
> > approach are also declared as inline thus their node trees would not be
> stored
> > inside the PPU and thus you could not work with them.
> I'm not sure I understand. Isn't the same true, with a new feature? The
> full
> node tree must be available, which basically means "has inlineinfo".
> Doesn't it?
> 
> Correct. 
> 
> > Additional benefit of the "pure" modifier: the compiler can check once
> the
> > method has been parsed whether it's pure or not and can thus error out
> in the
> > latter case.
> That is true, it is a more explicit declaration of intent.
> Maybe the codegen of pure can simply be implemented as generating inline
> info,
> but always replacing the calls with the simplified version. If it is
> already
> known that a function is pure, what I did with constexpr() would basically
> be
> guaranteed to work.
> 
> Pure functions would still have an ordinary generated body as there is no
> reason that their address shouldn't be able to be taken (otherwise one
> needs to declare wrappers for them). Other than that I personally favor the
> explicit declaration of intent for such functions without the need for a
> ConstExpr intrinsic. 
> Regards, Sven    _______________________________________________
> fpc-devel maillist - fpc-devel at lists.freepascal.org
> http://lists.freepascal.org/cgi-bin/mailman/listinfo/fpc-devel [1]
> 
> 
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