[fpc-devel] Progress on pure functions

[J. Gareth Moreton]

The next big milestone that I want to achieve is to make this a pure 
function:

procedure int_str_unsigned(l:longword;out s:shortstring); pure;
var
   m1 : longword;
   pcstart,
   pc2start,
   pc,pc2 : pchar;
   hs : string[32];
   overflow : longint;
begin
   pc2start:=@s[1];
   pc2:=pc2start;
   pcstart:=pchar(@hs[0]);
   pc:=pcstart;
   repeat
     inc(pc);
     m1:=l div 10;
     pc^:=char(l-(m1*10)+byte('0'));
     l:=m1;
   until l=0;
   overflow:=(pc-pcstart)-high(s);
   if overflow>0 then
     inc(pcstart,overflow);
   while (pc>pcstart) do
     begin
       pc2^:=pc^;
       inc(pc2);
       dec(pc);
     end;
   s[0]:=char(pc2-pc2start);
end;

This is essentially the core internal function that drives IntToStr and 
similar functions.  The challenges here include:

- A repeat...until loop with no obvious termination sequence.
- Using a pointer to access an array offset of a local variable.
- Writing characters (and the length field) one at a time to a shortstring.

The reason for wishing to make IntToStr a pure function is that for a 
given input, the output will never change, and it's perfectly feasible 
for some other function to call IntToStr as part of a string generation 
routine and which would otherwise itself be a pure function (if a pure 
function wishes to call another function, it must also be determined to 
be pure... see pure1b.pp for the recursive example where the actual 
parameter isn't even a constant, but is nonetheless deterministic).

Kit

On 13/12/2022 06:21, J. Gareth Moreton via fpc-devel wrote:
> 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
>
> _______________________________________________
> fpc-devel maillist  -  fpc-devel at lists.freepascal.org
> https://lists.freepascal.org/cgi-bin/mailman/listinfo/fpc-devel