[fpc-devel] Need patch for bugs : 0011503 / 0009472

[Jonas Maebe]

On 19 Jun 2008, at 13:21, Boian Mitov wrote:

> Thank you!
> I don't understand. 0011503 is a major failure. Is it in the  
> compiler or in some library?

Please read the comments to http://bugs.freepascal.org/view.php? 
id=9472. The whole issue is discussed there in great detail.

> This is a huge problem. This code must work the same way in both  
> cases but it does not. We are porting a huge library of over 4  
> million lines of code from Delphi. This is a major show stopper. I  
> can probably take a look in the compiler as it will be much easier  
> and faster for us to fix the compiler to be Delphi compatible rather  
> than rewriting the libraries. They rely heavily on interfaces and  
> the correct reference counting. Rewriting this even if desirable  
> will delay the project with at least a year at best. By definition  
> when there are no active references to an interface it must be  
> released. This is a total failure of the functionality.

There are active references, namely in an invisible temp. When this  
invisible temp is destroyed (at the function exit), the interface is  
freed.

It is wrong to assume that implicit temps which still contain  
references to interfaces (or ansistrings, or any other reference  
counted type) can never exist, because this is an implementation  
detail over which the compiler has full control. This can also happen  
in Delphi under certain circumstances by the way (namely if it cannot  
guarantee that optimising the interface assignment is safe without  
using a temp).

The purpose of automatic reference counting is simply that memory  
management is automated. It is not to guarantee the same semantics as  
what you would get in a manual reference counting scheme where you are  
in full control of when and where reference counts are increased and  
decreased. In principle, a periodic mark and sweep would also still be  
correct behaviour.

Newer versions of FPC optimise assignments of function results more  
aggressively so you will more often get reference counting behaviour  
like in Delphi, but we do not guarantee in any way that it will be  
100% identical. We only emulate the Delphi language, not the  
implementation details of the Delphi compiler or code generator.


Jonas