[fpc-devel] Progress on pure functions

[J. Gareth Moreton]

Hi everyone,

I've made a bit of a breakthrough with the development of pure 
functions.  I've successfully managed to get the compiler to calculate 
the factorial as a pure function in two different forms... one usig a 
for-loop, and one using a recursive call (which is where it differs from 
'inline').

It's nowhere near ready and I haven't tested floating point numbers or 
strings yet (and error messages and warnings etc. are still lacking).  
Only const and value parameters are accepted currently.  I haven't 
included support for "out" parameters because functions that return a 
value and have an "out" parameter aren't inlined and would be a little 
bit tricky to build a valid node tree for.

Find it here: https://gitlab.com/CuriousKit/optimisations/-/commits/pure

I welcome anyone to test it out and try to break it!  To confirm if the 
compiler has interpreted a subroutine as a pure function, the best way 
is to look at the node tree with DEBUG_NODE_XML, specifically the result 
of the "firstpass" section.

I've also attached two code examples that contain the factorial functions.

Kit
-------------- next part --------------
program pure1a;
{$MODE OBJFPC}
{$COPERATORS ON}

function Factorial(N: Cardinal): Cardinal; pure;
  var
    X: Integer;
  begin
    Result := 1;
	for X := N downto 2 do
      Result *= X;
  end;

begin
  WriteLn(Factorial(5));
end.
-------------- next part --------------
program pure1b;
{$MODE OBJFPC}

function Factorial(N: Cardinal): Cardinal; pure;
  var
    X: Integer;
  begin
    if N < 2 then
	  Result := 1
	else
      Result := N * Factorial(N - 1);
  end;
  
begin
  WriteLn(Factorial(5));
end.