[fpc-devel] A bug in FPC 2.4.x for accessing object methods from local functions?

[Kostas Michalopoulos]

Hi all,

I've found a bug that causes FPC 2.4.2/2.4.4 to create invalid code. It 
seems to be caused when a method of a packed* object is called from a 
local procedure and the object is declared on the procedure above 
(although it might be caused in other cases too - i just found it in my 
Lazarus program and trimmed it down enough to be usable for a bug report).

Note that this is caused only with -O2 or above enabled and with -O1 
works as expected.

(*=in my original program it also happens without the object being 
"packed" but i couldn't reproduce that)

Below is an example program that shows the issue:

-------------------------------------------------
program testprog;
{$MODE OBJFPC}{$H+}
type
   TThing = packed object
     a, b, c: Extended;
     procedure Dump;
   end;

   TTest = class
     procedure DoTest;
   end;

function Thing(a, b, c: Extended): TThing; inline;
begin
   Result.a:=a;
   Result.b:=b;
   Result.c:=c;
end;

procedure TThing.Dump;
begin
   Writeln(a:0:2, ' ', b:0:2, ' ', c:0:2);
end;

procedure TTest.DoTest;
var
   Some: TThing;

   procedure Add(a, b, c: Extended);
   begin
     Some.Dump;
   end;

begin
   Some:=Thing(1, 1, 1);
   Some.Dump;
   Add(10, 20, 30);
   Add(-10, 20, 30);
   Add(10, -20, 30);
   Add(10, 20, -30);
   Some:=Thing(1, 1, 0);
   Add(10, 10, 10);
   Add(20, 20, 20);
   Add(30, 30, 30);
   Add(40, 40, 40);
end;

var
   Test: TTest;
begin
   Test:=TTest.Create;
   Test.DoTest;
   Test.Free;
end.
-------------------------------------------------

The call of "Some.Dump" from inside "Add" seems to get the wrong values 
- or crash. It probably tries to read from invalid memory locations.

The bug exists in both 2.4.2 and 2.4.4. I checked out the code from the 
FPC SVN repository (revision 17644) and it seems to compile fine - or at 
least produce the expected results. However this might be accidental so 
i decided to report it.

Also if anyone knows the reason behind this and how to work around it, i 
would like to hear.

Kostas "Bad Sector" Michalopoulos