[fpc-pascal] About the bug report #9408...

[Peter Vreman]

> Compelling FPC 2.1.4 to behave like FPC 2.0.4 is not very difficult.
> In a program where you have "A := B - C;" (A,B,C being Longwords), it
> is sufficient to write "A := Longword(Longint(B) - Longint(C));" and all
> is right. The most difficult is to find which lines of code has to be
> modified. FPC signals most of them ("Possible loss of data ...") but not
> all. For instance, "if (A and (A-1)) = ..." is silently converted as a
> 64-bit operation.
>
> But the problem is not just an efficiency problem, it is also a 'result'
> problem. For instance, try this with both FPC 2.0.4 and 2.1.4 :
>
>    B := 1111;
>    C := 1112;
>    D := 311;
>    A := (B - C) div D;
>
> You will get two different values for A.
> With 2.0.4, you compute ((B - C) mod 2^32) div D; with 2.1.4, you
> compute (B - C) div D where (B - C) is a 64-bit signed parameter. These
> are two different operations.

We did a lot of experiments regarding this. There is no clear description how to calculate the
longwords.

The current behavior of 2.1.4 is the best effort we could do that is both the same on 32-bit and
64-bit platforms. In 2.0.4 there was a difference between 32-bit and 64-bit platforms and fpc was
also not always compatible with delphi.